Microsoft KB Archive/48720

Extended BIOS Data Area Reduces PS/2 RAM by 1K ID Number: Q48720

4.00 4.01 MS-DOS

Question:

All PS/2s and some OEM AT machines have only 639K available for MS-DOS, instead of 640K. Why is 1K not available?

Response:

This is a result of the ROM BIOS, not of MS-DOS. The IBM (and compatible) PS/2 series of machines have an “Extended BIOS Data Area,” an area of RAM that the ROM BIOS uses. The ROM BIOS lowers the amount of usable RAM by 1K (thus, 639K rather than 640K), reducing the amount returned by ROM BIOS Interrupt 12H, and stored in low memory at the address 40:13H.

The MS-DOS Version 4.00 MEM.EXE utility shows the amount used up for the Extended BIOS Data Area. This data area is almost exclusively used for the PS/2 pointing device (mouse). Some OEM AT (i.e., non-PS/2) machines (such as the Compaq 20e) have PS/2 pointing device hardware built in; they would have to support this Extended BIOS Data Area as well. For more information on the Extended BIOS Data Area, consult a ROM BIOS reference for Interrupt 15H, services C0H and C1H.