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1980s Computers Explanation of terms

The IBM 5150 Personal Computer

IBM 5150 system


International Business Machines (US)



Date Launched

12th August 1981 in US
Early 1982 in UK


$1565 (£1100) in US for the cheapest system, without a disk drive.
Approx. £2000 for the minimum usable system comprising the main processor box, keyboard, monochrome monitor and a floppy disc drive.

Microprocessor type

Intel 8088 @ 4.77 MHz

ROM size

40 kilobytes ?

Standard RAM

16 kilobytes originally.
Later 64 kilobytes.

Maximum RAM

Reported to be 64 kilobytes on early models.
Later increased to 640 kilobytes.

Keyboard type

Excellent quality typewriter style, separate to main unit, originally with only 83 keys.

Supplied language

Microsoft BASIC in ROM.
PC DOS 1.0 operating system.

Text resolution

40 columns x 25 lines.
80 columns as optional extra.

Graphics resolution

No graphics as standard.
320 x 200 or 640 x 200 pixels with optional expansion card.

Colours available

Basic model was monochrome only.
4 colours with optional graphics board.

Example Screenshot

IBM 5150 monochrome text screen
A monochrome display on an IBM PC, running a typical business program.


No sound except beeps from internal speaker.

Cassette load speed

The IBM 5150 incorporated a cassette interface but all except the earliest were instead supplied with one or two 5¼" floppy disc drives of 160 or 320 kilobytes capacity.

Dimensions (mm)
Weight (grams)

508 x 406 x 140 (main box)

Special features

Was specifically designed to be expandable and had five ISA expansion slots.
Had a very strong sheet metal case. This was actually to speed up the development since plastic cases required moulds to be prepared, which could take months.

Good points

Carried the IBM reputation for quality.
Expandability and use of standardised parts meant it could be adapted to many different jobs.
A wide range of operating systems, programming languages and application software was soon available.

Bad points

Very expensive, especially compared to home computers.
With only 16 KB of RAM, no colour or graphics as standard and only a 4.77 MHz processor its abilities were not even as good as the best of the home microcomputers (e.g. Atari 800 and BBC B).
The original PC DOS 1.0 was dire, with a clumsy syntax and no support for subdirectories.

How successful?

The IBM 5150 and derivatives sold in hundreds of thousands and created a de-facto 'PC standard' for business computers.
By the late 1980s 'PC compatibles' had become the dominant design of computer in business, and now account for about 90% of all computers in use.


IBM was already the largest manufacturer of mainframes and minicomputers and in 1980 decided to enter the new but growing market for microcomputers. However they were concerned that if their new micro was too good it could harm sales of the existing (and profitable) minicomputer range.
IBM therefore decided to limit the capabilities of the PC 5150, hence only 16 KB of RAM and a fairly slow microprocessor with awkward segmented memory access, rather than the much better Motorola 68000.

Purely on its merits it is hard to see why anyone bought the original PC 5150 other than because of the reputation of the IBM name. Desktop computers, even the Apple II and Commodore PET, had previously been seen as little more than executive toys in business, but IBM's entry into the market made them respectable. The large, bland, but solid appearance of the PC 5150 also helped it to be accepted into the workplace.

Microsoft was not the original choice to supply the operating system for the PC 5150. Gary Kildall's Digital Research had an operating system called CP/M-86 which could easily have been modified for the 8088 processor in the PC 5150. However it seems Digital Research were unwilling to sign the secrecy agreements that IBM insisted on, so IBM asked Microsoft to provide the operating system as well as the BASIC.
Unfortunately (especially for the users) Microsoft did not have the resources to write an operating system in time so they bought (not licenced) one from Seattle Computer Products.

This had been written by Tim Paterson, who apparently called it QDOS ('Quick and Dirty Operating System'), in just six weeks but it became the basis of PC-DOS 1.0 and thence all later versions of DOS.
Supplying DOS for PCs was mainly what allowed Microsoft to expand into the world's biggest software company, but they paid just $50,000 for QDOS.
Gary Kildall died in 1994 at the age of 52.

The 5150 was not in fact IBM's first desktop computer. In 1975 they had sold the IBM 5100 which was a semi portable containing the electronics, keyboard, 5 inch monitor and a tape drive all in one unit. However the 5100 cost around $10,000 and was not a commercial success.

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