MS-DOS Overview

MSDOS (Microsoft Disk Operating System) is designed for the IBM series (and compatible) range of computers. These computers were based on a new micro-processor range produced by Intel Corporation.

IBM Personal Computer (PC)
The IBM PC was produced in 1981, and featured 64K of RAM, a cassette interface and the BASIC language in ROM. Based on the 8088 microprocessor, the memory range was limited to 1Mbyte, 640K of which could be RAM, while the other 640K was reserved for hardware devices like video cards, ROM BASIC and expander cards. The 8088 processor was an 8 bit processor, handling data in 8 bit chunks (a single character). Its clock rate was 4.77Mhz. The PC machine was very quickly upgraded to 128K and then 256K of RAM on the motherboard. The two video systems supported were Monochrome (80 columns by 25 rows) and the Color Graphics Adapter (CGA). CGA systems had a text mode of 80x25, with two graphic modes, 320x200 in 4 colors and 640x400 in 2 colors. Introduced at the same time was MSDOS version 1.0 which did not support hierarchical directory structures, but was small in size and had limited functions. It was based on Digital Research’s CP/M operating system for the popular range of Z80 and 8080 microprocessor based computers.

IBM XT
The extended computer (XT) was introduced by IBM in 1983. The XT featured 512K of RAM (quickly replaced with 640K) and a possible expansion Hard Disk unit (10Mbyte). The processor was upgraded to the 8086, which processed data in 16bit chunks (two characters at a time), but the clock rate (speed of the processor) was still the same. Before long, other countries had cloned the hardware machine and was making it run faster and selling it cheaper than IBM.

IBM AT
The Advanced Technology computer range was released in 1984. This featured a new processor chip from Intel, the 80286. This processor was a 16 bit processor, handling data 16 bits at a time (two characters). Running at a faster clock rate (8MHz), it executed many more instructions per second than the 8088 processor. The memory range had expanded to a capacity of 16MB. MSDOS still imposed the restrictions of 640K of RAM for use by itself, and it was not until MSDOS version 5.0 was released where official support above 640K of RAM started to occur. EGA (Enhanced Graphics Adapter) emerged, supporting 16 colors in a higher resolution mode than CGA. Before long, clone manufacturers developed VGA (Virtual Graphics Adapter) with even higher resolutions and colors. VGA standard is 640x480 in 16 colors SVGA (Super VGA) 640x480 in 256 colors 800x600 in 16 colors 1024x768 in 16 colors Today, the new S3 video cards support the 1024x768 modes in 64K colors. Other standards like TIGA and XGA are also used.

The 386DX Computer System
The 386 range is based on the Intel 80386 processor. It is a full 32 bit processor, with a maximum RAM size of 4Gbytes. It supports many concepts that mainframe computers have used (virtual memory, task switching, pipelining, multiple instruction execution, and internal cache memory). It comes in various clock rates, the higher the clock rate, the faster it executes instructions. The clock rates are 16, 20, 25 and 33MHz. Several operating systems have been designed to exploit the hardware design features of this processor. They are: Microsoft Windows 3.x OS/2 v1.x and 2.x UNIX MSDOS is still written in the old 8086 code format, and does not take advantage of the better or faster instructions provided by the 80386 processor. The 80387 processor is an arithmetic processor for performing calculations. It can do these operations up to 300 times faster than doing the same calculations using software on the 80386. Typical operations are, multiply divide sine, cosine, tan, exponential

The 486DX Computer System
The 486 computer is very similar to the 80386 processor except for the following differences: The maths co-processor is internal to the chip, therefor maths operations are faster The internal cache size is 8k instead of 16 bytes Some instructions are optimised Most instructions are executed in half the time of an 80386 It can run at higher clock rates (up to 100MHz)

The SX series of Processor Chips
Intel also manufactures an SX version of the 80386 and 80486 processors. The 80386SX is the same as the 80386DX internally, but it talks to memory 16bits at a time rather than 32bits at a time. The 80486SX processor is the same as the 80486DX except for two notable features. The first is that it does not include a maths processor inside, and the second is that it is limited to addressing 16Mbytes of RAM.

Just what is MSDOS
MSDOS is a disk operating system for the IBM range (or compatibles) of computers. The MSDOS system resides on diskette (3.5" or 5.25") and the system files may be transferred to a hard disk unit. An operating system is a program which allows competing programs to share the resources of the computer (memory, disk, devices like printers and modems, and processing time). The operating systems first job is to initialise the hardware of the computer system, before running application programs.

For personal computers, MSDOS is a single user, single tasking operating system. Single user means only one person uses the computer at a time. Single tasking means that it essentially only runs one application program at a time, and has no inherent support for running more than one application program simultaneously. An operating system provides mechanisms for running programs, handling errors, controlling devices, configuring the computer system, and for MSDOS, a user interface (sometimes called a shell).

The main system part of MSDOS consists of three files:
MSDOS.SYS
IO.SYS
COMMAND.COM

IO.SYS implements MSDOS as seen by the hardware. It has default device drivers for:
console display and keyboard
printer
serial communications
clock
boot disk drive

MSDOS.SYS implements MSDOS as seen by application programs. It supports:
file and record management
memory management
character device input and output
execution of other programs
access to a real-time clock

COMMAND.COM is the shell program which interprets user commands, presents the shell prompt, and contains a set of internal commands. A diskette used to load MSDOS into the computer must contains these three files. In addition, these files must be first on the diskette.
The rest of MSDOS consists of utility programs. Examples of these are:
backup   backup files to another disk
format    format a disk for use by MSDOS
print       print a file
restore    restore a backed up file(s) from disk
xcopy     copy a range of files and sub-directories


Making an MSDOS diskette using the format command
These files are transferred from the MSDOS master diskettes using the format /s command. All previous files are erased and the disk is written with information which MSDOS uses to keep track of files.

Making a MSDOS system diskette using the sys command
The two .sys files are transferred to a blank diskette by using the sys command. The file command.com can then be transferred using the copy command. The following command sequence illustrates the commands necessary to create a system diskette which is used to boot the computer system. sys a: copy command.com a: