AT KBC - check other input bit for video mode select?

skiselev / 8088_bios

BIOS for Intel 8088 based computers

GNU General Public License v3.0

513 stars 61 forks source link

AT KBC - check other input bit for video mode select? #41

Closed SanguineBrah closed 12 months ago

SanguineBrah commented 1 year ago

Even though the Xi 8088 does not have a trace running to it, would it be possible to also check input port 17 (pin 34) on the 8242 for the other video mode bit in order to make it possible to solder a switch that would enable 40-column CGA mode on this board?

skiselev commented 1 year ago

It should be possible, at least looking at VT82C42N keyboard datasheet. It wasn't a standard feature in IBM AT though, and I am not positive that other keyboard controllers will support it. On the BIOS side, it would require modifications to this subroutine: https://github.com/skiselev/8088_bios/blob/3ca115140ac6d55d5f1d5a5031b89cb2bcfb74ed/src/at_kbc.inc#L468

SanguineBrah commented 1 year ago

Ah yeah, looking at the datasheet for the HT6542B I can see that it uses that pin for keyboard inhibit. Perhaps pin 32 (bit 5) is more appropriate, as it seems to be indicated as general purpose I/O across VT82C42N, 8242 and HT6542B as far as I can tell.

EDIT: What am I saying? P12 and P13 are undefined; those are obviously better candidates for this purpose.

SanguineBrah commented 12 months ago

Closing issue