Question

1. The MIPS instruction set includes several shift instructions. They include logical-shift-left (sll), logical-shift-right (srl), and arithmetic-shift-right (sra). [2 +2 4 points] a) Why does not MIPS offer an "arithmetic-shift-left (sla)" instruction? b) Write a MIPS code to implement the logical-shift-left (sll) instruction for a machine that did not have this particular instruction? In other words, you have to write the equivalent of the sll instruction using mul, add and other instructions. Try writing the equivalent of sll Ss1, Ss2, n where n is any positive integer. [Note: Again, here also you do not worry about declaration of variables or directives. Just work on the translation part]

Answer 1

(a)Arithmetic left shift is same as Logical left shift
A Left Arithmetic Shift of one position moves each bit to the left by one. The vacant least significant bit (LSB) is filled with zero and the most significant bit (MSB) is discarded. It is identical to Left Logical Shift. Therefore ,there is not any Arithmetic left shift.

(b)sll $s1 , $s2 ,n //$s1 = $s2<<n

let $t0<-n
for (; $t0 > 0; $t0--)
{
$s2 = $s2 + $s2; // or $s2 = 2 * $s2}

using instructions in mips

loop:
   beq $t0 $zero EXIT //if $t0==0 branch to EXIT
   add $s2 $s2 $s2 //$s2 = 2 * $s2
   addi $t0 $t0 -1 //$t0 = $t0-1
   j Loop //jump to loop
EXIT:
   add $s1 $s2 $zero //$s1 = $s2<<n