VECHO / V8Power relies on video rows count BIOS variable being initialized, but it is not guaranteed to be initialized

[skiselev]

I've received the a pull request for my 8088 BIOS: https://github.com/skiselev/8088_bios/pull/42

This pull request supposed to fix an issue where the 8088 BIOS does not initialize video row count BIOS variable at 0040:0084.

Upon closer investigation, it appears that the system BIOS is not guaranteed and not supposed to initialize this variable. VGA and EGA BIOS extensions do initialize it though.

The code here https://github.com/LoopZ/V8Power/blob/7852abe0198d265e77689d2e0fafa5d0787dda54/SOURCE/COMMON.INC#L2127 should be updated to validate that the BIOS indeed populates the right value in that variable, or use some another way to detect the screen height

[shidel]

@skiselev, I apologize for either not seeing or forgetting about this issue until today. While V8PT only officially supports EGA or better, I plan on trying to allocate some time this coming weekend to look into this variable. I will likely add a test for a "sane" value. If the value is not within a reasonable range (14-60 rows), then assume it has not been initialized and use 25 rows.

[shidel]

@skiselev, Again, I am sorry it took so long to get to this. But finally, I added a Sanity Check to the initialization of the Row Count in the last commit. Hopefully, this will help on sub-EGA hardware.

https://github.com/LoopZ/V8Power/commit/51750b6e7376dbb2759ec5b7abcdac2d6be5655c#diff-5ecac2b5b26fe54af95039c6cad832ac7cce727cee321d01f1f88b0bf281fb58R2149