[Rpcemu] Thoughts on RPCemu development

Tom Walker tommowalker at yahoo.co.uk
Sun Sep 2 18:05:47 BST 2007


Ed Avis wrote:

> Ah, zero page!  I remember that from the 6502.  Didn't realize it was used
> on ARMs but then I never did any ARM assembler.  Is it just a case of
> poking x and y coordinates into a couple of addresses?  Why not do that on
> every mouse movement rather than trapping the SWIs?

Because this doesn't always work properly for some reason. Also, if the mouse
coordinates change during POST then it's likely to fail to boot up.

> I remember various dodgy programs on Acorn User monthly discs and the like
> that would spit out mode definitions, and I used one to generate an extra
> large 800x288 mode or something to get the most screen space out of my
> CM8833.  I'll have a hunt round HENSA etc. to see which program is best.

If you knew the exact monitor type then you might be able to do this
reliably. The problem would come with defining arbitary modes for multisync
monitors.

> >You'd also run into the memory limit often - A5000 hardware 
> >has a limit of 480k on screen memory, and that can't be 
> >changed without rewriting the memory manager.
> 
> I was wondering about that.  There was the G8 from State Machine or
> something like that, a more upmarket rival to the ColourCard, which
> required special software to use the whole megabyte of VRAM.  It does
> appear that 480K is a hard limit.

I've just had a look at the ModeExtension service (having just coded a module
for 800x600x256), and I think there's a way around this for an emulator. You
can change the screen address used by the VDU drivers, so you could put the
screen buffer in an unused area of the ARM address space (such as
&2000000-&2FFFFFF, giving 16 megs of 'vram', or in IO space somewhere). Still
not sure why you'd want to - if you want that much video memory then use
RPCemu!

I suspect what the G8 did was map video RAM in podule space (16k at a time)
and the special software paged this around as VDU calls required. Would be
pretty slow, but on an A5000 you've got no other choice.

Cheers,
Tom


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