4 ms·

> Of course you're the expert here, but isn't this a little nitpicking? No, a PLA [1] and a microcode ROM are two very different animals. A PLA is a way of co

by pwg 6y ago

> Of course you're the expert here, but isn't this a little nitpicking? No, a PLA [1] and a microcode ROM are two very different animals. A PLA is a way of compactly specifying combinatorial logic circuits (arrangements of and/or/etc. gates) and in the form that gave it the name, it is end user programmable (the P stands for Programmable). But at its heart, a PLA is just a big arrangement of logic gates. A signal goes in, and after the necessary gate delays settle, the result signal comes out. Microcode is literally a "program" and is 'executed' in the same way that all other programs are executed, one instruction at a time, addressed by a program counter. There are also often branching instructions that generate new program counter values to use to sequence through the various steps. None of which exists in a PLA. The appearance of similarity arises because both look similar in a silicon die photo (rows and columns of transistors), but they are very different and perform quite different functions on the chip. [1] https://en.wikipedia.org/wiki/Programmable_logic_array https://en.wikipedia.org/wiki/Programmable_logic_array

monocasa 6y ago
The PLAs on the Z80 and 6502 aren't user programmable, but had their functions fixed in the mask just like regular logic or a mask ROM would be. Talking about the PLAs in these processors versus ucode ROM, the big difference is that in a PLA, typically several lines are enabled at a time. In contrast a ROM will only have one line active at a time with an address decoder sitting in front of it. You absolutely could use a microcode ROM in the place of how these chips use their PLAs, it'd just take more die area. You can see this logic/data equivalence in other places of hardware design too. For instance how FPGAs are a sea of SRAM, but are equivalent to standard cell chips. FPGAs are just slower, larger, and easier to modify.
- pwg 6y ago
  > The PLAs on the Z80 and 6502 aren't user programmable, I never said they were. I said: "and in the form that gave it the name, it is end user programmable (the P stands for Programmable)" which is in the context of generic PLA's, not the specific PLA in a Z80 or 6502.
jepler 6y ago
A consequence of this is the so-called "undocumented" opcodes of the 6502. Extra instructions such as LAX (load A and X from the same value) "work" because the instruction activates two different blobs of logic (the one for LDA plus the one for LDX). From my experience, it's less clear that microcode would function this way when confronted with an illegal opcode.
duskwuff 6y ago
The difference is less clearcut than you're assuming. A PLA and a ROM are both types of lookup tables, after all, and in this application they're both being used to control the next state of a FSM.
Taniwha 6y ago
I suspect that at the time microcode was copyrightable while masks (and therefore PLAs) were not (note the actual (C) message on the die explicitly calling out the microcode)
- pwg 6y ago
  In the context of IC chip layouts, the determining factor is almost always die area. Some functions will be smaller on the die in PLA form, other functions will be smaller on the die in microcode form. Since die area is almost always a scarce resource, the chip designer will usually pick the design that reduces die area consumption for any given function.
  - Taniwha 6y ago
    (puts on chip designer hat) yes and no, as I said upstream the copyright laws of the time did not protect designs, chip copying (stealing) was rampant. I can imagine that some CPU designers from a particular world view (especially in the 70s) might only design in terms of microcode - but also design tools were pretty minimal, I can imagine that it might have been far easier to design things in terms of microcode rather than as PLAs - what you might be seeing here is the differences between different teams' in house custom tooling
  - deleted 6y ago
    [deleted]
- kens 6y ago
  Protection for "mask works" was introduced in the Semiconductor Chip Protection act of 1984. You'll see a circled-M on modern dies indicating this protection. (I got the mask work symbol added to Unicode recently, but font support is lacking: 🆭.) The 8086 is the only chip I've seen with a copyright specifically designating the microcode.
- miohtama 6y ago
  Is 8086 microcode reverse engineered or released today for studying?
a1369209993 6y ago
> No, a PLA [1] and a microcode ROM are two very different animals. Technically a microcode ROM (or any ROM, really) is a (degenerate case of a) PLA. Hence the visual similarities, since that type of grid pattern will be observed with any PLA, whether ROM or general-logic.

by pwg 6y ago

monocasa 6y ago

The PLAs on the Z80 and 6502 aren't user programmable, but had their functions fixed in the mask just like regular logic or a mask ROM would be. Talking about the PLAs in these processors versus ucode ROM, the big difference is that in a PLA, typically several lines are enabled at a time. In contrast a ROM will only have one line active at a time with an address decoder sitting in front of it. You absolutely could use a microcode ROM in the place of how these chips use their PLAs, it'd just take more die area. You can see this logic/data equivalence in other places of hardware design too. For instance how FPGAs are a sea of SRAM, but are equivalent to standard cell chips. FPGAs are just slower, larger, and easier to modify.

pwg 6y ago
> The PLAs on the Z80 and 6502 aren't user programmable, I never said they were. I said: "and in the form that gave it the name, it is end user programmable (the P stands for Programmable)" which is in the context of generic PLA's, not the specific PLA in a Z80 or 6502.

jepler 6y ago

A consequence of this is the so-called "undocumented" opcodes of the 6502. Extra instructions such as LAX (load A and X from the same value) "work" because the instruction activates two different blobs of logic (the one for LDA plus the one for LDX). From my experience, it's less clear that microcode would function this way when confronted with an illegal opcode.

duskwuff 6y ago

The difference is less clearcut than you're assuming. A PLA and a ROM are both types of lookup tables, after all, and in this application they're both being used to control the next state of a FSM.

Taniwha 6y ago

I suspect that at the time microcode was copyrightable while masks (and therefore PLAs) were not (note the actual (C) message on the die explicitly calling out the microcode)

pwg 6y ago
In the context of IC chip layouts, the determining factor is almost always die area. Some functions will be smaller on the die in PLA form, other functions will be smaller on the die in microcode form. Since die area is almost always a scarce resource, the chip designer will usually pick the design that reduces die area consumption for any given function.
- Taniwha 6y ago
  (puts on chip designer hat) yes and no, as I said upstream the copyright laws of the time did not protect designs, chip copying (stealing) was rampant. I can imagine that some CPU designers from a particular world view (especially in the 70s) might only design in terms of microcode - but also design tools were pretty minimal, I can imagine that it might have been far easier to design things in terms of microcode rather than as PLAs - what you might be seeing here is the differences between different teams' in house custom tooling
- deleted 6y ago
  [deleted]
kens 6y ago
Protection for "mask works" was introduced in the Semiconductor Chip Protection act of 1984. You'll see a circled-M on modern dies indicating this protection. (I got the mask work symbol added to Unicode recently, but font support is lacking: 🆭.) The 8086 is the only chip I've seen with a copyright specifically designating the microcode.
miohtama 6y ago
Is 8086 microcode reverse engineered or released today for studying?

a1369209993 6y ago

> No, a PLA [1] and a microcode ROM are two very different animals. Technically a microcode ROM (or any ROM, really) is a (degenerate case of a) PLA. Hence the visual similarities, since that type of grid pattern will be observed with any PLA, whether ROM or general-logic.