6 Jan
2019
6 Jan
'19
11:24 a.m.
On 01/06/2019 01:19 PM, Noel Chiappa via cctalk wrote:
As far as
what other systems supported especially the 7094 and GE, that
is already out
of context as the focus was a Russian PDP-8 clone.? Any other machines
are then thread
contamination or worse.
In the early days a byte was the smallest recognized group of bits for
that system
and in some case its 9 bits, 6bits as they were even divisible segments
of the machine
word.? This feature was the bane of programmers as everyone had a
different idea
of what it was and it was poison to portability.
For PDP-8 and friends it was 6 bits and was basically a halfword, also
used as stated for
6bit subset of ASCII (uppercase, TTY codes).? Most of the 8 series had
the bit mapped
instructions (DEC called the microcoded) for doing BSW, byte swap,? swap
the
lower half of the ACC with the upper half.? Very handy for doing
character IO.
The PDP-8 and 12bit relations the instruction word and basic
architecture was 12bit word.
There were no instructions that were a half word in length or other
fragmentations.? The
machine was fairly simple and all the speculated concepts were well
outside the design
of the PDP-5/8 family.?? For all of those the instruction fetch, memory
reads and write
were always words of 12bits.?? I'd expect a Russian PDP-8 clone to be
the same.?? After
all DEC did widely gave out the data books with nearly everything but
schematics.? The
value of copying is software is also copied.? It happened here with the
DCC-112 a
PDP-8e functional clone.
While its possible to use half word ram with reconstruction the hardware
cost is high
(registers to store the pieces) and it would take more to do that than
whole 12bit words.
Any time you look at old machine especially pre-IC registers were costly
and only done
as necessity dictated as a single bit flipflop was likely 4 transistors
(plus diodes and other
components) or more to implement never minding gating.?
Minor history and thread relative drift...?
The only reason people didn't build their own PDP-8 in the early 70s was
CORE.? It was
the one part a early personal computer (meaning personally owned then)?
was difficulty
to duplicate and expensive outright buy.? Trying to make "random" core
planes that
were available work was very difficult due to lack of data, critical
timing, and the
often minimal bench (and costly) test equipment. ? The minimum gear for
seeing
the timing was a Tek-516 and that was $1169(1969$). ? Semiconductor ram was
either a few bits (4x4) or 1101 (three voltage 256x1) at about 8$ in
1972 dollars.? That
made the parts for a 256x12 a weeks pay at that time (pre-8008) and a
4Kx12 with parts
was nearly that of a new truck (2100$)!. ? Compared the basic logic of
the 8e (only
three boards of SSI TTL) core/ram was the show stopper.? About 7 years
later a 8K8
S100 ram was about? (early 1979) 100$, by 1980 64kx8 was 100$.?? Moore's
law was
being felt.
The small beauty of being there...?? FYI back then (1972) a 7400 was
about 25 cents
and 7483 adder was maybe $1.25.? Least that's what I paid.
Allison
From: Grant
Taylor
Is "byte" the correct term for 6-bits?
I thought a "byte" had always
been 8-bits.
I don't claim wide familiary with architectural jargon from the early days,
but the PDP-10 at least (I don't know about other prominent 36-bit machines
such as the IBM 7094/etc, and the GE 635/645) supported 'bytes' of any size,
with 'byte pointers' used in a couple of instructions which could extract and
deposit 'bytes' from a word; the pointers specified the starting bit, and the
width of the 'byte'. These were used for both SIXBIT (an early character
encoding), and ASCII (7-bit bytes, 5 per word, with one bit left over). I would have
blindly substituted "word" in place of "byte" except for
the fact that you subsequently say "12-bit words". I don't know if
"words" is parallel on purpose, as in representing a quantity of two
6-bit word.
I think 'word' was usually used to describe the instruction size (although
some machines also supported 'half-word' instructions), and also the
machine's 'ordinary' length - e.g. for the accumulator(s), the quantum of
data transfer to/from memory, etc. Not necessarily memory addresses, mind -
on the PDP-10, those were 18 bits (i.e. half-word) - although the smallest
thing _named_ by a memory addresses was usually a word.
Noel