Softcard dip switches

["Joel" <joelbuckley54.nospam.@hotmail.com>]

"Christian Groessler" > > The 4-position DIP switch is a giveaway--it's a
Microsoft SoftCard
> > clone.  > What are the dip swiches used for?

According to my softcard schematic:

s1 address offset disable
s2 Z80 DMA
s3 Z80 NMI
s4 Z80 IRQ

All are normally off.

s1 remaps Apple's bottom 4k memory to upper 4k space and i/o right below it.
This gives maximum contiguous RAM for CP/M and avoids conflict with Apple
text video memory and 6502 zero page, stack and other system memory areas.

s3+s4 send bus interrupts to the Z80 (if you we're going to have the
softcard to interrupt handling--unlikely)

s2 controls DMA. This is used by a few hard disk controllers, but I don't
think this is ever used when the Z80 is active.

Theoretically, if you didn't care about the 6502 (which is normally used to
handle all i/o), you could rewrite the BIOS to have the Z80 do all i/o
(which is normally handled by calls to 6502 routines), you could gain an
extra 4k of ram. You'd have to use an external terminal, because Apple's
video memory would be over-written by CP/M's transient program area.
I doubt anybody has ever done this.

The Softcard was toggled on or off by a read of its slot memory (not i/o)
space--i.e.$C4xx for slot 4.

The Softcard was a clever and efficien design to have the Apple double as a
CP/M computer.
It was one of Microsoft's few early ventures into hardware design. But it
served to expand their market for their CP/M based products (mainly computer
languages at that time).