Microsoft KB Archive/39370

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Addressing above 1 Megabyte Under MS-DOS


The information in this article applies to:

  • Microsoft Macro Assembler for MS-DOS, versions 5.0, 5.1a, 6.0, 6.0a, 6.0b


When a 80286 or higher processor is in real mode, addressing is done using a 16-bit segment and 16-bit offset. Therefore, it is only possible to address up to 1 megabyte of memory. To address memory above 1 meg under MS-DOS, you have to put the processor into protected mode by using IBM ROM BIOS Interupt 15 Function 89h. Addressing will be done using a selector into a descriptor table.

For more information about this interrupt, call IBM hardware support, (800) 426-2468. If ROM BIOS routines are not operating as expected, you may need to contact the company that supplied the BIOS in your particular machine.

For more information about protected mode, selectors and descriptor tables, see the Intel guides for the 80286, 80386, or 80486 or "Microsoft's 80386/80486 Programming Guide".

Additional query words: 5.00 5.10a 6.00 6.00a 6.00b

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Last Reviewed: January 5, 2000
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