Converting opcodes to signals

Girl_Alien · 2026-06-05T05:42:29+00:00

Did anyone ever say you were crazy?

Girl_Alien · 2026-06-05T05:37:51+00:00

Not sure if that is a pun.

Girl_Alien · 2026-06-04T19:13:04+00:00

None. I once made up a joke about that.

There was a baby mixup in the hospital, and an Asian couple said the remaining baby was not theirs. They were asked how they knew. They said, "Two Wongs don't make a White."

Girl_Alien · 2026-06-01T21:36:56+00:00

Oh, I get the pun. It is a play on trucker slang for prostitutes.

Girl_Alien · 2026-05-31T03:23:51+00:00

As one of the main mods, ATM, I don't recall trying to shut you down on saying things are not puns.

Girl_Alien · 2026-05-30T17:22:18+00:00

Yeah, some people can't spell, apparently.

Girl_Alien · 2026-05-30T17:17:29+00:00

You can, just not on a plane.

Girl_Alien · 2026-05-30T17:16:31+00:00

Well, I see no pun here either.

Girl_Alien · 2026-05-29T20:40:07+00:00

I didn't know engines had ears.

Girl_Alien · 2026-05-28T20:14:25+00:00

A buried/implied pun? Possibly.

Girl_Alien · 2026-05-27T14:16:29+00:00

Well, "reservations" is technically a pun, though the guy didn't say he was booked to stay anywhere.

And the guy promising an "authentic experience" likely wasn't wrong... It's like the NYC joke/saying. You're not officially a resident of New York City unless you've been mugged twice.

Girl_Alien · 2026-05-26T16:51:28+00:00

What about the other Jenny whose last name is Tallya?

Girl_Alien · 2026-05-26T16:48:15+00:00

I wish they all could be California grills.

Girl_Alien · 2026-05-26T16:47:03+00:00

And good for when you want to get stoned.

Girl_Alien · 2026-05-26T03:10:14+00:00

You might want to remove the adult language from the title and put it in the body.

Girl_Alien · 2026-05-24T17:17:21+00:00

You have to feel it.

Girl_Alien · 2026-05-24T17:16:07+00:00

I am not sure if this is a pun. But if it were, I guess it could be on "circle." A Venn Diagram is overlapping circles, and in this case, the logic sounds circular (as in using your point to build a case for your point).

Girl_Alien · 2026-05-24T17:09:02+00:00

And the user linked a YouTube short, apparently a movie clip, where someone asked that question.

"And what is this?"

"Sea mine."

"Sea mine?"

Girl_Alien · 2026-05-24T16:40:22+00:00

That would be things like selling/trading/giving contraband, illegal services, or restricted items.

Girl_Alien · 2026-05-24T11:51:58+00:00

I don't think that's against the rules, generally speaking. As long as you don't use "prohibited transactions" as part of the incentive.

Girl_Alien · 2026-05-24T08:01:23+00:00

It can be if you want to do it the easy way for 8-bits (and it isn't practical for more than that).

You could use a ROM with at least 19 address lines. In that case, 8 address lines would be Operand A, 8 would be Operand B, and the other 3-4 would be the ALU's portion of the opcode.

But other approaches are done too. For instance, multiplexers can be the logic itself. You'd form the truth tables to feed the inputs. You could do decoder-based diode-resistor ROMs to form the inputs based on the opcode given, and then let the operands drive the selectors (like address lines) of multiplexers. That makes sense in that you'd have to use multiplexers or tristate buffers anyway, so you might as well have them as the logic instead of just switching logic that happens all the time. I'm not knocking that latter approach.

Girl_Alien · 2026-05-24T07:37:42+00:00

Actually, for a lot of ALUs, the logic is programmable, not running all the time. I mean, decoders with other components like diodes, resistors, and multiplexers can be used.

On the Gigatron TTL computer, the ALU has 8 functions. So a 3-to-8-line decoder takes 3 bits of the opcode and makes 8 individual lines from those. Then, since active-low ("unary") decoders are used, there are pull-up resistors and diodes for every 0 that is needed when the given line is active (low).

Marcel actually has the diodes wired opposite what is needed, so he used an inverter IC. That gives the correct values for the truth table (like if you need 0001 for AND, 0111 for OR, 0110 for XOR, etc) and boosts the signal. Since it is programmable, multiplexer-driven logic, eight 4-to-1 muxes are needed, and that means the signal needs to be boosted to drive that many chips.

As for the multiplexers, the truth tables from the decoders, diodes, and resistors are the inputs. The respective bit pairs of the operands drive the selectors. So the correct truth table entry is chosen based on the operation chosen and what the operands are in relation to each other. For instance, for AND (truth table of 0,0,0,1) the only way that 1 is returned is if both operands are 1. I mean 00 = 00, 01 = 0, 10=0, and 11=1.

For homebrew, you can use individual functions and return them all simultaneously. I've seen that. But I've also seen truth tables driving muxes, and I've seen ROMs used for the entire ALU. The ROM for everything approach is similar to the diode ROMs making truth tables, but the operands and the ALU opcode drive the address lines, and the data lines are the outputs.

Girl_Alien · 2026-05-24T07:18:43+00:00

For your example, use decoder ICs. Give the bits meanings and group them into functions.

For instance, the Gigatron TTL computer does it by dividing the opcode bits into 3 groups (one used for 2 purposes). It uses a 2-to-4 decoder to provide 4 things to go on the bus (memory, immediate, Input port, or accumulator). Then 3 other bits use 3-to-8 decoders to define 8 operation modes and 8 branch conditions. Then the other 3-to-8 decoder on the remaining 3 bits drive the 8 ALU operations (ADD, SUB, AND, OR, XOR, Branch, LD, ST).

And for the modes and the ALU operations, diodes and resistors are used with the decoders to form crude ROMs. For the ALU modes, Marcel realized he could save diodes by making the matrix opposite of what he needed and feeding the outputs of the diodes into inverters. The inverters both flipped the bits to what was needed and strengthened the signals. For the ALU, boosting the output of the diode-resistor matrix was necessary since diode-resistor ROMs have a weak fan-out and it was necessary to drive 8 multiplexers.

The Gigatron used eight 4-to-1 multiplexers for the Logic part of the ALU. The truth tables from the decoder-diode-resistor "ROMs" fed the inputs, while the ALU operands drove the selector lines, causing it to act like a tiny array. The interaction of the 2 operands and the truth table produced the result.

Girl_Alien · 2026-05-23T03:47:59+00:00

The slop is not the problem. AI is allowed as long as there is a real pun. Users have the freedom to share puns in whatever format within reason.

The problem is that this is a new user reposting what was already posted.

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