Question

The C language permits a variable to be left- or right-shifted by a nonconstant amount (e.g. i >> j where i and j are variables), but the MIPS instruction set only supports shifts by a constant value (e.g. i >> 2). In words rather than code, describe how a variable-length right shift could be performed using MIPS instructions.

Answer 1

The MIPS instructions support the shifts operation by a constant value only. In C language the right shift is indicated by the symbol >>. In C, a variable can be left- or right-shifted by a variable value. In MIPS, a variable can be left- or right-shifted by a constant value. Example: i>>2. Here, the bits of the value i is right shifted by 2 positions.

• In MIPS, a variable-length right shift operation can be done by using the bit variable-length shift registers. The length of the variable-length shift registers can be varied between the bits 0 to 64.
• The variable-length right shift operation done by the instruction srlv (Shift word right logical variable).
• The instruction srlv uses the variable length registers. This instruction shifts the value of a register to the right by the amount specified in another variable length register.

The syntax of this instruction is given below:

srlv $d, $t, $s

This instruction shifts the value in register $t to the right with the number of bits specified in the register $s and stores the result in register $d.

Consider the example, the following instruction shifts the value $t2 with 2 bits to the right, and places the result in $t3:

li $t1, 2               #load register $t1with 2

li $t2, 9422          # load register $t1with 9422    

                                   #$t2 = 0000 0000 0000 0000 0010 0100 1100 1110(in binary)

srlv $t3, $t2, $t1   # shifts the value $t2 with 2 bits to the right, and places the result in $t3

                                  #$t3 = 0000 0000 0000 0000 0000 1001 0011 0011