19 Jun
2007
19 Jun
'07
12:50 p.m.
What is a "bit-slice" CPU? How does it
differ from, say, the 8086?
A bit slice CPU is noe made from bit slice chips. OK, that's no help at
all. It's probalby easiest to explain using a concrete example, the
AM2900 series (which are, AFAIK the most populer bit-slice chips).
The AM2901 is a 4 bit ALU + regitster 'slice'. It contains 16 4 bit
registers, a 4 bit ALU, and the clrcuitry to link them together. It's the
data path for a 4 bit CPU.
What makes it 'different' is that you can link several togther. 2 will
give you an 8 bit data path. 4 would give you a 16 bit data path. 8 would
give you a 32 bit data path, and so on. For example, the VAX11/730 has 8
2901s on one of the CPU cards for this reason.
Of course the data path is not the complete processor, you need the
control logic as well. There were microcode sequencer chips like the 2909
and 2911, which producde 4 bits of microcode address, and which could
easily be cascaerd to make as wide a microcode address as you wanted, but
which needed some external logic, and the 2910 which was a 12 bit-wide
adress seqencer, incldued the 'extra' logic, but which couldn't easily be
exteded (cue for the PERQ 2-bit-kludge story, I guess :-))
There;'s a useful book 'Bit Slice Microprocessor Design' by Mick and
Brick (that is not a joke!), whcih I am told is one the web soemwhere.
And of course the AM2900 series data sheets.
-tony