Re: Real Speed of GS RAM slot?

[dempson@actrix.gen.nz (David Empson)]

In article <384sba$nh6@cleese.apana.org.au>,
Jonathan Pratt <ntt@cleese.apana.org.au> wrote:
> dempson@actrix.gen.nz (David Empson) writes:
> > 
> > Any I/O device using the slot would have to be accessed outside the
> > normal RAM address space, i.e. in bank $80 to $DF, or $E2 to $EF.  It
> > would have to detect its address by monitoring the bank address on the
> > data bus, and would not work reliably with other DMA devices (except
> > possibly the RAMFast SCSI card).
> 
> None of these banks could be seen at the memory expansion slot because
> there isn't any signal lines to define the difference between bank 80 and
> 00 (the highest bit isn't confered to the slot).

Wrong.  The bank address is provided on the data bus by the processor
during the first half of the cycle.

A card could latch the address from the data bus and use it to extend
the multiplexed address bus provided for the RAM.  The big question is
what the FPI produces on the RAM address pins if you access memory
beyond bank $7F.

This technique is used by memory cards which support more than four
rows of memory.  For example, the OctoRAM latches one extra bit from
the data bus (bank address) to extend the addressable memory to 8
megabytes. 

This technique is not DMA compatible, because the bank address is not
provided during a DMA cycle (except with a RAMFast, which pulls a few
tricks).

In addition, this method cannot be used to add more IIgs-addressable
RAM in banks $80 and up, because the memory manager doesn't look past
bank $7F.  You might be able to patch in some additional memory after
startup time, but it would be ugly.

> The RAM and ROM addressing is quite different, and you could decode
> the ROMDISK area of memory (since there haven't been any ROMDISKs for
> the GS) and get control in banks F0 to FB (768k - enough room for a
> video screen ;^) 

No ROMDisks?  I know of at least two.  An early RAM card for the IIgs
had space for some on-card ROM.  I can't recall the brand.  There is
also a piggy-back static RAM card for the OctoRAM, called the OctoRAM
ESP, which acts like a ROMdisk.  I have one of these.

The ROMdisk isn't supposed to go past bank $F7 - banks $F8 to $FB are
"reserved for future ROM expansion".  We all know how likely _that_ is.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand