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A computer built from NOR gates: inside the Apollo Guidance Computer

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Re: A computer built from NOR gates: inside the Apollo Guidance Computer

#21

Post by basementcat »

bctnry wrote: post #19
anon91831837 wrote: post #18

NOR gate logic, like NAND and NOT/AND/OR gate logic, are functionally-complete. Combinational logic examples: - NOT gate -> N-input (N >= 1) NOR with all inputs tied together to the input - N-input OR gate -> N-input NOR gate with a NOT gate at the output - N-input AND gate -> N-input NOR gate with a NOT gate in front of each input - Bonus - sequential logic: S-R latch -> 2x 2-input NOR gates with one each of their i…

should be N >= 2 because you can't really have 1-input NOR gate...?

A 1-input NOR gate is more commonly known as a NOT gate.

Re: A computer built from NOR gates: inside the Apollo Guidance Computer

#22

Post by thewonderidiot »

ncmncm wrote: post #7

I was impressed to learn that when Margaret Hamilton programmed this machine, she wrote an interpreter, and coded the mission logic in the interpreted language. So, in a sense, it really was microcoded; but the microcode was what looks like the real instruction set, and the instructions actually used lived in ROM. The Xerox Alto, which was used for the original Smalltalk in 1972, also worked much this way: It had 16…

The Interpreter was written by Charley Muntz, not Margaret. Margaret wrote a large chunk of the DSKY display routines and a lot of the alarm/abort logic.

Re: A computer built from NOR gates: inside the Apollo Guidance Computer

#23

Post by FullyFunctional »

angel_j wrote: post #17

Why three inputs?

Most certainly a compromise. More inputs amortizes more of the overhead (power pins, input pads etc), but too many wastes density.

A similar issues shows up in FPGA where four-input lookup tables (LUT4) were the sweet-spot for many generations but in the past few years LUT6 (and hybrids like the ALM ~ 2xLUT6) have taken over.

Re: A computer built from NOR gates: inside the Apollo Guidance Computer

#24

Post by dfox »

fspeech wrote: post #20
not2b wrote: post #4

Back in the 1990s, UC Berkeley's computer hardware quals exam used to require the test taker to design a microcoded RISC processor out of NAND gates, from the bottom up. It was kind of like Little Alchemy, once you created every kind of primitive gate you needed, you could assemble them to make larger and larger things. This included building latches, registers, static RAM, bottom to top. NOR is just NAND with 0's an…

NAND or NOR preference is due to the underlying transistor technology. NPN transistors (with electron majority carriers) tend to be stronger than PNP types (with hole majority carriers) at comparable sizes. In the beginning we had mostly bipolar processes, where we used NPN transistors in parallel to pull down the output and pull-ups were simply resistors. NOR gate was the natural configuration efficiency/cost wise.…

Preference for NAND and negative logic (ie. active low) do not stem from early CMOS, but from TTL. TTL NAND is same structure as invertor, only with multiple emitters on the input transistor. And with TTL significant current flows only in the low state, typical TTL gate sees undriven input as logic 1.

By the way for CMOS, NOR is somewhat counter-intuitively still preferable as the two series connected PMOS pull-up transistors can be collapsed into one dual-gate PMOS, which leads to more desirable gate geometry.

This is the reason why CMOS CD4000 is NOR (and without looking it up I don't even know what is the P/N of 4000-series 2-input NAND), while TTL SN7400 is NAND.

On the other hand in modern CMOS processes and particularly in CMOS VLSI design this distinction is mostly moot.

Re: A computer built from NOR gates: inside the Apollo Guidance Computer

#25

Post by ncmncm »

thewonderidiot wrote: post #22
ncmncm wrote: post #7

I was impressed to learn that when Margaret Hamilton programmed this machine, she wrote an interpreter, and coded the mission logic in the interpreted language. So, in a sense, it really was microcoded; but the microcode was what looks like the real instruction set, and the instructions actually used lived in ROM. The Xerox Alto, which was used for the original Smalltalk in 1972, also worked much this way: It had 16…

The Interpreter was written by Charley Muntz, not Margaret. Margaret wrote a large chunk of the DSKY display routines and a lot of the alarm/abort logic.

Thank you for the correction.
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