On: How Many Computers Are in Your Computer?

[amelius]

... and how many are controlled by you?

[saagarjha → amelius]

None, probably, if you’re taking the definition of “open hardware” where you have full access to the microcode running on it and can modify it yourself.

[pkaye → saagarjha]

What if it has no microcode running?

[shawnz → pkaye]

Microcode is necessary to decode instructions. When you give the CPU instructions, it decodes them into microcode which is what actually specifies the sequence of electrical operations required to process that instruction.

[wnoise → shawnz]

CPUs without microcode exist. While microcode isn't exactly a new innovation, early mass-market CPUs were not implemented that way.

[shawnz → wnoise]

Right, but it wouldn't be possible for a modern CPU (for example: how would you retain compatibility with older architectures? how would you patch it if you find a critical bug in an instruction?)

Furthermore the parent was asking about having no microcode running on the CPU which is what I was speaking to. A CPU designed to use microcode can't be run without microcode. But yes, it is of course possible to design the CPU to not use microcode in the first place.

[pjmlp → wnoise]

Microcode free CPUs was mostly a RISC thing, and basic CPUs like Z80 and 6502, almost everyone else always had some form of microcode in them.

[shakna → pjmlp]

> CPUs without microcode exist. While microcode isn't exactly a new innovation, early mass-market CPUs were not implemented that way.

> Microcode free CPUs was mostly a RISC thing, and basic CPUs like Z80 and 6502

Those quotes aren't incompatible - the 6502 and Z80 are core to the early mass-market computer movement.

So whilst microcode-free may be rare now, there was a time when it was the norm.

[pjmlp → shakna]

The norm in the context of 8 bit home micros.