🚀 go-pugleaf

as my understanding goes, when you read from the VGA, 4 latches are set,
one from each plane, so, the reasoning goes, that if you want to prime
the latches to user the VGA to do your logical operations (AND/OR...)
then you will simply have to read ONCE per write, so it will take 2X
longer to do things, which if you are updating 4 planes at a time is
STILL 2X faster than mode 13h, assuming of course that 13h and ModeX
really do work at the same speed, as there is some debate and I haven't
done any experimenting myself yet.

on another note, does anyone know of a good way to debug programs that
use ModeX without killing the graphics display?  Turbo Debugger fails
miserably at this.  I would be happy with suggestions for other
debugging software that leaves the VGA registers alone.  I am mainly
interested in debugging graphics code on a system with one monitor, as I
can't really afford to go and buy a  new one right now.

dan reznick
reznick@acf3.nyu.edu

I just re read my previous message and realized that I am indeed wrong.
I went through this same dilemma - the VGA's ALU (Arithmetic Logic Unit)
is pretty useless, because it only works when 1) the latches are primed,
and 2) on data coming FROM the cpu, which means at most two bytes at a
time, and it also means that you CAN'T use the latches to copy from off
screen display memory in Mode X, IF you want to use the ALU.

daniel reznick
reznick@acf3.nyu.edu

In article <62430004@acf3.NYU.EDU>, reznick@acf3.NYU.EDU (Daniel Reznick) writes:
 > as my understanding goes, when you read from the VGA, 4 latches are set,
 > one from each plane, so, the reasoning goes, that if you want to prime
 > the latches to user the VGA to do your logical operations (AND/OR...)
 > then you will simply have to read ONCE per write, so it will take 2X
 > longer to do things, which if you are updating 4 planes at a time is
 > STILL 2X faster than mode 13h, assuming of course that 13h and ModeX
 > really do work at the same speed, as there is some debate and I haven't
 > done any experimenting myself yet.
 > 
 > on another note, does anyone know of a good way to debug programs that
 > use ModeX without killing the graphics display?  Turbo Debugger fails
 > miserably at this.  I would be happy with suggestions for other
 > debugging software that leaves the VGA registers alone.  I am mainly
 > interested in debugging graphics code on a system with one monitor, as I
 > can't really afford to go and buy a  new one right now.

I've heard this, but can't confirm it...

In an assembler source set up a segment in the VGA video buffer:
SEGMENT VGA PUBLIC AT 0A000h
data db 64000 dup (?)
ENDS VGA

or something similar.
This will force (?) TD to keep that area clean (again ?).

Link the resulting .obj file with your c prog or whatever.
 
 > dan reznick
 > reznick@acf3.nyu.edu

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> or something similar.
> This will force (?) TD to keep that area clean (again ?).

No, what he was referring to is that TD restores mode 13h on return...not
the tweaked registers.

Josh

I read, in _Power Graphics Programming_ (which, BTW, I bought at a local
chain bookstore off the shelf a few days ago - I think it is still in
print, by QUE), that Turbo Debugger STILL screws with the VGA's
registers, EVEN when using another monitor, so as to make it almost
impossible to use TD for Mode X.  Abrash suggessts hooking an entire
another computer into a serial port and doing it from there.. but I
don't have a computer to spare.  

I was actually looking for suggestions of ANOTHER source level debugger
that will be more polite with my precious VGA registers..

daniel reznick
reznick@acf3.nyu.edu