IBM Personal Computer System Board

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The IBM Personal Computer 5150 was equiped with one of two revisions of its main system board throughout its production:

  • Up to March 1983, the 16-64KB board
  • From March 1983, the 64-256KB board
A hand-made prototype IBM 5150 system board

16-64KB System Board

64-256KB System Board

Common Specification

  • Intel 8088 CPU at 4.77MHz
  • Socket for Intel 8087 FPU
  • 5 ISA slots
  • 4 banks of 9-bit (parity) RAM:
    • For 16-64KB board - using 16 kbit chips
    • For 64-256KB board - using 64 kbit chips
  • Physically 9" x 13"
  • DIN connectors for keyboard and cassette deck

BIOS Revisions

Configuration

The system configuration was set using two DIP switches.

Switch Block 1

This block is nearest the centre of the board.

Boot Intel 8087
Sw1 Boot from
ON ROM (Cassette) BASIC
OFF Floppy Disk
Sw2 Intel 8087
ON Not installed
OFF Installed


Memory Video Floppy Drives
Sw3 Sw4 Banks Installed
ON ON One bank
OFF ON Two banks
ON OFF Three banks
OFF OFF Four banks
Sw5 Sw6 Video Type
ON ON No video or special (EGA, VGA)
OFF ON 40 column CGA
ON OFF 80 column CGA
OFF OFF MDA (or Hercules) Video
Sw7 Sw8 Floppy Drives
ON ON 1 drive
OFF ON 2 drives
ON OFF 3 drives
OFF OFF 4 drives

Switch Block 2

This block is nearest the CPU. Switches 1-5 define the total amount of memory installed (including that on expansion cards), and switchs 6-8 are always off.

Total RAM Installed Sw1 Sw2 Sw3 Sw4 Sw5
16K ON ON ON ON ON
32K ON ON ON ON ON
48K ON ON ON ON ON
64K ON ON ON ON ON
96K OFF ON ON ON ON
128K ON OFF ON ON ON
160K OFF OFF ON ON ON
192K ON ON OFF ON ON
224K OFF ON OFF ON ON
256K ON OFF OFF ON ON
288K OFF OFF OFF ON ON
320K ON ON ON OFF ON
352K OFF ON ON OFF ON
384K ON OFF ON OFF ON
416K OFF OFF ON OFF ON
448K ON ON OFF OFF ON
480K OFF ON OFF OFF ON
512K ON OFF OFF OFF ON
544K OFF OFF OFF OFF ON
576K ON ON ON ON OFF
608K OFF ON ON ON OFF
640K ON OFF ON ON OFF


See Also