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

[Dave McGuire]

On Jun 18, 2007, at 11:39 PM, Jim Leonard wrote:
>>>   > Surely they didn't use the 6809 for the 3D calculations, did  
>>> they?
>>>
>>> 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  
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