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|Manufacturer ||DEC - Digital Equipment Corporation
|Date of first manufacture||1969
|Number produced ||-
|Estimated price or cost||-
|location in museum ||-
Contents of this page:
- The processor's instruction sets are PDP-8 and LINC. LINC stood for
"Laboratory Instrumentation Computer." That's why your 12, like most of
them, has nifty things like analog inputs and a point-addressable scope.
- The PDP-8 native mathematics is two's complement, however, the machine
actually does have some limited and cumbersome support for one's
complement. This was primarily for some form of data interchange
capability with the PDP-4/7/9/15, which were primarily one's complement.
- Neither the LINC or the PDP-8 actually HAS a concept of endianness.
Neither processor is big-endian, neither processor is little-endian.
Neither processor directly supports any data size bigger than its
wordlength, or smaller than its wordlength. If a routine uses a 24-bit
number, it's up to the programmer to decide whether MSW ordering or LSW
ordering gets used. This one is primarily a source of confusion because on
a DEC LINC machine, the individual bits were numbered with 0 as the most
significant bit, but on most other LINC machines, 0 was the least
significant bit. They're still LSB rightmost, MSB leftmost on the displays
and switches, only the numbers assigned to them differ. Since the PDP-8
doesn't actually include an instruction to, say, test bit 6, the
"endianness" difference was irrelevant.
- The instruction formats do differ significantly. Not surprising-- they
were completely different sets. :)
- With regards to the direction of writing on tape, this was actually not
something the PDP-12 can "choose" without modifying the hardware
significantly. A PDP-12 is always designed to read LINCtape. Later
versions of the PDP-8 OSes understood this and "fudged" it in software to
allow DECtape compatibility (the only real difference was writing
direction), but the PDP-12 can never read DECtape without the software
understanding the 12. The modification is one that should only be
performed on a 12 that will never run in LINC mode, by the way. The two
modifications I've seen were, always DECtape-mode, and mode selected by
CPU mode. The latter confuses the hell out of mixed-mode software which
jumps back and forth between 8 and LINC modes.
- Indexing: The PDP-8 and the LINC both have certain memory locations
which autoincrement or autodecrement on access. The 8 and LINC go opposite
directions here. This was, actually, a popular trick for mixed-mode 12
Mike Powell says
| - not direct quote - processor handles two instruction sets
which have different
arithmetic, one's complement vs. two's complement
endin ? big vs. little
normal direction of writing on tape
- something from Lincoln Labs
and everything else except word length
- The PDP-12 can use either NEGIBUS or OMNIBUS peripherals. (Not 100%
sure, actually, if it was NEGIBUS or POSIBUS, but OMNIBUS was allowed with
an attachment cabinet.)
- The PDP-12 has audio capabilities.
- The PDP-12 almost always had A/D converters.
- Despite two instruction modes, the instruction engine of the processor
was composed of under 700 logic gates.
- The PDP-12 is identical to an 8/I with the LINC processor added, with
the exception of B through D, and F. And, if I remember my 8/I specs
correctly, the 8/I only allowed one bus. :)
- The PDP-12, like most of the 8 family, retains not only its memory
contents, but all CPU registers, when powered off.
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Updated February 8, 2004