ewx | amd64

You're viewing

ewx's journal
Create a Dreamwidth Account Learn More

Reload page in style: site light

I’ve not written a nontrivial amount of assembler in over 15 years, but have over the past few days started doing some for a future version of my Mandelbrot set program.

A few random thoughts:

I did most of my assembler programming on the 68K, so the AT&T syntax preferred by gas (i.e. op src,dest) seems natural to me. But the AMD documentation uses the Intel syntax and turning everything back to front is mindbending, so I quickly gave up and switched to that.
Fortunately there’s a .intel_syntax directive, and I did some Z80 long ago, so the Intel order isn’t wholely alien.
The MUL instruction’s a bit lame, isn’t it? The 68K could multiply any pair of registers (or in fact memory, for the source) and store the result in any register. Intel/AMD’s MUL fixes the destination and one of the inputs.
I was disappointed to find that there doesn’t seem to be a high precision integer multiply (128*128->256 would be just the ticket) in the SSEn instructions.

Flat | Top-Level Comments Only

From:

simont

Operand order: I wavered between 68k and x86 in my childhood, so I've used both src,dest and dest,src order in assembly languages. Mostly I can get used to either reasonably quickly (or at least I could in the day – I might have more trouble now I've been writing extensive ARM assembler for twelve years solid), but one place where I really fell down is the compare instruction.

In x86 or ARM, with dest,src order, it's obvious how the compare instruction works with the subsequent conditional branch. If you CMP x,y and then branch-if-greater, it's clear that the 'greater' gets mentally inserted between the two compare operands, so you're branching if x > y. But on 68k, they flip the compare instruction's operands but don't flip the names of the branch conditions, so you always have to remember that comparing x,y followed by branch-if-greater means that you're branching if x is less than y.

Of course it makes sense if you're thinking of CMP as a trial subtraction, and in really complicated cases where you're abusing the condition codes to do fun things, that's the only way you can think of it. But for normal workaday code that isn't doing anything exciting, you really don't want to put your brain into that mode every time; you just want to say to yourself "now check if x > y and branch somewhere else if so", and then you want to translate that thought into a CMP and conditional branch in a basically trivial and mechanical manner, and mentally inverting the condition every time I did that was something I never got used to on 68k.