from
The Free On-line Dictionary of Computing (8 July 2008)
Intersil 6100
IMS 6100
<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.
(2003-04-04)