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Updated PC programming documentation

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CHAPTER 1.
DOS TECHNICAL INFORMATION
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
SOME HISTORY
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, which had been written by a man named Tim
Paterson for use on the company's line of 8086, S100 bus micros. This 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 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.
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, and the IBM
3270 PC, which ran DOS on a 68000 microprocessor. The Atari STs can run an
emulator program and boot MSDOS.
Specific Versions of MS/PC-DOS:
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 subdirectories, etc. 3.0 added file handles and network support.
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 for PC/AT, network support
PC-DOS 3.1 November 1984 bugfix for 3.0
MS-DOS 2.25 October 1985 compatible; extended foreign language support
PC-DOS 3.2 July 1986 3.5 inch drive support for Convertible
PC-DOS 3.3 April 1987 for PS/2 series
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 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 1100 and some Tandy
models have MSDOS 2.11 in ROM.
THE OPERATING SYSTEM HIERARCHY
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 STRUCTURE
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
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)
* The DOS Program File
The actual DOS program is 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. For device
operations, the functions translate the requirement into one or more calls to
IBMBIO.COM or MSDOS.SYS to complete the request.
* The Command Interpreter
The Command interpreter, COMMAND.COM, 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 of Abort, Retry, Ignore, Fail.
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.
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 Dos 2.x, the transient portion of the command processor contains the
EXEC routine that loads and executes external commands. For DOS 3.x, the
resident portion of the command processor contains the EXEC routine.
DOS Initialization
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 initializes interrupt vectors for interrupts 0Fh through 3Fh.
6. IBMBIO.COM uses the EXEC function call to load and start the top-level
command processor. The default command processor is COMMAND.COM.