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My first CPU, designed and built as a lab project in 1971, used 16-bit
registers and memory words, but the data paths were 1-bit serial. The memory was implemented with a ~9ms acoustic delay line, in which 8192 recirculating bits were stored. One of the neat aspects of this design is that one could continuously view all of memory with a single scope probe! It was unique in its use of a then-newfangled touchtone keypad for entering hexadecimal values into registers, replacing the individual toggle switches customarily used for that purpose. To prove its completion, we programmed it to perform BCD division; after literally seconds of flashing the lights attached to its registers, it halted with the correct result ablaze and the TA intoned "it lives". It was a defining moment... 73, Dave, AA6YQ wrote in message ... "Geoffrey G. Rochat" writes: ....[snip].... You can do an awful lot on a computer with only 8 instructions that can directly address a mere 256 12-bit words at a time - if you're willing to think a little bit. PDP-8s are to computers what regens are to radios: ....[snip].... You can also do an awful lot on a computer with only 8 instructions that can directly address only THIRTY-TWO NINE-bit words at a time: In 1969, while at Fairchild R&D Lab in Palo Alto, CA, I designed and built a 9-bit PDP-8 imitation (I called it "MINUS", since it was smaller than a mini-computer; if I had called it "MICRO", I might now be rich!) with 512 9-bit words of 200 nsec memory. Its instruction format used a: 3 bit opcode, 1 bit current page/page zero indicator, 1 bit indirect indicator, and a 4 bit address I also wrote a cross-assembler (in FORTRAN) for it, interfaced it to a 20Kbyte/second magnetic tape and a 3-foot x 5-foot flat bed plotter, and wrote a program (in MINUSASM) which, in a tight loop, read mag-tape printed-circuit wirelists produced on an IBM 360/44, buffered them in the upper half of memory, and then passed them to the plotter to draw large PC boards. Just before quiting time, we'd load a new mag tape, and 5-6 hours later another board had been drawn in three colors (horizontal, vertical, and vias). Ah, those were the heady days of youth! --Myron A. Calhoun. -- Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge PhD EE (retired). "Barbershop" tenor. CDL(PTX). W0PBV. (785) 539-4448 NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol) |