nanogui: Strange bug in ELKS and MicroWindows
Subject:
Re: Strange bug in ELKS and MicroWindows
From:
Ben Pfaff ####@####.####
Date:
28 Jul 1999 19:46:00 -0000
Message-Id: <87hfmolfyf.fsf@pfaffben.user.msu.edu>
Greg Haerr ####@####.#### writes:
I've been listening to the (sparse) comments about the need
for a faster VGA driver and am now responding with another
MicroWindows driver written in asm for ELKS. This one
operates faster, although the 8086 still doesn't exactly
blaze when it comes to graphics.
However, I've encountered a _strange_ bug that I can't quite
seem to get my hands around.... The microwindows
driver uses the VGA read-modify-write that sets the
VGA latches to draw a pixel. Assume that DS:BX is
the byte in screen ram, there's two ways to do this,
depending out OUTs that setup the VGA:
1) ...setup mask register with bit to write
or [bx],al ; rmw the vga
or 2)
mov al,[bx] ; read
mov al,[bp+8] ; modify
mov [bx],al ; write
There's no need to actually modify the byte: mov al,[bx]; mov [bx],
al; should work just as well.
If option two is used, which was my first version, then the mouse
leaves little "droppings" on the screen when it's being moved, every
once in a while.
Sure, it's possible that your code is being called reentrantly or some
other routine is reading or writing the screen.
Is there a reason you don't want to use version 1?
If I recode (2) with cli/sti around it, it works.
If I recode (2) as follows, (assume now ES:BX is screen byte)
mov al,es:[bx]
mov al,[bp+8]
mov es:[bx],al
It seems as though ELKS is trashing user DS during mouse movement!!!
However, if I use option (1), it works, although the screen is only accessed
with a single DS move.
That's truly bizarre, and I can't comment on why that would help.
