MIPS floating-point: swc1 vs. s.s
I'm doing some work involving MIPS assembly, and I keep coming across these four floating-point load/store pseudoinstructions: l.s, l.d, s.s, s.d. I found some documentation online and figured out that there are four "actual" instructions that seem to do the same thing: lwc1, ldc1, swc1, and sdc1.
My only question is, what's the difference? As far as I can tell, both sets of instructions do exactly the same thing. Do the pseudos maybe exist just because they're easier to read?
Thanks in advance for any insight.
Answer
My only question is, what's the difference? As far as I can tell, both sets of instructions do exactly the same thing.
Yes, you're right. The only difference that could appear is when a pseudo-instruction is translated to more than one "real" instruction.
Do the pseudos maybe exist just because they're easier to read?
Again, yes. That's why they exist. They give the illusion of a more expressive instruction set. Quoting Computer organization and design / Patterson & Hennessy:
... the assembler can also treat common variations of machine language instructions as if they were instructions in their own right. The hardware need not implement these instructions; however, their appearance in aassembly language simplifies translation and programming. ...
Given your example, it's more "clear" to say:
l.s $f2, 24(t1) # Load Single contained in 24(t1) to $f2
than
lwc1 $f2, 24(t1) # Load Word into Coprocessor 1 from 24(t1) to $f2
as well as you can understand better:
move $7, $18 # move contents of $18 to $7
than
add $7, $18, $0
For me, it's just be helped by mnemonics to get better legible code.