Inside old games machines, was: Re: Simulated CP/M-68K?

On Jun 18, 2007, at 11:39 PM, Jim Leonard wrote: "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 control operations. 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. -Dave -- Dave McGuire Port Charlotte, FL