nanogui: Drawing modes
Subject:
RE: Drawing modes
From:
Alex Holden ####@####.####
Date:
19 May 1999 19:31:07 -0000
Message-Id: <Pine.LNX.4.04.9905192006130.1891-100000@hyperspace>
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:
putpix_xor_mode:
READ_BANK() /* read pixel from screen */
xorb %es:(%eax, %ecx), %bl /* XOR */
movl ARG3, %eax /* re-read Y position */
jmp putpix_solid_mode
Surely it reads it from the screen, modifies it, then writes it back?
I've also looked hard at the GGI, W, and X low level framebuffer drivers,
and none of them even seem to even implement different draw modes. I've
looked at the kernel side code, and was unable to see any way to write to
a graphics device in anything other that the ordinary SET mode, though it
is pretty complex and I could have overlooked something.
--------------- Linux- the choice of a GNU generation. --------------
: Alex Holden (M1CJD)- Caver, Programmer, Land Rover nut, Radio Ham :
-------------------- http://www.linuxhacker.org/ --------------------
