Re: Apple and the Holy War

[regnirps@aol.com (Regnirps)]

pausch@saafNOSPAM.se  (Paul Schlyter) wrote:

>Also, you should add in the cycles to do the back switching.  Yes,
>you are here talking about a ROM memory space of 128K, on a CPU which
>has an address space of only 64K.

Ah, but there is no bank switching : ) An available Zpage address is decoded by
the (very simple) logic on a board in a slot. The byte is written to a latch
which isolates the ROM address lines from the system memory. The data is read
from a buffer that isolates the ROM data lines from the system memory. The same
address is used for read and write. In this way you can stuff huge amounts of
data into ROMs and get to it without bank switching. So I guess the 8 bit
multiply with 16 bit result took 12 cycles.

I used this to solve a lot of problems that could be mapped to a lookup table.
Before PALs came along we used ROMs to do complicated logic state machines. In
fact I think there was a small one in the IWM to get instant lookup of
something or stepper motor states or something like that. It is still very
handy. Input = address lines, output = data lines, states of lines = data at
the input address.

Charlie Springer