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** Programmer's Technical Reference for MSDOS and the IBM PC **
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³ Shareware Version, 12/16/88 ³
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Copyright (c) 1988 Dave Williams


C H A P T E R O N E

DOS TECHNICAL INFORMATION


SOME HISTORYÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

Development of MSDOS/PCDOS began in October 1980, when IBM began searching
the market for an operating system for the yet-to-be-introduced IBM PC.
Microsoft had no real operating system to sell, but after some research licensed
Seattle Computer Products' 86-DOS operating system, which had been written by a
man named Tim Paterson earlier in 1980 for use on that company's line of 8086,
S100 bus micros. 86-DOS (also called QDOS, for Quick and Dirty Operating System)
had been written as more or less a 16-bit version of CP/M, since Digital
Research was showing no hurry in introducing CP/M-86.
This code was hurriedly polished up and presented to IBM for evaluation. IBM
had originally intended to use Digital Research's CP/M operating system, which
was the industry standard at the time. Folklore reports everything from obscure
legal entanglements to outright snubbing of the IBM representatives by Digital,
irregardless, IBM found itself left with Microsoft's offering of "Microsoft Disk
Operating System 1.0". An agreement was reached between the two, and IBM agreed
to accept 86-DOS as the main operating system for thir new PC. Microsoft
purchased all rights to 86-DOS in July 1981, and "IBM PC-DOS 1.0" was ready for
the introduction of the IBM PC in October 1981. IBM subjected the operating
system to an extensive quality-assurance program, found well over 300 bugs, and
decided to rewrite the programs. This is why PC-DOS is copyrighted by both IBM
and Microsoft.

It is sometimes amusing to reflect on the fact that the IBM PC was not
originally intended to run MSDOS. The target operating system at the end of the
development was for a (not yet in existence) 8086 version of CP/M. On the other
hand, when DOS was originally written the IBM PC did not yet exist! Although
PC-DOS was bundled with the computer, Digital Research's CP/M-86 would probably
have been the main operating system for the PC except for two things - Digital
Research wanted $495 for CP/M-86 (considering PC-DOS was essentially free) and
many software developers found it easier to port existing CP/M software to DOS
than to the new version of CP/M.

After eight years the wheel has turned a full circle. Digital Research aided
IBM in writing DOS 4.0, which was subsequently licensed back to Microsoft,
which has dropped further development of the operating system to tilt at the
windmills of OS/2.

MSDOS and PC-DOS have been run on more than just the IBM-PC and clones. There
was an expansion board for the Apple ][ that allowed one to run (some) well -
behaved DOS programs. There are expansion boards for the Commodore Amiga 2000,
the Apple MacIntosh II, and the IBM RT PC allowing them to run DOS, the Atari
STs can run an emulator program called PC-Ditto and boot any version of MSDOS
with full functionality. Microsoft's OS/2 operating system incorporates the
so-called "DOS Compatibility Box" to run DOS software; QNX will run DOS as a
task, and a few Unix systems offer "DOS Merge" or control programs that allow
DOS to run as a task.


WHAT IS DOS?ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

DOS exists as a high-level interface between an application program and the
computer. DOS stands for "Disk Operating System", which reflects the fact that
its main original purpose was to provide an interface between the computer and
its disk drives.
DOS now lets your programs do simple memory management, I/O from the system
console, and assorted system tasks (time and date, etc) as well as managing
disk operations. Versions 3.1 and up also incorporate basic networking
functions.
With the introduction of installable device drivers and TSR (terminate but
stay resident) programs in DOS 2.0, the basic DOS functions may be expanded to
cover virtually any scale of operations required.


OTHER OPERATING SYSTEMSÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

Various other operating systems are availible for the IBM PC. These include:

Digital Research CP/M-86
Digital Research Concurrent CP/M-86 (multitasking)
Digital Research Concurrent DOS (PC-DOS compatible, multitasking)
Digital Research Concurrent DOS XM (PC-DOS compatible, multitasking, multiuser)
Digital Research Concurrent DOS 386 (same, for 80386 computers)
Digital Research DR-DOS 3.31 and 4.0 (PC-DOS clones)
Wendin-DOS (PC-DOS compatible, multitasking, multiuser)
QNX (multitasking, multiuser)
UNIX (various systems from IBM itself, Microsoft-SCO, Bell, and various UNIX
clones, single and multi user)
MINIX (multitasking UNIX clone)
PC-MOS/386 (PC-DOS clone, multiuser, multitasking)

"Shell" programs exist which use DOS only for disk management while they more
or less comprise a new operating system. These include:

DesQview Windows OmniView
GEM TopView TaskView


NEC V-series CPUs can execute Intel 8080/8085 8-bit instructions as well as
the 16-bit 8088-up instructions. They can run standard Digital Research 8-bit
CP/M and MP/M directly, as well as other operating systems developed for that
processor.



SPECIFIC VERSIONS OF MS/PC-DOSÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

DOS 1.x is essentially 86-DOS. DOS 2.x kept the multiple file layout (the two
hidden files and COMMAND.COM) but for all practical purposes is an entirely
different operating system with backwards compatibility with 1.x. I seriously
doubt there has been much code from 1.x retained in 2.x. DOS 3.x is merely an
enhancement of 2.x; there seems little justification for jumping a whole
version number. DOS 4.0, originating as it did from outside Microsoft, can
justify a version jump. Unfortunately, 4.x seems to have very little to reason
justify its existence - virtually all of its core features can be found in one
version or another of DOS 3.x.

DOS version nomenclature: major.minor.minor. The digit to the left of the
decimal point indicates a major DOS version change. 1.0 was the first version.
2.0 added support for subdirectorites, 3.0 added support for networking, 4.0
added support for Lotus-Intel-Microsoft EMS.
The first minor version indicates customization for a major application. For
example, 2.1 for the PCjr, 3.3 for the PS/2s. The second minor version does not
seem to have any particular meaning.

The main versions of DOS are:

PC-DOS 1.0 October 1981 original release
PC-DOS 1.1 June 1982 bugfix, double sided drive support
MS-DOS 1.25 June 1982 for early compatibles
PC-DOS 2.0 March 1983 for PC/XT, many UNIX-like functions
PC-DOS 2.1 October 1983 for PCjr, bugfixes for 2.0
MS-DOS 2.11 October 1983 compatible equivalent to 2.1
PC-DOS 3.0 August 1984 1.2 meg drive for PC/AT
PC-DOS 3.1 November 1984 bugfix for 3.0, implemented network support
MS-DOS 2.25 October 1985 compatible; extended foreign language support
PC-DOS 3.2 July 1986 720k 3.5 inch drive support for Convertible
PC-DOS 3.3 April 1987 for PS/2 series, 1.44 meg, multiple DOS partitions
PC-DOS 4.0 August 1988 over-32 meg DOS partitions, EMS support

IBM's PC-DOS is considered to be the "standard" version of DOS; Microsoft has
sold MS-DOS over the counter only since version 3.2 (previously, Microsoft
sold its versions only to OEMs). Most versions of DOS functionally duplicate
the external DOS commands such as DISKCOPY, etc. Microsoft has announced that
they will sell MS-DOS 4.0 only to OEMs, so it looks like the "generic" MS-DOS
was short-lived.

Some versions of MS-DOS varied from PC-DOS in the availible external commands.
Some OEMs only licensed the basic operating system code (the xxxDOS and xxxBIO
programs, and COMMAND.COM) from Microsoft, and either wrote the rest themselves
or contracted them from outside software houses like Phoenix. Most of the
external programs for DOS 3.x and 4.x are written in "C" while the 1.x and 2.x
utilities were written in assembly language. Other OEMs required customized
versions of DOS for their specific hardware configurations, such as Sanyo 55x
and early Tandy computers, which were unable to exchange their DOS with the IBM
version.

At least two versions of DOS have been modified to be run entirely out of ROM.
The Sharp PC5000 had MSDOS 1.25 in ROM, and the Toshiba 1000 and some Tandy
models have MSDOS 2.11 in ROM.

PC-DOS 3.0 was extremely buggy on release. It does not handle the DOS
environment correctly and there are numerous documented problems with the
batch file parser. The network support code is also nonfunctional in this DOS
version. It is recommended that users upgrade to at least version 3.1.

DEC MSDOS versions 2.11 for the Rainbow had the ANSI.SYS device driver built
into the main code. The Rainbow also supported a unique quad density, single
sided floppy drive.

IBM had a version 1.85 of PC-DOS in April 1983, after the introduction of DOS
2.0. It was evidently for internal use only, supported multiple drive file
searches (a primitive form of PATH), builtin MODE sommands for screen support,
a /P parameter for TYPE for paused screens, an editable command stack like the
public domain DOSEDIT.COM utility, and could be set up to remain completely
resident in RAM instead of a resident/transient part like normal DOS. It is a
pity some of the neat enhancements didn't make it into DOS 2.0. IBM also had
an internal use only version 3.4, evidently used while developing DOS 4.0.

Some versions of DOS used in compatibles do not maintain the 1.x, 2.x, .....
numbering system. Columbia Data Products computers labeled DOS 1.25 as DOS
2.0. Early Compaqs labeled DOS 2.0 as DOS 1.x. Other versions incorporated
special features - Compaq DOS 3.31 and Wyse DOS 3.21 both support 32-bit File
Allocation Tables.

AT&T DOS 3.1 differs from generic MSDOS 3.10 in its use of cluster-size and
file allocation table structures. AT&T DOS appears to use rules not from
version 3, but rather those from version 2.

Epson Equity III and ComputerLand 3.10 DOS's appear to use cluster techniques
that are a cross between versions 2 and 3. On type DOS partitions, these DOS's
use 3.x rules if the partition is larger than 32,680 sectors in total size.
This implies 16 bit FAT entries as well. On partitions below this size, they
will use 2.x rules, including the 12 bit FAT entries.

Zenith DOS 3.x and Wyse DOS 3.2 have a builtin internal device driver to
handle up to 4 32Mb DOS partitions on a single hard disk. Wyse DOS 3.31 will
handle single partitions up to 512Mb with a 32-bit FAT.

According to PC Week Magazine, July 4, 1988, Arabic versions of MSDOS are
shipping with a hardware copy-protection system from Rainbow Technologies.
This is similar to the short-lived system used by AutoCAD 2.52 and a very few
other MSDOS programs, where an adapter block is plugged into the parallel port
and software makes use of coded bytes within the block. This type of copy
protection has been common on Commodore products for several years, where it is
called a "dongle."
The AutoCAD dongle was defeated by a small program written within weeks of
version 2.52's debut. Version 2.62 was released 3 months later, without the
dongle. The DOS dongle will, however, prevent the system from booting at all
unless it is found.
This makes the Arabic version of MSDOS the first copy-protected operating
system, a dubious distinction at best. The modifications to the operating
system to support the dongle are not known at this time.

Versions of DOS sold in Great Britain are either newer than those sold in the
US or use a different numbering system. DOS 3.4, 4.0, 4.1, 4.2, and 4.3 had
been released there between the US releases of 3.3 and 4.0.
MSDOS 4.0 was introduced in mid-1987 in Europe (at SICOB in Paris and sometime
earlier by Apricot Computer in the UK). It apparently offers multitasking
PROVIDED applications are specially written for it.
David Fraser (UK MD of Microsoft) is on record saying that DOS 4 is unlikely
to set the world alight and is of interest only to specific OEM's who want its
features for networking/comms etc. Standard DOS applications will run under DOS
4.x as a foreground task according to uncertain information. It differs from
earlier versions only in allowing background tasks to run. No info about how
many tasks/memory requirements etc. at this time. It will run an MS-DOS 3.2 or
earlier type task in a foreground partition, and a specially written task in
the background.

Microsoft changed their OEM licensing agreements between DOS versions 2.x and
3.x. OEM versions of DOS 3.x must maintain certain data areas and undocumented
functions in order to provide compatibility with the networking features of the
operating system. For this reason, resident programs will be much more reliable
when operating under DOS 3.x.

IBM's release of DOS 4.0 (and the immediate subsequent release of a bugfix)
is a dubious step "forward." DOS 4.0 is the first version of DOS to come with
a warranty; the catch is that IBM warrants it only for a very slim list of
IBM-packaged software. 4.0 has some minor EMS support, support for large hard
disks, and not much else. With its voracious RAM requirements and lack of
compatibility with previous versions of DOS (many major software packages
crash under DOS 4.0), plus the increase in price to a cool $150, there has
been no great rush to go to the newest DOS.



THE OPERATING SYSTEM HIERARCHYÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

The Disk Operating System (DOS) and the ROM BIOS serve as an insulating layer
between the application program and the machine, and as a source of services
to the application program.
The system heirarchy may be thought of as a tree, with the lowest level being
the actual hardware. The 8088 or V20 processor sees the computer's address
space as a ladder two bytes wide and one million bytes long. Parts of this
ladder are in ROM, parts in RAM, and parts are not assigned. There are also
256 "ports" that the processor can use to control devices.
The hardware is normally addressed by the ROM BIOS, which will always know
where everything is in its particular system. The chips may usually also be
written to directly, by telling the processor to write to a specific address or
port. This sometimes does not work as the chips may not always be at the same
addresses or have the same functions from machine to machine.



DOS STRUCTUREÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

DOS consists of four components:

* The boot record
* The ROM BIOS interface (IBMBIO.COM or IO.SYS)
* The DOS program file (IBMDOS.COM or MSDOS.SYS)
* The command processor (COMMAND.COM or aftermarket replacement)


* The Boot Record

The boot record begins on track 0, sector 1, side 0 of every diskette formatted
by the DOS FORMAT command. The boot record is placed on diskettes to produce an
error message if you try to start up the system with a nonsystem diskette in
drive A. For hard disks, the boot record resides on the first sector of the DOS
partition. All media supported by DOS use one sector for the boot record.


* Read Only Memory (ROM) BIOS Interface and Extensions

The file IBMBIO.COM or IO.SYS is the interface module to the ROM BIOS.
This file provides a low-level interface to the ROM BIOS device routines and
may contain extensions or changes to the system board ROMs. Some compatibles do
not have a ROM BIOS to extend, and load the entire BIOS from disk. (Sanyo 55x,
Viasyn machines). Some versions of MSDOS, such as those supplied to Tandy, are
named IBMBIO.COM but are not IBM files.


* The DOS Program

The actual DOS program is the file IBMDOS.COM or MSDOS.SYS. It provides a high-
level interface for user (application) programs. This program consists of file
management routines, data blocking/deblocking for the disk routines, and a
variety of built-in functions easily accessible by user programs.
When a user program calls these function routines, they accept high-level
information by way of register and control block contents. When a user program
calls DOS to perform an operation, these functions translate the requirement
into one or more calls to IBMBIO.COM, MSDOS.SYS or system hardware to complete
the request.


* The Command Interpreter

The command interpreter, COMMAND.COM, is the part you interact with on the
command line. It consists of these parts:

Resident Portion:

The resident portion resides in memory immediately following IBMDOS.COM and its
data area. This portion contains routines to process interrupts 22h (Terminate
Address), 23h (Ctrl-Break Handler), and 24h (Critical Error Handler), as well as
a routine to reload the transient portion if needed. For DOS 3.x, this portion
also contains a routine to load and execute external commands, such as files
with exensions of COM or EXE.

When a program terminates, a checksum is used to determine if the application
program overlaid the transient portion of COMMAND.COM. If so, the resident
portion will reload the transient portion from the area designated by COMSPEC=
in the DOS environment. If COMMAND.COM cannot be found, the system will halt.

NOTE: DOS 3.3+ checks for the presence of a hard disk, and will default to
COMSPEC=C:\. Previous versions default to COMSPEC=A:\. Under some DOS
versions, if COMMAND.COM is not immediately availible for reloading
(i.e., swapping to a floppy with COMMAND.COM on it) DOS may crash.


All standard DOS error handling is done within the resident portion of
COMMAND.COM. This includes displaying error messages and interpreting the
replies to the "Abort, Retry, Ignore, Fail?" message.





DOS INITIALIZATIONÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

The system is initialized by a software reset (Ctrl-Alt-Del), a hardware reset
(reset button), or by turning the computer on. The Intel 80x8x series processors
always look for their first instruction at the end of their address space
(0FFFF0h) when powered up or reset. This address contains a jump to the first
instruction for the ROM BIOS.
Built-in ROM programs (Power-On Self-Test, or POST, in the IBM) check machine
status and run inspection programs of various sorts. Some machines set up a
reserved RAM area with bytes indicating installed equipment (AT and PCjr).
The ROM routine looks for a disk drive at A: or an option ROM (usually a hard
disk) at absolute address C:800h. If no floppy drive or option ROM is found, the
BIOS calls int 19h (ROM BASIC if it is an IBM) or displays error message.
If a bootable disk is found, the ROM BIOS loads the first sector of information
from the disk and then jumps into the RAM location holding that code. This code
normally is a routine to load the rest of the code off the disk, or to "boot"
the system.
The following actions occur after a system initialization:

1. The boot record is read into memory and given control.

2. The boot record then checks the root directory to assure that the first
two files are IBMBIO.COM and IBMDOS.COM. These two files must be the
first two files, and they must be in that order (IBMBIO.COM first, with
its sectors in contiguous order).
NOTE: IBMDOS.COM need not be contiguous in version 3.x+.

3. The boot record loads IBMBIO.COM into memory.

4. The initialization code in IBMBIO.COM loads IBMDOS.COM, determines
equipment status, resets the disk system, initializes the attached
devices, sets the system parameters and loads any installable device
drivers according to the CONFIG.SYS file in the root directory (if
present), sets the low-numbered interrupt vectors, relocates IBMDOS.COM
downward, and calls the first byte of DOS.
NOTE: CONFIG.SYS may be a hidden file.



5. DOS initializes its internal working tables, initializes the interrupt
vectors for interrupts 20h through 27h, and builds a Program Segment
Prefix for COMMAND.COM at the lowest available segment. For DOS versions
3.10 up, DOS also initializes the vectors for interrupts 0Fh through 3Fh.
An initialization routine is included in the resident portion and assumes
control during startup. This routine contains the AUTOEXEC.BAT file
handler and determines the segment address where user application programs
may be loaded. The initialization routine is then no longer needed and is
overlaid by the first program COMMAND.COM loads.
NOTE: AUTOEXEC.BAT may be a hidden file.

6. IBMBIO.COM uses the EXEC function call to load and start the top-level
command processor. The default command processor is COMMAND.COM in the
root directory of the boot drive. If COMMAND.COM is in a subdirectory
or another command processor is to be used, it must be specified by a
SHELL= statement in the CONFIG.SYS file.
A transient portion is loaded at the high end of memory. This is the
command processor itself, containing all of the internal command
processors and the batch file processor. For DOS 2.x, this portion also
contains a routine to load and execute external commands, such as files
with extensions of COM or EXE.
This portion of COMMAND.COM also produces the DOS prompt (such as "A>"),
reads the command from the standard input device (usually the keyboard or
a batch file), and executes the command. For external commands, it builds
a command line and issues an EXEC function call to load and transfer
control to the program.
NOTE: COMMAND.COM may be a hidden file.
NOTE: For IBM DOS 2.x, the transient portion of the command processor
contains the EXEC routine that loads and executes external commands.
For MSDOS 2.x+ and IBM DOS 3.x+, the resident portion of the command
processor contains the EXEC routine.
NOTE: IBMBIO only checks for a file *named* "COMMAND.COM". It will load
any file of that name if no SHELL= command is used.


IBM's original documentation spoke of installing alternate command
interpreters (programs other than COMMAND.COM) with the SHELL= statement in
CONFIG.SYS. Unfortunately, IBM chose not to document much of the interaction
between IBMDOS.COM and IBMBIO.COM. By the time much of the interaction was
widely understood, too many commercial software programs had been written to
use peculiarities of COMMAND.COM itself. There is only one commercial
replacement for COMMAND.COM (a program called Command Plus) and it is not
widely known or used.