> > Is there an Intersil 6100 chip on there somewhere? That would indicate it's
> > a PDP-8 clone. They should rather interesting!
>
> I do not think it had one, but they were awfully hard to look at
> internally. They also had an early 1970s look about them.
>
> Could a user fool around with the AC and PC directly on an Intersil part
> (as with a switch register)?
>
> William Donzelli
> aw288_at_osfn.org
Here is what I found on the net a little bit ago at this webpage:
http://www3.sk.sympatico.ca/jbayko/cpu2.html
Zane
Part VI: Intersil 6100, old design in a new package . . .
The IMS 6100 was a single chip design of the PDP-8 minicomputer (1965) from
DEC (low cost successor to the PDP-5 (1963)). The old PDP-8 design was very
strange, and if it hadn't been so popular, an awkward CPU like the 6100
would have never had a reason to exist.
The 6100 was a 12 bit processor, which had exactly three registers - the PC,
AC (an accumulator), and MQ. All 2 operand instructions read AC and MQ, and
wrote back to AC. It had a 12 bit address bus, limiting RAM to only 4K.
Memory references were 7 bit (128 word) offset either from address 0, or the
PC.
It had no stack. Subroutines stored the PC in the first word of the
subroutine code itself, so recursion wasn't possible without fancy
programming.
4K RAM was pretty much hopeless for general purpose use. The 6102 support
chip (included on chip in the 6120) added 3 address lines, expanding memory
to 32K the same way that the PDP-8/E expanded the PDP-8. Two registers, IFR
and DFR, held the page for instructions and data respectively (IFR always
used until a data address was detected). At the top of the 4K page, the PC
wrapped back to 0, so the last instruction on a page had to load a new value
into the IFR if execution was to continue.
The PDP-8 itself was succeeded by the PDP-11 (though a version called the
PDP-12 was produced, it was part of the PDP-8 series, not a replacement).
The IMS 6120 was used in the DECmate (1980), DEC's original competition for
the IBM PC, but lacked the processor and RAM capacity (a Z-80 or 8086 card
could be added (reducing the 6120 to an I/O coprocessor) but lacked IBM PC
compatability). DEC also tried competing with the 8086 based Rainbow, and
the PDP-11 based PRO-325 personal computers, but none caught on.
Intersil was eventually bought by Harris Semiconductors, which produces
versions of the 8088 and 8086, 1802, and 68HC05.
Received on Fri Jun 21 2002 - 23:01:47 BST