On 01/06/2019 01:19 PM, Noel Chiappa via cctalk wrote: > > 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). 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. > > 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 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
