Date   : Fri, 19 Oct 2007 20:47:19 +0100 (BST)
From   : debounce@... (Greg Cook)
Subject: 12 kB Sideways RAM in a B+?

On Fri, 19 Oct 2007 17:49:59 +0100, Daniel Beardsmore
<public@...> wrote:

> The manual confirms it -- the B+ has 12 kB Sideways RAM. It's not
> enough to
> host a ROM image, being 4 kB short of one page, so why did Acorn only
> fit 12
> kB instead of 16?

See http://beebwiki.jonripley.com/ANDY

It's not really a sideways RAM slot; even if you put an 8K ROM image in
there the MOS will ignore it.

> The B+ manual doesn't seem to cover it at all.

There's a tiny mention of this area on page 452 under OSWORD 5,  but
nothing on the 'ordinary' SWRAM banks found in the B+ 128K.

> I've never seen
> Sideways RAM
> documented anywhere -- like an OS call to find out how many banks you
> have
> of it, which they are, and how large each one is. Clearly you'd need
> dedicated calls like this if you can't even rely on having a complete
> bank!

There *might* be something of the sort in the SRAM utility bundled with
the 1770 DFS ROM.  Presumably it would be documented in the DFS user
guide, which I don't have.  But certainly software running on a Model B
would have to scan for sideways RAM itself.

> I don't know for example how Quest Paint recognises Sideways RAM when
> looking for the extra 5 kB buffer it needs to back up the screen
> behind a
> menu. Or how you detect Shadow RAM for that matter ...

Off the top of my head:

.testShadow
    LDX #0:LDA #&72:JSR osByte  \force shadow on next MODE change
    TXA:PHA                     \save current shadow setting
    LDX #6:LDA #&85:JSR osByte  \what would HIMEM be in MODE 6?
    PLA:TAX:TYA:PHA             \save high byte & fetch X
    LDA #&72:JSR osByte         \restore current shadow setting
    PLA:RTS                     \restore result and exit

On exit A=&80, N=1 iff shadow RAM is installed.  Tested on all models
in BeebEm 3.8.

Regards,

Greg Cook
debounce@...
http://homepages.tesco.net/~rainstorm/



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