🚀 go-pugleaf

zonker@diku.dk (Claus Engdahl) writes:

> Hi.
> 
> I'm desperately seeking help from one of you VGA gurus out there. 
> 
> I am trying to write to screen, using writemode 2. I manage to get
> something written, but the bitmask register seems to work in strange
> and wonderous ways :-(
> 
> Having read "Programmers guide to the EGA and VGA cards", I managed to
> cook up the following routine:
> 
Whoa! theres your problem right there-- lousy docs! I wasted my money on 
this book too. Information is duplicated through the whole book, with each 
duplication leaving something different out. I eventually ended up 
realising what was going on, but it took far more skull sweat than it 
should have. The way to fix your program is to do a "dummy read" of the 
location containing the pixels you want to change. This read has the 
effect of initializing the internal 32-bit latch to the contents of screen 
memory. During the write operation, the ALU operates on what is in the 
latch, not what is in the memory location in question. After the operation 
is complete, then the new contents of the latch are written to video 
memory.

------------------------------------------------------------------
 Andy Vaught (Fuzzy Andy)  ::  C Code. C Code Run. Run C code, Run...
 Grad Student on Vacation  ::  before I whip out my 12-Gauge
    andy@bluemoon.uucp     ::  Dynamic Debugging Tool!