Inside old games machines, was: Re: Simulated CP/M-68K?
dkelvey at hotmail.com
Tue Jun 19 09:29:07 CDT 2007
>From: Jim Leonard <trixter at oldskool.org>
>Ethan Dicks wrote:
>>On 6/18/07, Ensor <classiccmp at memory-alpha.org.uk> 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
>What is a "bit-slice" CPU? How does it differ from, say, the 8086?
Bit-slice refers to the fact that the ALU is designed to be stacked for
whatever word size you'd like. Most come with chips that are 1, 2,4 or 16
bits wide. Say you wanted a 20 bit computer, 5 ea of a 4 bit wide would
Also, these are truly microcoded. These usually have there own microcode
ROMs and can have any number of bits wide to run them. Often when used
for hardware control ( such as disk drive controllers ) many of the bits
in the micro code directly control I/O functions so that the processor part
could be doing something with data at the same time the I/O was doing
The ROM was usually controlled by a chip called an address sequencer. This
part usually handled things like conditional jumps, subroutine calls and
The AMD 2900 series was relative straight forward and seemed like an address
sequencer used in any number of todays uP's. The 3000 series sequencer
by Intel was a strange beast. It had all kinds of row and column jumps.
instruction had bits that defned where the next instruction was. It was
quite complicated to program and most likely why it was not that popular.
I know that there is someplace a design that used 2900 parts to emulate
a 8080 processor. Remember, the micro code was a separate structure
from the instruction code in the actual 8080 memory.
Bit-slice machines were the way to go for speed until more recent times.
Now, the time lost in driving external I/O lines between chips means that
uP's can actually run much faster.
Still, for hardware control, knowing when an instruction executes relative
to external hardware can be important. This means that there are still
that bit-slice methods are still needed. Today it is often handled with a
Although, it looked like one could build most any computer model with
these bit-slice chips, most were register oriented and did poorly at
other computer models, such as stack processors.
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