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Is there a DOS memory documentation available? (Users)

posted by Rugxulo Homepage, Usono, 08.01.2024, 09:46

8086 could address 1 MB (conventional memory), but the IBM PC specifically (contrary to SCP's boards) would only use 640 kb (minus OS overhead) with ROM data (e.g. BIOS) in higher parts. EMS with bankswitching was optional, but some people (e.g. Jim Leonard) have physical hardware EMS, allowing a few extra MB.

XMS required at least a 286 (v2) or 386 (v3). The 286 could only address 16 MB (or 1 GB virtual, allegedly, IIRC). It fragmented a bit worse than EMS.

EMM386 was optional but helped get old EMS apps to work using V86 mode. VCPI (ring 0, 386 only) was a superset of this, supported by Win 3.x Standard mode (win.com /s), but DPMI was overall better.

DPMI was originally invented for Win 3.0 (Enhanced mode) in 1990 but later standardized and became common elsewhere (OS/2 2.1, Novell DOS 7). Unfortunately, most people only supported the 0.9 spec (usually for 32-bit apps only). But Borland did support 16-bit DPMI. So, just to be clear, unlike ring 0 32-bit exclusive VCPI, DPMI could also be ring 3 and either 286 or 386 compatible. Ring 3 is less prone to OS instability.

There are various bugs and limits in implementations of all of the above. Testing is more important than blind fidelity to specs.

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