Acorn A4 Extension ROM

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Pernod
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Acorn A4 Extension ROM

Postby Pernod » Wed Nov 23, 2016 12:16 am

The Acorn A4 is the only machine that utilises an extension ROM to handle battery power management. Does anyone know where it is mapped in memory?

I believe it resides somewhere in the region 3400000-37fffff but that is a large region for a 64K ROM. Since RISC OS 3 apparently scans for extension ROMs from top down I'd guess it's mapped high in the region. I guess it doesn't really matter to be exact for emulation purposes but if anyone knows exactly where it sits would be appreciated.
- Nigel

BBC Model B, ATPL Sidewise, Acorn Speech, 2xWatford Floppy Drives, AMX Mouse, Viglen case, etc.

poink
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Re: Acorn A4 Extension ROM

Postby poink » Wed Nov 23, 2016 1:05 am

Pernod wrote:I believe it resides somewhere in the region 3400000-37fffff but that is a large region for a 64K ROM. Since RISC OS 3 apparently scans for extension ROMs from top down I'd guess it's mapped high in the region

The circuit diagram suggests to me that it's actually mirrored/repeated across that address space; so it appears every 256KB (it only uses the high byte; bits 24-31; of each word) ie., it appears 16 times in the 4MB space.

steve3000
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Re: Acorn A4 Extension ROM

Postby steve3000 » Wed Nov 23, 2016 1:17 am

poink wrote: so it appears every 256KB (it only uses the high byte of each word). ie., 16 times in the 4MB space.

As he said... It's an 8-bit ROM on a 32-bit bus, so each byte is addressed on a 32-bit word boundary (hence 64kb becomes 256kb). The content is extracted by riscos at boot up and run from RAM.

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Pernod
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Re: Acorn A4 Extension ROM

Postby Pernod » Wed Nov 23, 2016 2:59 am

Thanks both, I now see it with *ROMModules, but is Dormant. It doesn't seem to matter which byte of each word it's loaded into, RISC OS still finds it. Doesn't need to be repeated either.

Really wish I could read circuit diagrams #-o
- Nigel

BBC Model B, ATPL Sidewise, Acorn Speech, 2xWatford Floppy Drives, AMX Mouse, Viglen case, etc.


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