MIPS assembly assignment - issue with a load word

Question

The following code used to implement the instruction

sllv $s0, $s1, $s2

which uses the least significant 5 bits of the value in register $s2 to specify the amount register $s1 should be shifted left:

          .data
   mask:  .word  0xfffff83f
          .text
  start:  lw     $t0, mask
          lw     $s0, shifter
          and    $s0,$s0,$t0
          andi   $s2,$s2,0x1f
          sll    $s2,$s2,6
          or     $s0,$s0,$s2
          sw     $s0, shifter
shifter:  sll    $s0,$s1,0

I know what most of those instructions are doing.

I don't however understand how the second load word is loading something from shifter which itself is an instruction and not a word.

Also the value of the mask in hex when converted to binary doesn't have zeroes in the least 5 significant places as the question says so I am not sure how it will mask the least 5 sig places.

you need to tell us what architecture and what processor this is for. Without that we can't tell you what it does. — Spence
You'll likely get more help if you make your question more specific. Is there a particular instruction you're having trouble understanding? — Charlie Salts
Are you familiar with any of those instructions? A good faith attempt at translating as much as you can will go a long way — Drew Dormann
its for MIPS architecture en.wikipedia.org/wiki/MIPS_architecture — anon
I know what most of those instructions are doing. I don't however understand how the second load word is loading something from shifter which itself is an instruction and not a word. Also the value of the mask in hex when converted to binary doesnt have zeroes in the least 5 significant places as the question says so I am not sure how it will mask the least 5 sig places — anon

sybreon sybreon · Accepted Answer · 2009-06-02T04:04:16

That's kind of a round-about way of doing it. It's actually modifying the instruction in-memory to perform the shift! If you follow the code, you will see that the sll $s0,$s1,0 instruction is loaded, has its sa field modified from 0 to $s2 and then saved back into memory and executed.

MIPS assembly assignment - issue with a load word

1 Answers