Inside old games machines, was: Re: Simulated CP/M-68K?
mcguire at neurotica.com
Tue Jun 19 01:11:31 CDT 2007
On Jun 18, 2007, at 11:39 PM, Jim Leonard wrote:
>>> > Surely they didn't use the 6809 for the 3D calculations, did
>>> No, the 3D calculations were done by the so called "math box".
>>> This is
>>> essentially a math co-processor made up of several (4?) AMD bit-
>>> slice CPUs.
>> Yes, four.
> What is a "bit-slice" CPU? How does it differ from, say, the 8086?
"Bit slice" chips are building blocks that can be used to
construct a processor. They typically contain things like an ALU and
stuff to help with the decoding of microcode. The neat thing about
them is that they can be cascaded to make a processor of any data bus
width that you want. The ubiquitous AMD Am2901, for example, is four
bits wide. Many 3rd-party disk controllers for DEC machines used two
or four of them to implement an 8- or 16-bit processor to handle disk
They've also been used for much more ambitious things: The
VAX-11/730 and DECsystem-2020, for example, are both built around
Am2901 bit-slice chips, running microcode from fast RAM which
implements the VAX and PDP-10 instruction sets, respectively. That
microcode is loaded at boot time, from TU58 in the case of the
VAX-11/730, or from a TU45 9-track magtape on the DECsystem-2020.
Before that microcode is loaded, the machines aren't really VAXen or
PDP-10s...as they have no idea how to implement the architecture or
execute the instructions, until the microcode is loaded. With a good
bit-slice chip like an Am2901, one can build a processor to implement
any architecture one wants.
Many, many machines were built in this way...I only use the
VAX-11/730 and DECsystem-2020 as examples because I have them sitting
here. Further, the Am2901 isn't the only bit-slice CPU
around...there were many others. Intel made at least one, I think it
was the 3002.
Port Charlotte, FL
More information about the cctech