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PC-SIG Diskette Library (Disk #1408)
[PCjs Machine "ibm5170"]
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Information about “CLONEROM”
Watch out, Big Blue! CLONEROM helps the dedicated hobbyist make his
PC/XT/AT absolutely, totally IBM compatible, right down to the
programming of the chips in the motherboard. To obtain system-level
compatibililty, you must install a true blue ROM-BIOS. The full text
from the IBM-PC Read-Only-Memory Backup Manual is ready to print to
tell you how. Two utility programs are available to help you finish the
job.
If you already own an IBM or clone, how would you like to have backup
ROM and EPROM chips that cost less than IBM-programmed ICs? You can do
it with CLONEROM. Save your entire ROM on disks as insurance against
disaster.
CORELOOK.DOC
CORELOOK.EXE
PC Memory Scan and Search
This program is a simple tool for viewing the contents of the
PC's memory. It will display any memory address within the 1024 K
bytes that are addressable in the Intel segmented addressing scheme.
Thus includes the 640 K bytes of user memory and the 360 K bytes
defined for system and hardware support functions.
This program also has a string search function that will find
the memory locations that contain a string of up to five characters
that you define.
When CORELOOK is loaded, a screen frame appears that includes
the viewing "window" into the memory contents as well as the
address/string input fields and all required help information.
MEMORY CONTENTS DISPLAY
CORELOOK displays the Hex values of 256 bytes of memory in the
viewing "window", arranged in 16 rows of 16 bytes, similar to DOS's
DEBUG utility. Each row is preceded by the 16-bit segment and
offset components of the 20-bit segmented address of the first byte
in the row. At the end of each row, the ASCII representations of
the 16 bytes are displayed. Unprintable low-ASCII and extended
characters are shown as blank spaces, much as DEBUG displays
periods.
When CORELOOK first starts, the viewing window displays the
memory beginning at address 0000:0000. You can then change to other
addresses by using the several scanning options, as follows.
>0000:0000< ENTER BEGINNING DISPLAY ADDRESS
This option provides an input field to specify the segment
and offset of the beginning of the desired block of memory.
The cursor starts under the leftmost position: enter valid
hex values (0-9 and A-F) across the field, skipping over
unchanged positions with the <Right> cursor. The Backspace
and <Left> cursor do not work, but entry will wrap around
to the beginning if you need to correct a value. At any
place in the field, pressing Enter will put the indicated
memory block in the viewing window.
F7 - BACK PAGE
Changes the contents of the viewing window to begin at an
address 256 bytes lower than the current beginning address.
F8 - FORWARD PAGE
Changes the contents of the viewing window to begin at an
address 256 bytes greater than the current beginning
address.
F9 - BACK LINE
Changes the contents of the viewing window to begin at an
address 16 bytes lower than the current beginning address.
F10 - FORWARD LINE
Changes the contents of the viewing window to begin at an
address 16 bytes greater than the current beginning
address.
When these function key commands are used to scroll the viewing
window, the address in the BEGINNING DISPLAY ADDRESS input field is
updated automatically, and will always be consistent with the first
memory location displayed in the viewing window.
The viewing window scroll functions also have a wrap capability
at the ends of the addressable memory. If the viewing window
beginning address is 0000:0000 and you use F7 to move back one page,
the new viewing window will begin at the address of FFF0:0000.
STRING SEARCHING
CORELOOK provides a string search feature that will find memory
locations that hold byte values matching a five character ASCII
string. The match must be exact, including leading and trailing
spaces that you have in the input field.
Alt F - ENTER FIND STRING
Press ALT F to access the string search function. The
cursor will move to the input field where you type the five
characters to be matched. If the search string is less
than five characters, you need to enter leading and
trailing spaces exactly as you expect them to be located.
This may take a little experimentation if you are not sure
how the search string is delimited in its memory storage.
When the input field is to your liking, press <Enter> to
begin the search.
F5 - REPEAT FIND
This will search for the next occurrence of the string
currently displayed in the input field. Memory addresses
are searched in ascending order.
PROGRAM TERMINATION
Pressing <ESC> at any time will terminate CORELOOK and take you
back to DOS.
Rev 0 (docs only)
TAM 9/3/88
FILE1408.TXT
Disk No: 1408
Program Name: CLONEROM version 1.0
PC-SIG version 1
Watch out, Big Blue! CLONEROM will help the dedicated hobbyist make his
PC/XT/AT absolutely, totally IBM compatible, right down to the
programming of the chips in the motherboard. To obtain system level
compatibililty, you must install a "true blue" ROM-BIOS. The full text
from the IBM PC READ-ONLY-MEMORY BACKUP MANUAL is ready to print out to
tell you how. Two utility programs are available to help you finish the
job.
If you already own an IBM or clone, how would you like to have backup
ROM and EPROM chips that cost less than IBM-programmed ICs? You can do
it with CLONEROM. Save your entire ROM on disks as insurance against
disaster.
Synopsis: Shows you how to program PC/XT/AT motherboards to insure 100%
IBM compatibility. A hands-on, "under-the-hood" program for the
dedicated hobbyist.
Usage: System Utilities/ROM-BIOS/Upgrade.
Special Requirements: None.
How to Start: Type GO (press enter).
Suggested Registration: $9.95
File Descriptions:
READ ME File descriptions.
GO BAT Tells how to display READ.ME and ROMBAK texts.
ROMBAK BAT Prints ROMBAK.
ROMBAK TXT Text of THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL.
ORDER TXT Order form for printed ROMBAK.
PC-DUMP EXE Program to download any part of a PC's memory.
PC-DUMP DOC Documentation for PC-DUMP.
CORELOOK COM Program to view PC memory contents.
CORELOOK DOC CORELOOK documentation.
PC-SIG
1030D E Duane Avenue
Sunnyvale CA 94086
(408) 730-9291
(c) Copyright 1989 PC-SIG, Inc.
ORDER.TXT
=========================== ORDER FORM ================================
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL
To obtain the hardcopy manual, please complete the information
listed below and send this form and $9.95 to;
PC TECHNOTES
P.O. Box 1574
San Juan Capistrano, CA 92693
___________________________________________________________________
Name (Please Print)
___________________________________________________________________
Company
___________________________________________________________________
Street
___________________________________________________________________
City State Zip
================================================= PC TECHNOTES 8/88 =====
PC-DUMP.DOC
PC-DUMP.EXE
PC Memory Download to Disk File
Rev. 2.0 11-23-87
This program will download the contents of a block of PC system
memory - either RAM or ROM - and write it to a disk file. The byte
image in the disk file is suitable for editing using DEBUG, or for
blasting EPROM's.
As selected from the options menu, you need to input the memory
segment address, the offset of the end of the memory block that is
to be written to the disk file, the pattern (all, even or odd) of
the bytes that are written to the disk file and the disk file name.
Input prompting and a preset response format will fairly well
confine your responses to acceptable ones.
(F1) MEMORY SEGMENT ADDRESS : The four-hexadecimal-digit memory
segment (or paragraph) address of the beginning of the block
of memory to be downloaded. This address must conform to the
20-bit segmented addressing scheme used with the 8088.
(F2) MEMORY BLOCK END OFFSET : The four-hexadecimal-digit offset
address of the end of the memory block that is being written
to the disk file. This is essentially one less than the total
number of bytes in the overall memory block being downloaded,
since the downloading begins at an offset of &H0000 from the
memory segment address. The maximum offset is &HFFFF, for a
total memory block size of 64K bytes.
(F3) DISK FILE NAME : The filespec of the disk file in which the
memory block contents will be written. The filespec should
follow the DOS convention (d:filename.ext) for format.
(F4) BYTES WRITTEN TO FILE : Within the designated memory block
you may select the pattern of bytes to be written to the disk
file. The default is to write all bytes. To facilitate work
with interlaced address ROMs, options are available to write
only the even or odd numbered bytes to the disk file. Note
that pressing F4 rolls the option stack: no other keyboard
entry is necessary.
When you input the hexadecimal data for the memory segment
address and the memory block edd offset, PC-DUMP routines will
collect the input characters one at a time and display them in the
preset range. Upon receiving four characters, the program will put
the full hexadecimal number in the status line and return control
to the options menu. If you make a mistake during entry of these
data items, go ahead and input four characters and come back and
try again. Also, you should note that this program rigidly checks
for allowable keystrokes (the disk filespec is the one exception).
EXAMPLE #1
MEMORY SEGMENT ADDRESS (4 hex digits) = FE00
MEMORY BLOCK END OFFSET (4 hex digits) = 1FFF
DISK FILENAME (d:filename.ext) = A:ROMBIOS.DAT
BYTES WRITTEN TO FILE = ALL
The input shown above would download an 8K (8,192 bytes) block
of memory, beginning at segment address FE00(hex), to a disk file
on the A: disk drive. All bytes in the indicated block are written
to the file.
EXAMPLE #2
MEMORY SEGMENT ADDRESS (4 hex digits) = F000
MEMORY BLOCK END OFFSET (4 hex digits) = FFFF
DISK FILENAME (d:filename.ext) = A:ATROM_LO.DAT
BYTES WRITTEN TO FILE = EVEN
The input shown above would download 32K (32,768) bytes making
up the even numbered (offsets 0, 2, 4, 6 ...) bytes in the 64K block
of memory, beginning at segment address F000(hex), to a disk file
on the A: disk drive. This data is typical of the low-byte ROM in
the interlaced address ROMs of the PC-AT.
Rev 1 (docs)
TAM 11/23/87
ROMBAK.TXT
THE IBM PC
READ-ONLY-MEMORY
BACKUP MANUAL
************************************************************************
************************************************************************
This copy of the manual is printed from the file ROMBAK.TXT which
is distributed on disk through the user supported software distribution
network. This manual is a copyrighted work, with permission granted for
electronic media distribution ONLY; printed media distribution without
the expressed consent of the copyright holder is prohibited.
This electronic media distribution of this manual is an experiment
in user-supported publishing. This is a specialty publication and the
audience most interested in the subject matter is probably quite
comfortable with the distribution and use of disks of user supported
software.
This file contains all of the text from the printed manual, as
published by PC TECHNOTES. The figures and certain compresed-format
tables from the printed manual are not reproduced in this file. The
absence of this graphic material should not make a major reduction in
the instructional value of this material, as most of the information
could be assembled from other sources.
The user supported part of the deal works the same as for software.
IF YOU READ THIS VERSION OF THE MANUAL AND FIND IT TO BE USEFUL, PLEASE
SHOW YOUR SUPPORT BY ORDERING THE PRINTED MANUAL. The price of $9.95 is
nominal for a specialty publication like this, and you'll like having
all of the figures and tables in one convenient place. A handy order
form is contained in the file ORDER.TXT, which is included on the
distribution disk.
***********************************************************************
***********************************************************************
_____________________________________________________________________
PC TECHNOTES
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL
Copyright (C) 1986, 1987 by PC TECHNOTES
All rights reserved. No part of this manual may be reproduced or
transmitted in any form or by any means without the prior written
permission of the copyright holder.
PC TECHNOTES is an independent publisher, not affiliated in any way with
International Business Machines Corp. As used in the title and text of
this manual, "IBM PC" refers solely to a recognized industry standard
for computer hardware design and operation, and not to any authorization
or license from the International Business Machines Corp.
Disclaimer
PC TECHNOTES makes no warranties as to the contents of this manual or to
the fitness of the information for any particular use. Although every
effort has been made to insure that the information contained in this
manual is reliable, PC TECHNOTES cannot be held responsible for any
damage or loss suffered from, or any liability incurred as a consequence
of the use of the information contained herein.
Trademarks/Owners: IBM, IBM PC, PC-XT and PC-AT/ International Business
Machines Corp. Intel/Intel Corporation. MS-DOS and GW-BASIC/Microsoft
Corp. Hercules/Hercules Computer Technology. BYTE/McGraw-Hill Inc.
PC TECHNOTES
P.O. Box 1574
San Juan Capistrano, CA 92693
- i -
THE IBM PC
READ-ONLY-MEMORY
BACKUP MANUAL
TABLE OF CONTENTS
1. Introduction .............................................. 1
2. PC Memory Organization .................................... 2
3. PC Memory Addressing ...................................... 5
4. PC ROM Software and Hardware .............................. 7
4.1 Software Addresses and Markers ........................ 7
ROM-BIOS
ROM-BASIC
4.2 ROM Hardware on the System Board ...................... 8
5. The Making of a ROM Chip .................................. 11
5.1 EPROM Integrated Circuit Chips ........................ 11
5.2 EPROM Programmers ..................................... 12
6. Putting It All Together ................................... 14
7. So You Bought a PC-Clone .................................. 16
7.1 PC-Clone ROM Hardware ................................. 17
8. Helpful Hints ............................................. 21
Appendix A. Hexadecimal Arithmetic .......................... 22
Appendix B. ASCII Characters ................................ 25
Appendix C. Useful Software ................................. 27
Appendix D. EPROM Programmer Sources ........................ 29
- ii -
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 1
1. INTRODUCTION
You are a cautious computer user. Should your important PC software
be damaged, you know that you have backup copies of everything in a safe
place. Everything, that is, except the most critical item of all ....
the system software residing in the read-only-memory (ROM) chips
installed on your IBM PC's motherboard.
The built-in software in the IBM PC's ROM chips contains the
primitive system startup and initialization routines, the Basic
Input/Output System (ROM-BIOS) and the core of the BASIC and BASICA
language interpreters (ROM-BASIC). Without the ROM-BASIC your IBM BASIC
Interpreter disk files are inoperative. Without a functioning ROM-BIOS
your applications software is useless. Without the system startup
routines your PC turns into an expensive paperweight.
So what? These ROM chips are standard IBM parts and you can always
obtain replacements from your dealer. Right? Well, maybe .....
The IBM PC family of microcomputers is known for an open
architecture built around commercially available components, but IBM has
carefully guarded the ROM-based system software that distinguishes them
from all of the compatibles and clones. Under the right circumstances,
you may obtain a ROM chip replacement through an authorized IBM dealer,
but you will probably have to deliver your IBM computer to such a dealer
and experience a wait for the replacements to be installed.
If your dealer is a mail order house in another state, or is local
but uncooperative, a malfunctioning ROM chip could spell major
inconvenience or loss of business. At worst, you could find yourself
replacing the motherboard, or even the system unit itself, at a cost
much greater than the price of a backup ROM chip.
You can make a backup set of system software ROM chips for your IBM
PC. Even if you have no prior computer circuitry or systems programming
experience, you can readily master the few necessary technical concepts.
This manual is intended to be a straightforward compilation of the
key technical information, and the practical applications of such, that
a PC owner should understand before attempting to backup his PC's ROM
software. All of the technical details are in the public domain.
The material is logically organized as a tutorial on the structure
of the PC's RAM and ROM memory, the way that the PC addresses memory
locations, the arrangement and variations of the ROM software and
hardware, and the types of ROM chips and programming hardware that are
on the market today. Also included is a guide to the all important
subject of hexadecimal arithmetic (don't leave home without it),
information about two very helpful pieces of public domain software, and
notes on sources of ROM programming hardware for the PC owner.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 2
2. PC MEMORY ORGANIZATION
This chapter describes the organization of random-access-memory
(RAM) and read-only-memory (ROM) inside the PC and the predefined
functions of various blocks of this memory. (If you are not familiar
with hexadecimal arithmetic, please go to Appendix A. before reading any
further, as you will need this to understand the notation for memory
block references.) Note that this is a general structure discussion, and
that the specifics of memory addressing will be covered in the next
chapter.
The memory organization in the PC is a result of the capability of
the Intel 8088 CPU to address the memory. The 8088 can address up to
1,024K (or 1,048,576) bytes of memory. Each byte is referred to by a 20-
bit address which may be expressed as an equivalent 5-digit hexadecimal
number. With such an addressing scheme, the full 1,024K bytes may be
covered by address values in the range from hex 00000 to hex FFFFF.
By definition, the PC's addressable 1,024K bytes of memory are
divided into 16 blocks of 64K bytes each. Each block is then identified
by the first, or high order, hex digit of the hexadecimal addresses of
all bytes in that block. For example, the first 64K byte block of memory
at addresses hex 00000 through 0FFFF is called the 0 block. Similarly,
the last of the 64K byte memory blocks at addresses F0000 through FFFFF
is called the F block.
In theory, there is no functional boundary between these 16 blocks
of memory. The block convention is used partly due to the specifics of
the addressing scheme (more on this in the next chapter) and partly
because the overall memory usage scheme developed by IBM assigns
distinctly different uses block by block. The outline of IBM's memory
block assignments is shown in Figure 2-1.
The first ten memory blocks (blocks 0 through 9) are defined as RAM
in all of the PCs. This is ordinary working memory, up to 640K bytes.
All PCs have memory installed in at least the first 64K block, or block
0, the lowest addresses of which are used by the operating system
software and are not available for general user applications. Any
additional RAM occupies a contiguous address space from block 1 up to
the top of block 9. If the PC is not fully populated with RAM, the
corresponding address blocks are left idle with no function.
The last six memory blocks are reserved for various system and
hardware support functions. The A and B blocks, physically located on
the video controller circuit board, are used for video memory in the PC,
PC-XT, and PC-AT models. The B block is divided into two 32K byte
sub-blocks used by the conventional Monochrome Display Adapter and the
Color/Graphics Adapter. The A block of memory is reserved for video
memory expansions, such as those recently introduced as part of the
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 3
Enhanced Graphics Adapter and the Professional Graphics Adapter.
┌──────────────── figure goes here ─────────────────────┐
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
└───────────────────────────────────────────────────────┘
Figure 2-1. IBM PC Memory Block Assignments
The C block of memory is set aside for any additions to the ROM
system software that is permanently installed on the motherboard. These
additions are usually BIOS extensions to support new types of peripheral
devices. This memory block was first used for the ROM-BIOS routines
supporting the hard disk drive that appeared with the PC-XT model.
Similar to the A and B blocks' video memory, this C block memory is
typically installed in the PC as part of an expansion circuit board.
According to the IBM memory block outline, the D and E blocks are
set aside for ROM memory in "plug-in" cartridge software, such as that
used with the PCjr model. Cartridge software is not presently supported
in the regular PC, PC-XT, and PC-AT models, although it could
conceivably be added to any of these models.
The F block of memory is set aside for the ROM software that is
permanently installed on the PC's motherboard. This memory area is home
to the system startup and diagnostics routines, the ROM-BIOS functions,
and the ROM-BASIC (or "cassette" BASIC) routines. It is because of the
contents of the F block of memory that this manual exists: this is where
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 4
the PC owner may seek to backup the most important software used by his
computer. The organization and versions of the several software
(actually "firmware", if you want to be particular) components housed in
the F block ROM are detailed in Chapter 4.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 5
3. PC MEMORY ADDRESSING
As mentioned in the previous chapter, the Intel 8088 microprocessor
is capable of addressing up to 1,048,576 memory locations, which is the
largest value representable by a 20-bit binary number (2 raised to the
20th power). The 8088, as a 16-bit processor that cannot work directly
with binary numbers containing more than 16 bits, would not be able to
address more than 65,536 (2 raised to the 16th power) memory locations
if it were not for a clever scheme known as segmented addressing.
Segmented addressing divides the total addressable memory into a
number of segments containing 64K bytes each. Each segment begins at a
memory location that is evenly divisible by 16 bytes. This beginning
location is known as the segment address, or segment paragraph.
Individual bytes within this memory segment are referenced by an
additional address known as the offset address. The offset address,
which is always measured relative to the beginning of a segment, points
to an exact location within the 64K byte segment.
Physical memory addresses are created by combining the 16-bit
segment paragraph with the 16-bit offset address, as illustrated by the
binary addition shown in Figure 3-1. In representing memory addresses
that are multiples of 16 bytes, the segment paragraph is thus shifted
four binary bits to the left. Adding the segment paragraph to the offset
address gives a 20-bit address, which now can access the full 1,024K
bytes of addressable memory in the PC.
┌───────────────── figure goes here ────────────────────┐
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
└───────────────────────────────────────────────────────┘
Figure 3-1. Binary Addition of 16-Bit Segment Paragraph and
Offset Address to Yield a 20-Bit Segmented Address
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 6
The previous chapter stated that 1,024K bytes of addressable memory
may be referenced by means of a 5-digit hexadecimal number. Since the
segment paragraph is always a multiple of 16 bytes, its hexadecimal
value will always have a zero in the last place, such as hex FE6A0 or
hex 34560. In use, the segment paragraph is expressed as a 4-digit
hexadecimal number, with the units place zero dropped, in order to yield
an equivalent 16-bit binary value for manipulation by the 8088
microprocessor. The offset address is likewise expressed as a 4-digit
hexadecimal number, which also thoroughly covers the 64K bytes contained
in the memory segment.
As for notation, the segmented address is written as nnnn:nnnn,
with the segment paragraph address on the left side of the colon and the
offset address on the right.
To put all of this together, consider the evaluation of a segmented
address that is written as 1234:6789
12340 Segment Paragraph Address (hex), shifted left four bits
+ 6789 Offset Address (hex)
───────
18AC9 Full 20-bit Segmented Address (hex)
This expanded 20-bit address is for a byte physically located a little
more than half-way into block 1 of the PC's addressable memory.
Since a segment can begin as frequently as every 16 bytes, it is
quite easy to express any particular memory location by means of a
variety of segmented addresses, depending upon the choice of the segment
paragraph. When discussing PC memory in terms of IBM's block outline, it
is easiest to use segment paragraph addresses that correspond to the
beginning addresses of the sixteen memory blocks. For the remainder of
this manual, the F block ROM memory will be described using segmented
addresses of the form F000:nnnn, where nnnn is the offset address within
this 64K byte block.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 7
4. PC ROM SOFTWARE AND HARDWARE
In this chapter we will describe the basic organization of the F
block ROM software (firmware) in the PC, and note some of the typical
software markers and IDs that will be useful to know about before you
explore on your own. You will also learn about the typical ROM hardware
configuration in the various PC models, and how this relates to the
software organization.
Beginning in this chapter, references will be made to the contents
of specific memory locations in the F block ROM. You can scan these
addresses using the DOS utility DEBUG or a simple BASIC program to
display the value of each byte. Much more conveniently, you can zoom
around the ROM contents by using a memory display utility such as the
excellent public domain program CORELOOK. Appendix C describes the
necessary DEBUG commands and BASIC program statements, as well as the
features and use of CORELOOK.
4.1 ROM SOFTWARE ADDRESSES AND MARKERS
The F block ROM software is arranged to occupy memory address
ranges consistent with the start/end addresses of eight sequential 8K
(2000 hex) byte sub-blocks. This is not totally arbitrary, as the ROM
chips are made in various capacities that are compatible with this
sub-block size. In the IBM PC ROM set, the separate functional sections
of the ROM software (BIOS and ROM-BASIC) start at the beginning of an 8K
byte sub-block and spread over one or more of these sub-blocks. There
may be sections of empty memory addresses scattered throughout these
sub-blocks.
In the PC and PC-XT models, the system startup and ROM-BIOS
routines are located in the highest address 8K byte sub-block, from
address F000:E000 through F000:FFFF. If you display the character
representations of the hex code, you can find some bits of code at the
beginning of this memory segment that appear to identify IBM's
handiwork. At the end of this sub-block, IBM has encoded a ROM version
release date and a machine ID marker.
The ROM version release date is located in an 8-byte memory area
from F000:FFF5 through F000:FFFC. This date is in the common American
date format with two digits apiece for the month, day, and year. As
listed below, this date marker is useful for identifying the particular
IBM new machine release and/or ROM update. Note that the ROM release
date is essentially the ROM-BIOS version date since the ROM-BASIC code
has remained unchanged at least since the 10/27/82 release.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 8
Release Date IBM Machine / Update
──────────── ────────────────────────────────────────
04/24/81 Original PC
10/19/81 Revised PC (bug fixes)
08/16/82 Original PC-XT
10/27/82 Upgrade of PC to PC-XT BIOS level
11/08/82 Original Portable PC (used in later XTs)
01/10/84 Original PC-AT (6 MHz Model 068)
06/10/85 Upgraded PC-AT
11/15/85 Upgraded PC-AT (8 MHz Model 339, with
101-key enhanced keyboard)
01/10/86 Upgraded PC-XT **
05/09/86 Upgraded PC-XT **
** Revision for 101-key enhanced keyboard.
See the note at the end of this Chapter.
The machine ID is a single byte located at F000:FFFE. The published
values for the IBM machine ID markers are listed below. Note that these
ID: Dec Hex Machine
────────────── ──────────────────────
255 FF Original PC
254 FE PC-XT and Portable PC
252 FC PC-AT
251 FB Upgraded PC-XT **
machine ID markers are apparently not always consistent: there are
reported instances of PC-XTs bearing the PC marker. In any event, you
may want use this ID marker in the ROM-BIOS as a secondary means of
identifying the code's vintage.
Also in the PC and PC-XT models, the ROM-BASIC code resides within
a 32K byte area of F block memory from addresses F000:6000 through
F000:DFFF. As mentioned above, this code has been unchanged for quite
some time. As of now, there is no particular BASIC release date marker
to be concerned about as long as you deal with a ROM release date of
10/27/82 or later.
In the PC-AT model, the ROM-BIOS routines have been expanded
considerably to accommodate the additional capabilities of this machine.
The primary BIOS routines and the ROM-BASIC are in the same locations as
in the PC and PC-XT models, but the expanded BIOS routines have been
"wrapped around" to occupy the lowest 24K bytes of F block memory from
addresses F000:0000 through F000:5FFF.
4.2 ROM HARDWARE ON THE SYSTEM BOARD
In keeping with the 8K byte sub-block convention described above,
the system motherboard in the IBM PC and PC-XT models carries five 8K
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 9
byte ROM chips for a total of 40K bytes (32K bytes of ROM-BASIC and 8K
bytes of BIOS) of system ROM.
Components on the motherboard are indexed by means of alphanumeric
codes printed on the circuit board. The ROM sockets on the IBM PC/XT
motherboard are labeled with a code sequence such as U29 through U33,
for a five ROM chip setup. The highest number socket typically
corresponds to the highest F block memory addresses. Additional
information on the location of the ROM sockets on the motherboard may be
found in Chapter 7.
The socket containing the PC/XT ROM-BIOS will be the highest
address portion of memory in the F block. This BIOS is typically carried
in an 8K byte ROM chip, using memory addresses F000:E000 through
F000:FFFF, as described earlier. The full set of 8K byte ROM chips fill
up the IBM PC/XT ROM sockets as shown below. Note that the eight
possible 8K byte ROM chip/sockets are numbered 0 to 7, from lowest to
highest memory address.
ROM Socket No. Memory Addresses Function
────────────── ───────────────────── ──────────
7 (U33) F000:E000 - F000:FFFF ROM-BIOS
6 (U32) F000:C000 - F000:DFFF ROM-BASIC
5 (U31) F000:A000 - F000:BFFF "
4 (U30) F000:8000 - F000:9FFF "
3 (U29) F000:6000 - F000:7FFF "
As shown above, additional ROM chips for the ROM-BASIC routines will be
located below (address-wise) the ROM-BIOS in contiguous memory.
The IBM PC-AT utilizes a quite different ROM hardware configuration
to make optimal use of the 16-bit data bus on the PC-AT's 80286
microprocessor. The ROM chips are typically designed as "by 8" devices,
meaning that they can be accessed 8 bits at a time. Since 16-bit data
read accesses are faster if the system doesn't have to read the same ROM
chip twice in sequence, IBM arranged the PC-AT ROMs as two 8-bit ROMs
with their addresses interleaved. In other words, all of the even
numbered bytes (at offsets 0,2,4,6,....) are on one chip and all of the
odd numbered bytes (at offsets 1,3,5,7,....) are on the other chip.
With this arrangement, each 16-bit data access can set a single
segmented address and read one "slice" out of the pair of interleaved
ROM chips.
Even though the ROM chip hardware addresses are interleaved on the
data bus in the PC-AT, the F-block ROM-BIOS and ROM-BASIC code is still
located within the same memory address areas described in section 4.1.
The original model IBM PC-AT motherboard used four 16K byte ROM
chips to carry the full 64K bytes of ROM available in the F block. The
four-chip ROM arrangement in this model IBM PC-AT is as follows;
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 10
ROM Socket No. Memory Addresses Function
────────────── ───────────────────── ───────────────
3 F000:8001 - F000:FFFF ROM-BIOS, BASIC: Odd Bytes
2 F000:8000 - F000:FFFE ROM-BIOS, BASIC: Even Bytes
1 F000:0001 - F000:7FFF ROM-BIOS, BASIC: Odd Bytes
0 F000:0000 - F000:7FFE ROM-BIOS, BASIC: Even Bytes
Although the current IBM PC-AT motherboard has four ROM sockets as
in the original model, apparently only two of these sockets are mapped
to the F block of memory. A pair of the newer 32K byte ROM chips are now
used to carry the F block memory. This two-chip ROM arrangement in the
IBM PC-AT is as follows;
ROM Socket No. Memory Addresses Function
────────────── ───────────────────── ───────────────
1 F000:0001 - F000:FFFF ROM-BIOS, BASIC: Odd Bytes
0 F000:0000 - F000:FFFE ROM-BIOS, BASIC: Even Bytes
════════════════════════════════════════════════════════════════════════
NOTE: Upgraded PC-XT ROM-BIOS for Enhanced Keyboard
The PC-XT ROM-BIOS versions dated after 01/10/86 and containing a
machine ID marker of FB hex are upgrades to accommodate the new 101-key
enhanced keyboard. The new features of this keyboard can be used only
with systems that support it by means of a substantially enhanced
interrupt 16H handler in the BIOS. This additional code now causes the
PC-XT's BIOS to spill out of the previous F000:E000 to F000:FFFF memory
space and into the 8K byte sub-block at F000:0000 to F000:1FFF. This is
similar to the manner in which the PC-AT's expanded BIOS routines have
been "wrapped around" to occupy the lowest F block memory addresses.
Working with the ROM software from these latest model PC-XT's will
require that extra steps be taken, similar in theory and technique to
those described in this manual, to provide for the additional ROM-BIOS
8K byte sub-block at addresses F000:0000 to F000:1FFF. This may include
using a different capacity ROM chip and a modified allocation of code
among the chips than what is described elsewhere based on the earlier
model PC-XT.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 11
5. THE MAKING OF A ROM CHIP
This section describes the Erasable Programmable Read Only Memory
(EPROM) chips used to backup the PC's ROM software, and the hardware
used to burn the program code into these chips.
5.1 EPROM INTEGRATED CIRCUIT CHIPS
The types of EPROM chips used to backup the PC, PC-XT and PC-AT ROM
software are members of what is known as the 27xxx series. The chips in
this series are similar in physical appearance, as shown in Figure 5.1,
but vary in capacity, access speed, and programming characteristics.
┌────────────────── figure goes here ───────────────────┐
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
│ │
└───────────────────────────────────────────────────────┘
Figure 5-1. Features of the 27xxx EPROM Family
You will need to obtain and work with the correct capacity 27xxx
EPROMs for your ROM backup. The "xxx" characters in the 27xxx chip
number stand for the EPROM capacity in kilobits, or eight times the
kilobyte capacity. The table below shows the capacities for several
members of this series.
Chip Capacity
Chip Number bits bytes
─────────── ─────── ───────
2764 65,536 8,182 (8K)
27128 131,072 16,384 (16K)
27256 262,144 32,768 (32K)
27512 524,288 65,536 (64K)
Throughout this manual, the PC ROM software has been described in terms
of kilobytes, so the capacities shown in parentheses are the ones that
you need to work with.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 12
In addition to capacity, the 27xxx series EPROMs are available in
different access speeds of 200, 250, or 450 nanoseconds. This access
speed is typically designated by some form of suffix (usually 2 or 3
digits) to the basic chip number. For instance, a number 2764-25 would
indicate an 8K byte capacity with a 250 nanosecond access time. You
should use EPROMs with access speeds that at least meet your PC's
original specifications, but faster chips will also work.
Other letters may appear in the chip number, such as 27xxxA (a
lower programming voltage) or 27Cxxx (low power consumption CMOS
technology chips). Consult with your EPROM vendor for issues of chip
compatibility. Also, some EPROM manufacturers may add characters to
these basic chip numbers, and, again, your vendor can help in making
sure that you obtain the functional type that you need.
As the "E" in EPROM stands for erasable, a few words about the
erasure window are definitely in order. If you look through the small
circular window on the EPROM you can see the tiny semiconductor chip
that constitutes the memory. While the 27xxx series EPROMs are
programmed by applying varying voltages to the device's pins, they are
erased by ultraviolet light shining through the erasure window onto the
integrated circuit chip. This erasure technique typically involves
exposing the unshielded EPROM for 10 to 20 minutes under a high
intensity ultraviolet bulb in a specially built EPROM eraser. An EPROM's
contents can, however, be lost by prolonged exposure to the low level of
ultraviolet light present in common light sources. Before you burn your
code onto an EPROM, permanently cover the erasure window with a piece of
opaque tape or other such material.
In addition to normal erasure, the EPROM's contents can be zapped
by static charges developed during handling. Be cautious about grounding
your equipment and yourself before handling these devices. Your vendor
should place the EPROMs in a protective anti-static tube typically used
for transporting integrated circuit chips.
As far as EPROM sources and prices, a good place to start is with
the advertisements in the back sections of major computer magazines such
as BYTE or PC MAGAZINE where numerous vendors offer fairly low mail
order prices. If you can find a local discount electronics supplier, you
may well be able to beat the mail order houses' total cost.
5.2 EPROM PROGRAMMERS
The programming hardware used to record code onto a blank EPROM
chip is commonly referred to as a "blaster" or "burner". Originally,
these devices were available as costly stand-alone professional
equipment, usually designed to program a wide variety of ROM chips. If
you have access to one of these commercial style EPROM burners and plan
to use it for your ROM backup procedures, then you can probably skip the
rest of this section. Continue reading if you need to obtain an EPROM
programmer.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 13
Recently, a number of very specific and relatively inexpensive
EPROM blasters have appeared that are obviously targeted for the PC
hobbyist market. These new PC-based EPROM blasters are designed as
plug-in circuit cards that fit into one of the PC's expansion slots and
are typically capable of burning only the 27xxx series of EPROMs. These
blasters come with software that runs on an IBM PC or compatible. While
these blasters are pretty much "bare bones" hardware for one-at-a-time
burning, the accompanying software usually offers the following
convenient features;
o Programs various 27xxx series EPROMs
o Menu driven selection of EPROM type sets programming voltage
o Read/write EPROM to/from buffer
o Read/write buffer from/to disk file
o Verify EPROM is blank
o Compare EPROM and buffer
o Variable buffer and EPROM offsets and read/write length
o Copy PC memory into buffer
o Edit buffer contents
You should look for at least these features in a PC-based EPROM blaster
that you would obtain to use to backup your PC's ROM. You can do without
the last two features on this list if you make use of other available
software. For example, you could use the public domain program PC-DUMP
(see Appendix C.) to download PC memory to a disk file which could then
be edited using the DOS utility DEBUG.
With reasonable user documentation and good menu driven software,
the process of burning an EPROM is actually quite simple. At the risk of
appearing to stop short of giving out the really meaty information on
the actual process of blasting the EPROMs, we can only advise you that
using one of these PC-based menu driven EPROM programmers is a pretty
uninspiring operation. Other than some of the helpful hints contained in
Chapter 8, there is not much more that this manual can do to prepare you
to use one of these devices.
Appendix D. discusses some of the sources where you can find these
low cost PC-based EPROM blasters for sale, as well as some typical
prices at the time of this writing.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 14
6. PUTTING IT ALL TOGETHER
Now that you have covered all of the background details related to
backing up your ROM software, its time to develop a summary of the steps
that you might typically go through.
The first step, obviously, is to prepare your backup disk files of
the ROM software and store these away for safekeeping. The whole point
of this manual is protection against a failure of, or damage to, the ROM
chips, so download your backup now.
1. Download specific portions of the ROM code to individual disk
files. Depending on the capacity of the ROM chips used in your
particular model of PC (Chapter 4), the code in these downloaded
disk files should correspond exactly to the ROM memory addresses
that would be occupied by the replacement EPROM chips. (You could
download the entire F block contents to a single file and later
break this up into the appropriate sub-blocks using the software
provided with some EPROM programmers, but the specific file
approach will assure flexibility and reduce the risk of errors down
the road.)
As an example, you might backup your PC-XT ROM using the following
parameters with the PC-DUMP program (Appendix C).
Segment Paragraph Bytes Written
Address to Disk File Disk File Name
───────────────── ───────────── ──────────────
FE00 2000 (hex) FE00ROM.DAT
FC00 " FC00ROM.DAT
FA00 " FA00ROM.DAT
F800 " F800ROM.DAT
F600 " F600ROM.DAT
The five backup files created in this example correspond to the
five 8k byte type 2764 EPROM chips that could be programmed as
replacements.
In the event that you someday need to replace a damaged or faulty ROM
chip(s) in your PC, the following steps are typical.
2. Determine, if possible, which of the original ROM chips is faulty.
If this is not possible, plan on replacing the full set. If you
replace the full set and your curiosity is high, you can return the
individual original chips on a trial-and-error basis and isolate
the culprit chip.
3. Purchase the type of blank EPROM chip(s) that are to be used with
your PC's particular ROM hardware capacity and speed (Chapter 5).
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 15
For a full ROM set in a PC-XT, you would buy five type 2764 EPROMs
with an access speed of 250 nanoseconds (or faster).
4. Using either a PC-based or a stand-alone EPROM programmer, burn the
code from the backup disk file(s) onto the blank EPROM(s). If you
have prepared your backup disk files as described in step (1)
above, this operation will involve one EPROM per disk file. Label
the programmed EPROM(s) with the corresponding ROM software memory
address to assure matching with the proper ROM socket(s) during
installation.
5. Open up your PC's case, locate the row of motherboard sockets
holding the ROM chips, and look for the individual socket labels
printed on the circuit board (Chapter 4). Determine the ROM sockets
corresponding to high and low memory addresses, and make certain
that you can identify the memory addresses of all involved sockets.
Note the orientation of the casing notches on the existing ROM
chips.
6. Remove the existing ROM chip(s) and insert the replacement
EPROM(s), making certain that all of the EPROM's pins properly
contact in the socket. If the two pin rows seem to be wider than
the socket holes, tilt the chip and lightly insert one row then
press the chip towards the inserted row until the other row can be
inserted. Be sure that the replacement EPROM and the socket are
properly matched according to the ROM memory addresses described in
Chapter 4.
7. After installing the replacement EPROM(s) and successfully booting
the PC, verify the contents of the replacement chip(s). In the same
manner that the original ROM software backup disk file was made,
download a disk file copy of the memory contents on the replacement
EPROM(s). Check this validation disk file against the original
backup disk file using the DOS file comparison utility program
COMP.COM. The two files should compare with no reported mismatches.
════════════════════════════════════════════════════════════════════════
IMPORTANT NOTE
The ROM chips used in some models of the IBM PC are not pin-compatible
with the 27xxx series EPROM chips. If you encounter this situation, the
EPROM usage described here will not allow you to install the backup
read-only memory on your original IBM motherboard. Obtaining and
installing an inexpensive clone motherboard to carry your 27xxx
EPROM-based backup ROM set may be an attractive option in the event of
an original ROM malfunction, depending upon the price of the IBM
replacement ROMs and how you value inconvenience and/or loss of business
due to delays.
════════════════════════════════════════════════════════════════════════
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 16
7. SO YOU BOUGHT A PC-CLONE
The personal computer marketplace has seen a proliferation of new
machines that are often described as IBM "compatibles" or "clones". Some
of these models, usually from major manufacturers of computer hardware
and peripherals, are unique hardware designs offering IBM compatibility
by running the MS-DOS operating system and the applications software
originally written for the IBM PC. More common now are the PC-Clone
computers that clearly attempt to duplicate the look, feel, and
circuitry design of the real IBM models. Assembled from generic
components, these low price PC-Clones are being sold by hundreds of
vendors, from chain computer stores and large mail order outlets to the
garage and swap meet crowd.
These PC-Clones are available due to the fact that IBM chose to
design its family of personal computers around readily available
components and to openly disclose the details of the system architecture
and operation. This open architecture cleared the way for the avalanche
of third-party hardware and software enhancements that are on the market
today. It also allowed a number of enterprising printed circuit board
designers and manufacturers to provide the inexpensive IBM compatible
motherboards for the PC-Clones.
One part of the PC design, however, that IBM did not release into
the public domain is the system software that is permanently recorded in
read-only-memory (ROM) integrated circuit chips mounted on the
motherboard. This built-in software contains the primitive system
startup and initialization routines, the Basic Input/Output System
(ROM-BIOS), and the core of the BASIC and BASICA programming languages
(ROM-BASIC). By copyrighting the ROM software and declining to license
its use in any computers but its own, IBM managed to retain control of
the key to this otherwise open system.
Over the years, PC systems programmers have been able to write
legal functional equivalents of the system startup routines and the
ROM-BIOS machine language routines that provide support services for the
operation of the computer. Every PC-Clone comes equipped with a
work-alike ROM-BIOS, some of which are reputed to be very good and some
of which are homebrew hacks of unknown reliability and compatibility.
The frequently seen PC-Clone advertising claim of "fully IBM compatible"
may be a compromise: while the well behaved popular software may work
fine, the more quirky and/or aggressive system utilities and
applications programs may fail due to an unsupported or incomplete BIOS.
The PC-Clones do not, as a rule, come with a hardware functional
equivalent of the ROM-BASIC software, even though most of the PC-Clone
motherboards are built with the appropriate sockets to hold these ROM
chips. (This may be a measure of the degree to which clone components
are carbon copies, when circuit board space is taken up by sockets that
the merchants can not possibly fill with the intended chips.) Instead,
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 17
PC-Clone owners wishing to run interpreted IBM BASIC or BASICA programs
must use an all-disk-based functional equivalent, such as Microsoft's
GW-BASIC, which combines the disk and ROM routines of the original IBM
BASIC and BASICA. Not all of the disk-based BASIC interpreters are fully
compatible with the IBM BASIC interpreters, and disk versions will not
usually support add-on language extensions, such as the Hercules HBASIC
graphics extension, designed to look for and modify IBM BASICA.
You do not, however, have to pass up the tremendous price advantage
of a PC-Clone just because it does not come equipped with a True Blue
set of ROM software. If you are willing to be a do-it-yourself computer
experimenter, you can make a full set of 100% IBM compatible ROM chips
and install them in your PC-Clone. You can begin by exploring the
structure of ROM-based software on a PC using convenient public domain
software. And your ROM programming experiment can be completed with
fairly inexpensive and easy-to-use hardware advertised in the major
computer magazines or available through local electronics suppliers
(yes, and even at some swap meets).
Since the marketplace in filled with clones of the IBM PC, PC-XT,
and PC-AT models, this manual describes the differences in ROM software
and hardware among these three original IBM computers. In keeping with
the theme of this manual as an educational text, you will not find
specific references or instructions for any particular brand or model of
PC-Clone. In mastering this material, you should acquire an
understanding of ROM software and hardware anatomy and learn what to
look for to determine if your PC-Clone is a suitable candidate for
surgery. It is up to you, however, to perform the operation to find out
if the patient will be better than new.
One final note. It should be perfectly clear that, in making your
own experimental set of compatible ROM chips, you will be dealing with
software copyrighted by IBM. YOU ARE RESPONSIBLE FOR ASSURING THAT YOUR
USE DOES NOT VIOLATE THE COPYRIGHT LAWS PROTECTING IBM'S SOFTWARE. The
PC-Clone vendors already know all that there is to learn from this
manual, and they are not foolish enough to make illegal copies of ROM
sets for sale or trade. Keep your activities educational and hobby
oriented, and enjoy the experience of learning about ROM technology and
the inner workings of your computer.
7.1 PC-CLONE ROM HARDWARE
The PC/XT-Clone motherboards typically contain at least five
sockets for Erasable Programmable Read Only Memory (EPROM) chips, only
one of which will be used for the clone's BIOS. Figure 7-1 illustrates
the general location of the ROM chips/sockets on an PC/XT-Clone
motherboard. Although the position of the EPROM sockets is different on
some of the clone motherboards, the row of sockets is easily identified
by the presence of the EPROM chip that carries the ROM-BIOS. (The EPROM
chips themselves are described in Chapter 5.)
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 18
┌────────────────── figure goes here ───────────────────┐
│ │
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└───────────────────────────────────────────────────────┘
Figure 7-1. Typical XT-Clone Motherboard
Component Layout
Components on the motherboard are indexed by means of alphanumeric
codes printed on the circuit board. Depending on the manufacturer, the
EPROM sockets on the PC/XT-Clone motherboard will be labeled with a code
sequence such as U30 through U34 (for a five-socket setup), or perhaps
something a little more obvious like ROM3 through ROM 7. Look at the
label for the socket that holds the ROM-BIOS, it should be the highest
numbered socket.
The socket containing the PC/XT-Clone ROM-BIOS will be the highest
address portion of memory in the F block. This BIOS is typically carried
in an 8K byte EPROM chip, using memory addresses F000:E000 through
F000:FFFF, as described earlier. If you continue to use 8K byte EPROMs
for your experiment, you would fill up your PC/XT-Clone's EPROM sockets
as follows;
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 19
ROM Socket No. Memory Addresses Function
────────────── ───────────────────── ──────────
7 F000:E000 - F000:FFFF ROM-BIOS
6 F000:C000 - F000:DFFF ROM-BASIC
5 F000:A000 - F000:BFFF "
4 F000:8000 - F000:9FFF "
3 F000:6000 - F000:7FFF "
2 (not used)
1 "
0 "
As shown above, additional EPROMs for the ROM-BASIC routines will be
located below (address-wise) the ROM-BIOS in contiguous memory. Note
also that if your PC/XT-Clone motherboard has more than five ROM
sockets, these extras will not be used when installing a compatible ROM
set.
Although the new IBM PC-AT model uses two F block ROM sockets in
what appears to be a bank of four sockets, many of the AT-Clones are
built with only two EPROM sockets on the motherboard. For this reason,
the AT-Clone ROM will be described for a two-chip two-socket setup. If
you find that your AT-Clone has two EPROMs residing in a four-socket
bank, you can adapt the following descriptions to the even and odd
address sockets of the pair that are used.
Since the PC-AT hardware design uses interlaced address ROMs to
cover the entire F-block of memory addresses, your AT-Clone will have
EPROMs in both of the available sockets. You will have to look at the
printed component labels and/or consult your machine's documentation to
determine which is the even or odd address socket. When you experiment
with your compatible ROM set, you will fill up your AT-Clone's EPROM
sockets as follows;
ROM Socket No. Memory Addresses Function
────────────── ───────────────────── ───────────────
1 F000:0001 - F000:FFFF ROM-BIOS, BASIC: Odd Bytes
0 F000:0000 - F000:FFFE ROM-BIOS, BASIC: Even Bytes
Unused ROM sockets are not an issue in those AT-Clones designed to use a
pair of 32K byte EPROMs.
For those of you receiving your initial introduction to the 27xxx
family of EPROM chips used on the PC-Clone motherboard, a good place to
start is "under the hood" of your computer. Remove the case from your
PC-Clone and look for the ROM sockets, as shown in Figure 7-1, and the
ROM-BIOS chip(s). The ROM BIOS chip supplied with your PC/XT-Clone is
typically a type 2764 EPROM. In an AT-Clone, the ROM-BIOS will typically
be provided on a pair of type 27256 EPROMs.
If you are "under the hood", note the chip number and speed
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 20
designation printed on the ROM-BIOS chip(s). The numbers may be covered
up by a piece of tape or other opaque material placed over the erasure
window. If so, you should CAREFULLY peel back enough of the tape so that
you can read the chip number. If you have correctly located the ROM
chip(s), and your PC-Clone is a candidate for the type of ROM software
backup covered by this manual, then the chip designations should
correspond to the descriptions in Chapter 5.
You should obtain and use a uniform set of 27xxx series EPROMs with
an access time equal to, or less than, that of the ROM-BIOS EPROM that
was supplied with your PC-Clone. You should probably stay with the same
type of 27xxx chip as provided for your PC-Clone's ROM-BIOS, or consult
with your EPROM vendor for issues of chip compatibility.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 21
8. HELPFUL HINTS
This section contains an assortment of additional helpful hints and
practical ideas related to PC ROM backup procedures, based on the ROM
programming experiences of others.
A. First and foremost, there is NO CERTAINTY that either the IBM BIOS
or ROM-BASIC will work on all PC motherboards - although it seems
more likely that the ROM-BASIC should work in cases where system
hardware incompatibility prevents a BIOS replacement.
B. Compatibility of the ROM-BIOS can require a specific release date
(See Chapter 4.). For example, a motherboard designed as an XT may
not work with a BIOS associated with one of the original/updated
IBM-PC release dates.
C. Buy an extra EPROM chip or two. These devices are sensitive to
static charges, and you can easily zap one during handling and
installation. Having an extra one on hand may save a lot of
frustration, especially if you buy them through the mail.
D. Make a disk copy of the applicable memory contents before you
attempt to remove any ROM chip from the motherboard of a PC. This
is easy insurance in the event that you damage the original chip in
removing it from its socket and later decide to use it again. With
a backup disk copy on hand, you can always program a replacement.
E. The variations in the 27xxx series EPROMs (27xxx, 27Cxxx, 27xxxA)
require different programming voltages to be applied, either 21 or
12.5 volts. The EPROM programmer hardware or software should
clearly allow you to either specify exactly which chip variation is
to be programmed or allow you to specify which voltage level to
use. Using too high of a programming voltage can damage a chip. If
in doubt, ask your EPROM vendor about the proper voltage for the
chips that you buy.
F. Pulling EPROM chips from their sockets without damaging them can be
a little tricky if you aren't using the proper tool. The fingers
often aren't enough to do the job and you can easily mutilate the
chip's pins or fracture the casing by extracting it with pliers
large enough to span the chip. If you are going to do much
reinstalling of EPROMs, you should consider obtaining a special
chip puller tool. These are available at electronics suppliers for
about the same price as two or three ruined EPROM chips.
G. The EPROM chips must be oriented properly when installed in the
sockets on the motherboard. Before removing your PC's ROM chips,
note the location of the notched end of the chip casing. Any EPROMs
that you install should be inserted into the sockets with the
casing notches aligned in this same direction.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 22
APPENDIX A. HEXADECIMAL ARITHMETIC
Every operation in a computer is based on binary numbers, which
tend to become cumbersome to represent in print when the numerical
values are large. Hexadecimal notation is a convenient way to represent
large binary numbers.
Hexadecimal numbers are base-16, just as the familiar decimal
numbers are base-10. Since we still are limited to the ten digits (0
through 9) in our decimal system, we need to define an additional six
characters to use to represent the sixteen digits (0 through 15)
necessary for a base-16 numbering system. In hexadecimal notation, the
characters A through F are used to represent the values 10 through 15,
as shown below.
Hex Decimal Hex Decimal
─────── ─────── ─────── ───────
0 0 8 8
1 1 9 9
2 2 A 10
3 3 B 11
4 4 C 12
5 5 D 13
6 6 E 14
7 7 F 15
Hexadecimal numbers are constructed from hex digits in the same
manner that you build decimal numbers. For example, the decimal number
5500 would be expressed as 157C in hex notation, as follows;
1 times 4096 (16x256) = 4096
5 times 256 (16x16) = 1280
7 times 16 = 112
12 times 1 = 12
─────
5500
Just as each successive place in decimal notation represents the next
higher power of ten, each successive place in hexadecimal notation
represents the next higher power of sixteen.
Hex numbers work so well as substitutes for binary numbers because
each hex digit represents four binary bits. As shown in Figure A-1, a
four bit binary number has sixteen possible combinations, requiring a
base-16 system to cover all combinations with one character. The PC
generally handles data as either eight bit bytes or two byte (16-bit)
words requiring two or four digit hex representations, respectively. The
decimal and hex values of the bit positions are shown in Figure A-2.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 23
┌────────────── reduced table goes here ────────────────┐
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└───────────────────────────────────────────────────────┘
Figure A-1. Binary Bit Values and Hexadecimal Digits
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Figure A-2. Decimal and Hexadecimal Values of the Bit
Positions in an 8-Bit Byte and a 2-Byte Word
As described in Chapter 3, the segmented addressing scheme of the
PC's 8088 microprocessor employs a pair of 16-bit binary numbers to
construct a complete 20-bit memory address. Each of these 16-bit binary
numbers can be represented by a four digit hex number from 0000 through
FFFF. You can verify this range for yourself by adding up the decimal
values listed in Figure A-2.
As an aid in converting hexadecimal numbers to decimal numbers, and
vice versa, the decimal equivalents of each hex digit in the first five
places are shown in Figure A-3. To convert a hex value into a decimal
value, you can work in either direction across the five places. To
convert a decimal number to hex notation, start with the fifth (or left)
position and work to the first place successively finding the largest
hex digit that is less than, or equal to, the remaining decimal value.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 24
┌────────────── reduced table goes here ────────────────┐
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Figure A-3. Decimal Equivalents of the
Hexadecimal Digit Positions
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 25
APPENDIX B. ASCII CHARACTERS
The 8-bit bytes of data stored and manipulated by the IBM PC family
have values ranging from 0 through 255 (0 through FF hex), which also
represent the numeric codes for the 256 distinct characters used by
these computers. These characters and numeric codes are known as ASCII
characters and codes.
The first 128 characters (0-127 decimal, or 0-7F hex) are the
standard or true ASCII characters, which date from the pre-PC days.
These standard characters are handled consistently among many computers,
except for variations in the first 32 characters which have uniform
ASCII meanings as codes for printer and console control, but which IBM
also uses for special display characters. The second 128 characters
(128-255 decimal, or 80-FF hex) make up the extended ASCII character set
that is unique to the IBM PC family and, of course, the PC-Clones.
Memory display utility programs such as CORELOOK and DEBUG will
display both the hex code and the standard ASCII character representing
each byte of memory. As a ready reference for translation between these
byte representations, the ASCII codes, in hex notation, and the screen
display are shown for the full standard and extended IBM character set
in Figure B-1. For quick reference, Figure B-2 translates hex notation
to decimal values. In both of these figures, the most significant hex
digits are at the top of the columns and the least significant hex
digits are at the beginning of the rows. For example, the "?" character
has the ASCII code 3F hex or 63 decimal.
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Figure B-1. Standard and IBM Extended Character ASCII Codes
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 26
┌──────────────── figure goes here ─────────────────────┐
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Figure B-2. Hex-to-Decimal Conversion Chart
Note that some of the special and extended characters will not
always appear as a screen display when a program produces video output
using a print function rather than poking the character code into the
display memory. For instance, CORELOOK's ASCII display shows a blank and
DEBUG substitutes a period for the special and extended characters.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 27
APPENDIX C. USEFUL SOFTWARE
In order to easily browse through the contents of the PC's memory
or download a portion of those contents to a disk file, you should
consider collecting a minimum set of software tools. These tools range
from the DOS utility DEBUG and a few lines of BASIC programming to some
more powerful and convenient public domain programs such as CORELOOK and
PC-DUMP.
One simple way to look at the contents of the PC's memory is to use
the DOS utility program DEBUG. For example, you can display the ROM-BIOS
release date by means of the following commands from the DOS prompt;
A>DEBUG
- D F000:FFF5 L 8
This will list (the L) the ASCII codes and characters for the eight
contiguous bytes (the 8) of memory beginning at the address F000:FFF5.
Consult the DOS manual for descriptions of the other features of DEBUG
that you can use to view the memory contents.
You can also use a short BASIC program to display the ASCII
characters representing the contents of specific memory locations. The
following code will display the ROM-BIOS release date.
10 DEF SEG = &HF000
20 FOR I = 0 TO 7
30 PRINT CHR$(PEEK(&HFFF5 + I));
40 NEXT I
50 END
If, rather than the ASCII characters, you want to display the two-digit
hex values of the memory locations, substitute HEX$ for the CHR$ in line
30.
A much more convenient way to quickly scan through the memory
contents is to use the public domain program CORELOOK, which is
available on many computer bulletin boards. CORELOOK is a menu driven
program that displays both the hex and ASCII representations of the
contents of a block of memory. Using single keystrokes, you can
literally fly through memory locations to any block that you desire. The
display is formatted for a logical sixteen bytes per line, and you have
full control in selecting the basis to be used for the segmented
addresses. You don't have to do much scanning of PC memory to really
appreciate the value of using this great little program.
If you are an experienced BASIC programmer, you can see how the
simple program listed above could be expanded to include the provision
for writing the contents of a specific portion of PC memory to a disk
file. You need not bother, however, as this function is already
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 28
available in the public domain program PC-DUMP, which is also available
for downloading on some computer bulletin boards. PC-DUMP is a menu
driven program that lets you select the segment paragraph address, the
number of bytes to be written to the disk file and the disk file name.
The program provides rigorous input formats and checking of the input
data for allowable and/or expected values. The latest update (Rev. 2.0)
of PC-DUMP also contains the option for downloading either the even
numbered or the odd numbered bytes from a given block of memory
addresses, as required for the PC-AT ROM hardware design. In the event
that you do not obtain an EPROM blaster with software that includes a
full featured download function that can be used without the blaster
being installed, you may save a good deal of time and frustration by
using PC-DUMP.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 29
APPENDIX D. EPROM PROGRAMMER SOURCES
At the time of this writing (October 1986), the type of PC-based
EPROM blasters described in Chapter 5 are available by mail order from
several computer components suppliers and manufacturers that advertise
in the major computer magazines. Two sources that currently offer models
in the $130 price range are:
Apparat, Inc.
4401 So. Tamarac Parkway
Denver, Colorado 80237
(303) 741-1778
JDR Microdevices
110 Knowles Drive
Los Gatos, California 95030
800-538-5000
Both of these models include software that provides the necessary
functions described in Chapter 5.
No-name versions of this type of PC-based EPROM blaster, some
selling for around $100, are starting to appear at computer shows and
swapmeets where vendors are selling various PC-Clone components.
Apparently nothing is immune to the cloning phenomena. Since these
devices are fairly new and are just catching on with the PC hobbyist
crowd, future price reductions are sure to be seen.
For you hobby purists, the perfect compliment to your ROM software
experimentation is an EPROM programmer that you build yourself. An
intelligent serial EPROM programmer is the topic of Steve Ciarcia's
Circuit Cellar feature in the October 1986 issue of BYTE magazine. Like
the stand-alone commercial programmers, this type of design interfaces
via the PC's serial I/O port rather than through an expansion card
connector. This arrangement is convenient if your PC has an available
serial port and would even allow you to do your EPROM burning using
computers other than the IBM/clone family. Ciarcia's project includes
the menu-driven programming system software to make using the programmer
a breeze. Typical of the exceptional quality of the Circuit Cellar
build-it-yourself projects, this update of an earlier design is a real
gem.
The PC-based EPROM blasters may see limited use and it may be
possible to pick up a used unit from another PC owner. You might check
the specialized computer/electronics classified add publications, the
posted notices at your local computer and electronics retailers, or the
PC User's Group in your area.
THE IBM PC READ-ONLY-MEMORY BACKUP MANUAL Page 30
NOTES
Directory of PC-SIG Library Disk #1408
Volume in drive A has no label
Directory of A:\
READ ME 1275 8-22-88 11:48p
GO BAT 283 5-31-89 3:46p
ROMBAK BAT 956 9-04-88 4:42p
ROMBAK TXT 75648 9-04-88 1:29p
ORDER TXT 919 8-22-88 9:32p
PC-DUMP EXE 46116 11-23-87 10:27p
PC-DUMP DOC 3708 11-23-87 11:50p
CORELOOK COM 4608 1-01-80 2:39a
CORELOOK DOC 4324 9-04-88 12:09p
FILE1408 TXT 1610 5-31-89 3:39p
10 file(s) 139447 bytes
18432 bytes free