VGA INT 10h mode snapshot code: Text modes 0-3 & 7 should also call INT 10h functions to set 200-line, 350-line, and 400-line emulation.

[joncampbell123]

There's a INT 10h function on VGA BIOSes that changes whether the VGA text modes are 200-line 8x8, 350-line 8x14, or 400-line 8x16 text modes. We need snapshots for each.

[MrMadguy64]

IBM VGA uses following logic:

1. 400-lines and 200-lines bits are handled separately (I guess, because MCGA doesn't support 200-lines bit).
2. 400-lines bit overrides any other settings in ALL modes (including 0x07) - 400-lines mode is forced.
3. 200-lines bit is actually used for informational purposes only.
4. 200-lines bit is changed, when switching from mono->color or color->mono, if no secondary adapter present.
5. EGA switches are analyzed instead to determine if 200-line or 350-line mode should be used.
6. 200-lines mode can't be set in mono mode, if secondary adapter is present.
7. 350-lines mode is forced for mode 0x07 anyway.

Algorithm is following:

1. If adapter is inactive - do nothing.
2. There are 4 routes - if secondary adapter is present or not and if it's mono mode or not.
3. Secondary adapter is present: do not change 200-lines bit, 400-lines bit = 0 for 350, 1 for 400.
   3.1) Mono mode: fail, if 200-lines mode is chosen, EGASW = b.
   3.2) Color mode: EGASW = 8 for 200-lines mode, 9 for 350/400.
4. No secondary adapter: set 400-lines bit for 400, otherwise 200-lines bit if 200, otherwise both bits are 0.
   4.1) Mono mode: EGASW = b.
   4.2) Color mode: EGASW = 8 for 200-lines mode, 9 for 350/400.

I've looked into S3 BIOS code: it uses 3x2x2 jump table instead.

[MrMadguy64]

I have related question. I've got stuck with this problem too. What is the most reliable documented way to determine, what mode is selected? Can I rely on BIOS data or BIOS data exist for internal use only and only what is provided by Int 10h API can be reliable? The biggest problem - text resolution mode can be set, but no API returns this setting, except EGA switches, that don't distinguish 350 and 400 modes.

[MrMadguy64]

Things are even more complicated. 200-lines bit can't be relied on, because IBM VGA uses it to temporary store 200-lines state, while in mono mode, where 200-lines aren't supported. I.e. IBM VGA actually uses EGA switches to determine, if 200 or 350-lines mode should be used (either 8 or 9), but this information is lost while in mono mode, where EGA switches are set to 0x0b. So, in order not to lose it, VGA stores it in 200-lines bit. This bit is always cleared during mono->color mode switch and it's always set according to current EGA switches during color->mono mode switch. Bit it preserved in all other cases, including unsupported EGA switch values. Bit is unused, if secondary adapter is present, as mono<->color mode switches aren't possible in this case.

Plus, I guess, I've found bug in IBM VGA BIOS. Feat bits can be destroyed during mono<->color switches, because somebody has forgotten to add one "and bl, 0fh" instruction.