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I remember using this chip in college in a "build a CPU from scratch" course. Most teams implemented these elaborate instruction decoding methods, mapping 000

by payne92 10y ago

I remember using this chip in college in a "build a CPU from scratch" course. Most teams implemented these elaborate instruction decoding methods, mapping 000 -> addition, 001 -> and, etc. We just jammed 4 bits from our opcode right into the ALU select, so we ended up with all of the weird operations this writeup documents. Worked great; our CPU was almost "too simple". Woz would have approved, I think.

jacquesm 10y ago
> We just jammed 4 bits from our opcode right into the ALU select That's how it is meant to be used, so props to you for picking the right (and shortest) path.
- kens 10y ago
  It's one way to use the 74181, but there are others. For instance the Xerox Alto uses a PROM to map opcode bits to the 16 most useful 74181 operations, calling the others "relatively useless".
  - jacquesm 10y ago
    Space/speed trade-off.
zeta0134 10y ago
I built a working computer in Minecraft, and ended up going with exactly that because the decoding circuits would have been too complex for the project otherwise: https://www.youtube.com/watch?v=s9XT-dyfEiU https://www.youtube.com/watch?v=s9XT-dyfEiU This build could be improved in a lot of ways as I've grown as a developer and engineer in the years since I built it, but back then, "Jam the opcode bits into the ALU" was the simplest thing I could think of at the time, and worked quite well. If I ever tackle something like this again (a year's worth of Redstone is... I'm not sure I have the patience for that anymore) I'd like to try an 8-bit system just for the challenge of packing the opcodes down a bit. The 16-bit opcodes I used here were rather inefficient.
- kbenson 10y ago
  I'm seriously wondering at this point how many college CS assignments meant to be completed in Logic Works (or some equivalent) have instead been implemented in Minecraft. If Minecraft existed when I went through college, I imagine it's one of the things I might have attempted (at least for the simpler assignments). Kudos to any teachers that use Minecraft as a neat visual aid. That would have been awesome.

by payne92 10y ago

jacquesm 10y ago

> We just jammed 4 bits from our opcode right into the ALU select That's how it is meant to be used, so props to you for picking the right (and shortest) path.

kens 10y ago
It's one way to use the 74181, but there are others. For instance the Xerox Alto uses a PROM to map opcode bits to the 16 most useful 74181 operations, calling the others "relatively useless".
- jacquesm 10y ago
  Space/speed trade-off.

zeta0134 10y ago

I built a working computer in Minecraft, and ended up going with exactly that because the decoding circuits would have been too complex for the project otherwise: https://www.youtube.com/watch?v=s9XT-dyfEiU https://www.youtube.com/watch?v=s9XT-dyfEiU This build could be improved in a lot of ways as I've grown as a developer and engineer in the years since I built it, but back then, "Jam the opcode bits into the ALU" was the simplest thing I could think of at the time, and worked quite well. If I ever tackle something like this again (a year's worth of Redstone is... I'm not sure I have the patience for that anymore) I'd like to try an 8-bit system just for the challenge of packing the opcodes down a bit. The 16-bit opcodes I used here were rather inefficient.

kbenson 10y ago
I'm seriously wondering at this point how many college CS assignments meant to be completed in Logic Works (or some equivalent) have instead been implemented in Minecraft. If Minecraft existed when I went through college, I imagine it's one of the things I might have attempted (at least for the simpler assignments). Kudos to any teachers that use Minecraft as a neat visual aid. That would have been awesome.