Re: SCSI, RAM, etc.

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

In article <97853@cup.portal.com>
ScottM@cup.portal.com (Scott - Maxwell) writes:
> My main question was how the memory (more than 64k) on an aux slot
> card is different from a slot 1-7 memory card (a la the Apple Memory
> Expansion Card).

Memory in an auxiliary-slot card is accessed using bank switching
techniques.   Here is a quick summary of how it works:

A 64k memory expansion card is simplest.  The IIe includes several
soft-switches which allow software to select whether to access the
64k of RAM on the motherboard or the 64k of RAM on the card in the
auxiliary slot.  The area of memory from $0000 to $01FF and $D000 to
$FFFF (zero page, stack and language card) is switched independently
from the rest of the memory ($0200-$BFFF).

With a larger auxiliary-slot RAM card such as RamWorks, there are
multiple 64k banks which can be switched into the auxiliary memory
area, using a bank select soft-switch.

The disadvantage of this type of card is that there is no ROM on it,
and you need special software to access its memory (whether as a RAM
disk or as additional memory for use by a program).  Accessing the
various banks is also rather clumsy.  A major advantage is that you
can execute code directly from the memory card (but it usually cannot
access the video buffers or call the operating system).


A standard-slot RAM card uses a completely different technique.  The
memory on the card is accessed via a single-byte "window" which can be
set to any location on the card (using a three-byte address pointer
register and a one-byte data register).  Whenever the data register is
accessed, the address pointer is automatically incremented, so it is a
simple task to read or write a consecutive block of memory.

These cards have a ROM which includes code to make the card act as a
RAM disk for ProDOS, DOS 3.3 or Pascal 1.3.  The RAM disk is
automatically recognised by ProDOS or Pascal, and a simple IN#n
command from DOS 3.3 will make the card available (by patching itself
into the RWTS routine).

If you format the RAM disk (ProDOS or Pascal only) and copy appropriate
software onto it, you can then boot from it (this is not possible with
an auxilary slot memory card, even if it has RAM disk software).

No special software is required to use the card as a RAM disk.  To use
it as memory expansion does require special software.

The main disadvantage of this type of RAM card is that you cannot have
code execute directly from the card - it has to be copied into main
memory first.

Also note that if you have one of these cards in a IIe, you also
require a 64k "extended 80-column card" if you want to be able to run
software that requires a 128k IIe.  Standard-slot RAM cards do not
provide the auxiliary 64k RAM.
-- 
David Empson                                                               
dempson@swell.actrix.gen.nz                                                
Snail mail: P.O. Box 27-103, Wellington, New Zealand