Paul Brook wrote:
Will this work also for the CL542x adaptor? (Does that fall in the
category of vga?) My current hack works for with/without -std-vga and I
think that's because it lives "underneath" both, in the connection to
SDL.
Each adapter will have to do it's own minimization but that's sort of
the write thing anyway IMHO. How granular each update is really only
depends on the adapter. For instance, the VMware adapter really
shouldn't need to do any minimization at all.
It would be nice if we could share the framebuffer blitting routines. We've
currently got 3 different implementations (vga/cirrus, tcx and pl110) of
basically the same framebuffer rendering routines.
Take a look at the video code in BasiliskII / SheepShaver, a 68k/PPC
classic MacOS emulator written by Gwenolé Beauchesne:
http://gwenole.beauchesne.info/projects/sheepshaver/
It contains optimized code (source level) for blitting between various
bit depths and endiannesses. See
SheepShaver-2.3/src/Unix/video_blit.{h,cpp} in the sources.
It also uses a technique called "video on segfault" (VOSF) to improve
performance on platforms which support it: rather than testing each
store to see if it modifies the framebuffer, it keeps the framebuffer
write-protected (via mprotect(), or the equivalent on non-POSIX systems)
and uses a SIGSEGV handler to catch stores to the buffer. When a page
receives a store, the handler unprotects the page and updates a bitmap
of modified pages. Every so often a display update thread wakes up,
consults the bitmap, calculates the updated region, blits it to the
screen (using the optimized blitters), and clears the bitmap. See
SheepShaver-2.3/src/Unix/video_vosf.h,
SheepShaver-2.3/src/Unix/video_x.cpp, and other files.
The emulators also have alternative techniques for tracking update
regions on systems for which VOSF is not supported. But VOSF is almost
always a big win. And most modern OSes can support it without trouble.
The code is in C++ so it can't be dropped in directly, but Some of the
techniques may be useful in qemu.
Brian J. Johnson
