Date   : Mon, 02 Mar 2009 11:49:55 +0100
From   : kortink@... (John Kortink)
Subject: Fwd: New Screen Modes with ARM7 Co-Processor?

On Mon, 2 Mar 2009 10:06:23 +0000, Rob <robert@...> wrote:

>2009/3/2 Ian Wolstenholme <bbcmailinglist@...>:
>
>> So, with memory in abundance on the ARM7 co-processor, would there
>> be a way of creating new screen modes, say an 8-colour mode at
>> Mode 0 resolution, or even higher?
>>
>
>The memory for the display has to be accessible by the crt controller,
>so has to be in the host machine, i.e. the Model B or Master. ?It
>can't use memory on the co-processor (of any type). ?This means,
>typically, that it needs to come out of the 32K RAM available. ?Mode 0
>already takes 20K - to make it 8 colours would take 60K ...
>
>It did cross my mind about paging in a SRAM bank, and using that to
>give a few more contiguous KB upwards, but I think the circuitry is
>set to hard-wire a wrap-around at the 32K boundary (so as to allow
>hardware scrolling) which would preclude this, plus it'd keep getting
>paged out for filing system access...

It wouldn't matter per se that it gets paged out, since
the CRTC has it's own access to memory, independent from
the processor. That's why shadow memory works. But there
is a 32K addressing limitation in the current hardware.

One somewhat unexploited trick is the fact that the 6845
sends out odd and even frames from the same memory source,
which halves the potential vertical resolution. Switching
back and forth between two framebuffers on every frame is
a solution, but there's no hardware support, so this has
to be done by software, on interrupt. I.e. shadow screen
memory, and switch banks (for 6845, not 6502 access) per
frame. Plus some convoluted screen drawing code (every
other screen line is in the shadow bank) ...


John Kortink

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