241 lines
12 KiB
Text
241 lines
12 KiB
Text
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CHAPTER 1.
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DOS TECHNICAL INFORMATION
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Programming Technical Reference - IBM
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Copyright 1988, Dave Williams
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SOME HISTORY
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Development of MSDOS/PCDOS began in October 1980, when IBM began searching
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the market for an operating system for the yet-to-be-introduced IBM PC.
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Microsoft had no real operating system to sell, but after some research licensed
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Seattle Computer Products' 86-DOS, which had been written by a man named Tim
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Paterson for use on the company's line of 8086, S100 bus micros. This was
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hurriedly polished up and presented to IBM for evaluation. IBM had originally
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intended to use Digital Research's CP/M operating system, which was the industry
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standard at the time. Folklore reports everything from obscure legal
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entanglements to outright snubbing of the IBM representatives by Digital,
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irregardless, IBM found itself left with Microsoft's offering of "Microsoft Disk
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Operating System 1.0". An agreement was reached between the two, and "IBM PC-DOS
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1.0" was ready for the introduction of the IBM PC in October 1981. IBM subjected
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the operating system to an extensive quality-assurance program, found well over
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300 bugs, and decided to rewrite the programs. This is why PC-DOS is copyrighted
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by both IBM and Microsoft.
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It is sometimes amusing to reflect on the fact that the IBM PC was not
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originally intended to run MSDOS. The target operating system at the end of the
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development was for a (not yet in existence) 8086 version of CP/M. On the other
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hand, when DOS was originally written the IBM PC did not yet exist! Although
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PC-DOS was bundled with the computer, Digital Research's CP/M-86 would probably
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have been the main operating system for the PC except for two things - Digital
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Research wanted $495 for CP/M-86 (considering PC-DOS was essentially free) and
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many software developers found it easier to port existing CP/M software to DOS
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than to the new version of CP/M.
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MSDOS and PC-DOS have been run on more than just the IBM-PC and clones. There
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was an expansion board for the Apple ][ that allowed one to run (some) well -
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behaved DOS programs. There are expansion boards for the Commodore Amiga 2000,
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the Apple MacIntosh II, and the IBM RT PC allowing them to run DOS, and the IBM
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3270 PC, which ran DOS on a 68000 microprocessor. The Atari STs can run an
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emulator program and boot MSDOS.
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Specific Versions of MS/PC-DOS:
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DOS version nomenclature: major.minor.minor. The digit to the left of the
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decimal point indicates a major DOS version change. 1.0 was the first version.
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2.0 added subdirectories, etc. 3.0 added file handles and network support.
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The first minor version indicates customization for a major application. For
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example, 2.1 for the PCjr, 3.3 for the PS/2s. The second minor version does not
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seem to have any particular meaning.
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The main versions of DOS are:
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PC-DOS 1.0 October 1981 original release
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PC-DOS 1.1 June 1982 bugfix, double sided drive support
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MS-DOS 1.25 June 1982 for early compatibles
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PC-DOS 2.0 March 1983 for PC/XT, many UNIX-like functions
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PC-DOS 2.1 October 1983 for PCjr, bugfixes for 2.0
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MS-DOS 2.11 October 1983 compatible equivalent to 2.1
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PC-DOS 3.0 August 1984 for PC/AT, network support
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PC-DOS 3.1 November 1984 bugfix for 3.0
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MS-DOS 2.25 October 1985 compatible; extended foreign language support
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PC-DOS 3.2 July 1986 3.5 inch drive support for Convertible
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PC-DOS 3.3 April 1987 for PS/2 series
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Some versions of MS-DOS varied from PC-DOS in the availible external commands.
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Some OEMs only licensed the basic operating system code (the xxxDOS and xxxBIO
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programs, and COMMAND.COM) from Microsoft, and either wrote the rest themselves
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or contracted them from outside software houses like Phoenix. Most of the
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external programs for DOS 3.x are written in "C" while the 1.x and 2.x utilities
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were written in assembly language. Other OEMs required customized versions of
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DOS for their specific hardware configurations, such as Sanyo 55x and early
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Tandy computers, which were unable to exchange their DOS with the IBM version.
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At least two versions of DOS have been modified to be run entirely out of ROM.
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The Sharp PC5000 had MSDOS 1.25 in ROM, and the Toshiba 1100 and some Tandy
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models have MSDOS 2.11 in ROM.
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THE OPERATING SYSTEM HIERARCHY
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The Disk Operating System (DOS) and the ROM BIOS serve as an insulating layer
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between the application program and the machine, and as a source of services
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to the application program.
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The system heirarchy may be thought of as a tree, with the lowest level being
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the actual hardware. The 8088 or V20 processor sees the computer's address
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space as a ladder two bytes wide and one million bytes long. Parts of this
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ladder are in ROM, parts in RAM, and parts are not assigned. There are also
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256 "ports" that the processor can use to control devices.
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The hardware is normally addressed by the ROM BIOS, which will always know
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where everything is in its particular system. The chips may usually also be
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written to directly, by telling the processor to write to a specific address or
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port. This sometimes does not work as the chips may not always be at the same
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addresses or have the same functions from machine to machine.
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DOS STRUCTURE
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DOS consists of four components:
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* The boot record
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* The ROM BIOS interface (IBMBIO.COM or IO.SYS)
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* The DOS program file (IBMDOS.COM or MSDOS.SYS)
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* The command processor (COMMAND.COM or aftermarket replacement)
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* The Boot Record
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The boot record begins on track 0, sector 1, side 0 of every diskette formatted
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by the DOS FORMAT command. The boot record is placed on diskettes to produce an
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error message if you try to start up the system with a nonsystem diskette in
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drive A. For hard disks, the boot record resides on the first sector of the DOS
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partition. All media supported by DOS use one sector for the boot record.
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* Read Only Memory (ROM) BIOS Interface
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The file IBMBIO.COM or IO.SYS is the interface module to the ROM BIOS.
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This file provides a low-level interface to the ROM BIOS device routines and
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may contain extensions or changes to the system board ROMs. Some compatibles do
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not have a ROM BIOS to extend, and load the entire BIOS from disk. (Sanyo 55x,
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Viasyn)
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* The DOS Program File
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The actual DOS program is file IBMDOS.COM or MSDOS.SYS. It provides a high-
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level interface for user (application) programs. This program consists of file
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management routines, data blocking/deblocking for the disk routines, and a
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variety of built-in functions easily accessible by user programs.
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When a user program calls these function routines, they accept high-level
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information by way of register and control block contents. For device
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operations, the functions translate the requirement into one or more calls to
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IBMBIO.COM or MSDOS.SYS to complete the request.
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* The Command Interpreter
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The Command interpreter, COMMAND.COM, consists of these parts:
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Resident Portion:
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The resident portion resides in memory immediately following IBMDOS.COM and its
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data area. This portion contains routines to process interrupts 22h (Terminate
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Address), 23h (Ctrl-Break Handler), and 24h (Critical Error Handler), as well as
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a routine to reload the transient portion if needed. For DOS 3.x, this portion
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also contains a routine to load and execute external commands, such as files
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with exensions of COM or EXE.
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When a program terminates, a checksum is used to determine if the application
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program overlaid the transient portion of COMMAND.COM. If so, the resident
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portion will reload the transient portion from the area designated by COMSPEC=
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in the DOS environment. If COMMAND.COM cannot be found, the system will halt.
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NOTE: DOS 3.3 checks for the presence of a hard disk, and will default to
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COMSPEC=C:\. Previous versions default to COMSPEC=A:\. Under some DOS
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versions, if COMMAND.COM is not immediately availible for reloading
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(i.e., swapping to a floppy with COMMAND.COM on it) DOS may crash.
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All standard DOS error handling is done within the resident portion of
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COMMAND.COM. This includes displaying error messages and interpreting the
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replies of Abort, Retry, Ignore, Fail.
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An initialization routine is included in the resident portion and assumes
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control during startup. This routine contains the AUTOEXEC.BAT file handler and
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determines the segment address where user application programs may be loaded.
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The initialization routine is then no longer needed and is overlaid by the first
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program COMMAND.COM loads.
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NOTE: AUTOEXEC.BAT may be a hidden file.
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A transient portion is loaded at the high end of memory. This is the command
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processor itself, containing all of the internal command processors and the
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batch file processor. For DOS 2.x, this portion also contains a routine to load
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and execute external commands, such as files with extensions of COM or EXE.
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This portion of COMMAND.COM also produces the DOS prompt (such as "A>"), reads
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the command from the standard input device (usually the keyboard or a batch
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file), and executes the command. For external commands, it builds a command line
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and issues an EXEC function call to load and transfer control to the program.
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NOTE: COMMAND.COM may be a hidden file.
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NOTE: For Dos 2.x, the transient portion of the command processor contains the
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EXEC routine that loads and executes external commands. For DOS 3.x, the
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resident portion of the command processor contains the EXEC routine.
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DOS Initialization
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The system is initialized by a software reset (Ctrl-Alt-Del), a hardware reset
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(reset button), or by turning the computer on. The Intel 80x8x series processors
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always look for their first instruction at the end of their address space
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(0FFFF0h) when powered up or reset. This address contains a jump to the first
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instruction for the ROM BIOS.
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Built-in ROM programs (Power-On Self-Test, or POST, in the IBM) check machine
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status and run inspection programs of various sorts. Some machines set up a
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reserved RAM area with bytes indicating installed equipment (AT and PCjr).
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The ROM routine looks for a disk drive at A: or an option ROM (usually a hard
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disk) at absolute address C:800h. If no floppy drive or option ROM is found, the
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BIOS calls int 19h (ROM BASIC if it is an IBM) or displays error message.
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If a bootable disk is found, the ROM BIOS loads the first sector of information
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from the disk and then jumps into the RAM location holding that code. This code
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normally is a routine to load the rest of the code off the disk, or to "boot"
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the system.
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The following actions occur after a system initialization:
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1. The boot record is read into memory and given control.
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2. The boot record then checks the root directory to assure that the first
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two files are IBMBIO.COM and IBMDOS.COM. These two files must be the
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first two files, and they must be in that order (IBMBIO.COM first, with
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its sectors in contiguous order).
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NOTE: IBMDOS.COM need not be contiguous in version 3.x.
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3. The boot record loads IBMBIO.COM into memory.
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4. The initialization code in IBMBIO.COM loads IBMDOS.COM, determines
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equipment status, resets the disk system, initializes the attached
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devices, sets the system parameters and loads any installable device
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drivers according to the CONFIG.SYS file in the root directory (if
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present), sets the low-numbered interrupt vectors, relocates IBMDOS.COM
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downward, and calls the first byte of DOS.
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NOTE: CONFIG.SYS may be a hidden file.
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5. DOS initializes its internal working tables, initializes the interrupt
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vectors for interrupts 20h through 27h, and builds a Program Segment
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Prefix for COMMAND.COM at the lowest available segment. For DOS versions
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3.10 up, DOS initializes interrupt vectors for interrupts 0Fh through 3Fh.
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6. IBMBIO.COM uses the EXEC function call to load and start the top-level
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command processor. The default command processor is COMMAND.COM.
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