<programming> (IMS 6100) A single chip design of the DEC
PDP-8 minicomputer. The old PDP-8 design was very
strange, and if it hadn't been popular, an awkward CPU like
the 6100 would never been designed.
The 6100 was a 12-bit processor, which had three registers:
the PC, AC (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, offset either from address 0, or from
the PC page base address (PC AND 7600 oct).
It had no stack. Subroutines stored the PC in the first
word of the subroutine code itself, so recursion required
fancy programming.
4K RAM was pretty much hopeless for general purpose use. The
6102 support chip (included 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 was 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.
