[monocasa]

> The fact that instructions have some 'octal' structure doesn't make the thing an 'octal machine'

Basic concepts like the 8 GPRs are rooted in it's octal decoding roots. MOD/RM is still octal decoded, SIB is still octal decoded, etc. These fields aren't just three bits long, but also aligned to a three bit boundary within the byte being decoded.

> and as importantly, a Datapoint 2200 is not an x86. The x86 is not an octal machine.

The x86 traces its lineage to that and the points still hit. For instance, even when they added more registers in x86_64, it's still a three bit bank with simply a new prefix to select whether it's referring to the top or bottom 8 register bank out of now 16 total registers. There's some awkward places where you can't address different 8 register banks in the way you'd want to from an encoding perspective because of these continued restrictions going back to the Datapoint 2200.

Having written the HDL for a simple x86_64 decoder, it is very much still an octal machine.

[pvg]

They are still bits, not octdigits or whatever. When 4 bits are used, nobody calls these 'hex machines'. We can definitely spend a lot of time pedanti-digging into the details but at the end of the day, it's just an early example of 'viral title'.

[monocasa]

> They are still bits, not octdigits or whatever.

Once again, it's not just that they're just groups of three bits, but the fields are also three bit aligned.

> When 4 bits are used, nobody calls these 'hex machines'.

I mean, most systems aren't aligned nearly as well on clear repeated boundaries the same way. The only other one that I can think of (the SH series) I for one have absolutely called a hex machine because you can read most of the machine instructions directly from the 4-bit nybbles. A four bit opcode and three address RISC instructions out of 16 GPRs means you can read the hex just about as easily as ASM.

The fact that most other machines correctly take a more bit level almost huffman coding route doesn't make x86 any more less octal derived at it's base.

[anyfoo]

On a machine where opcodes were easily decodeable by just looking at their individual nybbles, calling the machine a “hex machine” would be relatively natural. Obviously the term is to be seen in context, and not (necessarily) the defining characteristic of the machine.