assembly code help

Question

Can someone please help me with this assembly code? an explanation of what goes in with each step will be great. thanks!

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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.

Answer 1

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.