[Qemu-devel] Purpose of Cirrus VGA BIOS

[Qemu-devel] Purpose of Cirrus VGA BIOS

[Bartosz Fabianowski]

Hi all

I am trying to figure out how the new Cirrus emulation works. One thing 
I am still not sure about is the purpose of the Cirrus BIOS. The 
emulation coded inside QEMU seems to be trapping the graphics card 
interrupt as well as port and memory accesses. It also performs lots of 
operations and reacts to registers being changed or the interrupt being 
invoked. So, what exactly is the purpose of the graphics card BIOS in 
this scenario? What purpose does it serve? Obviously, a real Cirrus BIOS 
contains lots of code that would control a real Cirrus graphics card and 
that is totally unnecessary in QEMU. What other stuff does it contain 
that can't be done in native C code?

- Bartosz

Re: [Qemu-devel] Purpose of Cirrus VGA BIOS

[Antony T Curtis]

On Tue, 2004-06-08 at 00:02, Bartosz Fabianowski wrote:
> Hi all
> 
> I am trying to figure out how the new Cirrus emulation works. One thing 
> I am still not sure about is the purpose of the Cirrus BIOS. The 
> emulation coded inside QEMU seems to be trapping the graphics card 
> interrupt as well as port and memory accesses. It also performs lots of 
> operations and reacts to registers being changed or the interrupt being 
> invoked. So, what exactly is the purpose of the graphics card BIOS in 
> this scenario? What purpose does it serve? Obviously, a real Cirrus BIOS 
> contains lots of code that would control a real Cirrus graphics card and 
> that is totally unnecessary in QEMU. What other stuff does it contain 
> that can't be done in native C code?

For starters, the real Cirrus BIOS would initialize the reserved
scratchpad registers (they do have specific uses even if it is not fully
documented).

As an aside, looking at the ROP codes, I have worked out the
relationship of what the bits mean...

ROP[0] for bit 0
ROP[1] for bit 1 
  etc

DST = ((ROP[0] & SRC) | ROP[1]) & ((ROP[2] & DST) | (ROP[3] & ~DST)) 
  & ((ROP[4] & ~SRC) | ROP[5]) & ~((ROP[6] & ~DST) | (ROP[7] & DST));
-- 
Antony T Curtis <antony.t.curtis@ntlworld.com>