Microsoft KB Archive/40489

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Total Memory Versus Available Memory and the MEM Command PSS ID Number: Q40489 Article last modified on 03-25-1993 PSS database name: O_MSDOS

4.00 4.01


The information in this article applies to:
- Microsoft MS-DOS operating system versions 4.00 and 4.01


The MEM.EXE command in MS-DOS Version 4.00 reports on the amount of conventional memory, the amount of available conventional memory, and the size of the largest executable program that can be run in the system.

The total amount of memory and the amount of available memory are the same for non-Micro Channel Architecture (MCA) machines. For MCA machines, the available memory is slightly less than the total system memory, due to the presence of the extended BIOS Data Area. (This is somewhat of a generalization. IBM PS/2 systems support this extended BIOS data area. There are some OEM systems that are not complete PS/2 compatibles and are not MCA systems, but nonetheless DO support this extended BIOS data area.)

When a PS/2 runs its POST (Power On Self Test) routine, it sets aside some memory at the top of conventional memory for the extended BIOS data area. The location in memory that contains the total system memory is updated to reflect the reduced size of conventional memory.

When MEM determines the memory configuration of any machine, a call is made to the BIOS to determine the total amount of memory. This value is actually the total amount of available memory. A call is then made (interrupt 15H, function C1H) to get the address of the extended BIOS data area. The amount of memory in the extended BIOS data area is then added to the total system memory.

With systems that do not support the extended BIOS data area call, MEM will return the same amount for total and available memory.

For more information on IBM (and 100 percent compatible) ROM BIOS information, please refer to the “IBM PS/2 and PC BIOS Interface Technical Reference,” part number 68X2260, available from IBM by calling (800) IBM-PCTB. Another reference that contains similar material is the Microsoft Press book, “Programmer’s Quick Reference Series: IBM ROM BIOS,” by Ray Duncan, ISBN 1-55615-135-7. For OEM ROM BIOS extensions, refer to your OEM’s hardware technical reference manuals.

Additional reference words: 4.00 4.01 noupd

Copyright Microsoft Corporation 1993.