nanogui: Drawing modes
Subject:
Re: Drawing modes
From:
Ben Pfaff ####@####.####
Date:
19 May 1999 23:15:16 -0000
Message-Id: <877lq4sl36.fsf@pfaffben.user.msu.edu>
Alex Holden ####@####.#### writes:
On Wed, 19 May 1999, Greg Haerr wrote:
> Don't get confused by the "read-modify-write" cycle that the EGA/VGA
> controllers require for the damned hardware. It think this is what
> you're looking at. The EGA/VGA doesn't require an rmw cycle for XOR, it
> requires it for all writes, and the SET, AND, OR, XOR stuff is
> controlled in yet another hw register.
Since you keep insisting that it is possible to get around the requirement
to do RMW, I've looked hard for any code which demonstrates this, but I'm
sorry to say I've been unable find any. As an example, here is an extract
from __linear_putpixel8 (the basis of pretty much all operations on 8 bit
linear devices- the other bit depths all seem to work the same way) from
allegro:
He might be referring to one (or both) of two different things:
1. Under VGA planar (16-color) modes, there is hardware assistance for
logical operations, which supports NOP, AND, OR, XOR at the
hardware level. You write the desired logical operation to a
couple of bits in the VGA Data Rotate Register, then perform the
operations like you would usually, and the device automatically
does the desired logical operation.
Unfortunately, you still need to do a read and a write, unless
there's some clever combination of write mode and register settings
that I'm missing right now. You don't necessarily need to do a
rmw, though: often, due to the VGA architecture, the following does
just fine:
volatile char *x;
...set up registers...
y = *x; *x = 0x42;
...finish up...
i.e., you don't care what gets read from the VGA, and you don't
care what byte gets written to the VGA; the 0x42 above may be any
arbitrary byte, which produces identical results.
2. Video RAM is often quite a bit slower than system RAM, so sometimes
it's faster to keep `video RAM shadow' as a bitmap in system RAM.
Then, to update the screen image, you rmw the bitmap and just do
the write to video RAM (as opposed to a read and a write to video
RAM and none to a bitmap): three memory accesses instead of two,
but only one really slow memory access rather than two.
--
"Let others praise ancient times; I am glad I was born in these."
--Ovid (43 BC-18 AD)
