[brucehoult]

> The historical reason for the ordering is because that’s how it was on the PDP-11

True, I think. I mean, that certainly was the case for PDP-11 and VAX asm. 68000 too (pretty much a 32 bit, 16 register PDP-11). Whether that was the actual reason is more debatable.

> And all AT&T/GNU versions preserve this ordering regardless of the vendor format.

False. GNU `as` puts the destination register first for all of Arm32, Arm64, MIPS, PowerPC, RISC-V. Every RISC ISA, as far as I know. Except for store instructions, where the source register is first.

[epcoa]

Yes absolutely right about the modern RISC archs. I should have just said AT&T not AT&T/GNU. The difference is that x86, VAX and SPARC were AT&T UNIX porting efforts prior to or alongside GNU, and GNU sort of chose to inherit that. The newer RISC architectures did not have the hand/influence of AT&T as far as I know.

As far as the operand ordering with the x86, the thing is they did flip it from the Intel syntax and the beginnings of this port date back to Version 7 (1978), a few years before GNU. So what was the reason other than it looked like the incumbent/precedent PDP-11? I haven't heard it being much more than circumstance.