15 Nov
2007
15 Nov
'07
midnight
I don't know PAL/HAL chips, but if you have reason
to think the circuit
Basically they're the counterpart to a PROM. Both consists of an AND
matrix follwed by an OR matrix. In a PROM, the AND matrix is fixed (it's
called an 'address decoder' :-)), while you program the OR matrix
(determine which addresses contain '1's). In a PAL the OR matrix is fixed
(each output is a given number of terms ORed together), you program the
AND matrix (determine which inputs/their inverses appear in each term).
is combinatorial, it's easy enough to just
exhaustively probe its truth
table, and once you have that, you don't care how it's built up in
terms of gates (except, I suppose, for cases where delay times and
output glitches matter).
I've never seen inside a Mac 512K, but I spent some time poking about
inside a Mac+, and I assume the design is similar. The logic board
cotains a 68000 processor, RAM (SIMMs in a Mac+, I believe individual RAM
chips in a Mac512K), RONs, a small amount of TTL and about 8 or so PALs/HALs.
I am darn sure that most, if not, all of them have sequential functions.
Either by being 'registered PALs' (meaning they have D-type flip-flops on
some or all of the outputs) and/or by using 'feedback terms' (the outputs
of the device can be taken as inputs into the AND matrix.
One thing that makes life a little easier is that there's no 'hidden
state'. There are no internal flip-flops, you can't have a feedback term
from anything other than an output. So if the state of
the device
changes, it is visible on the pins. It doesn't make life _easy_
though.
Alas I can't quickly find the notes I made on the intnerals of the Mac+
-tony