Question

Write MIPS code for each of the following instructions,

Your assembly should implement the C code directly – i.e.,do not ’optimize’ the C code to change the order of operations or reduce computations.

Use commands only like add, sub, lw, sw, immediate

Part 1. x = 3-13*x; Do not use multiply. One way of doing the multiply without a multiply instruction is by using many add instructions (x+x+...+x). For this problem, you should do it with fewer additions. Hint: We know how to express any integer as a sum of powers of 2 (e.g.,7 = 1+2+4), and we also know how to multiply by powers of 2 using repeated addition. This gives us a method to multiply by 13 (applying distributivity).

Part 2. a[j-3] = a[2j]+j

Answer 1

Part 1. x = 3-13*x

.data
   msg1:   .asciiz   "Enter x: "
   msg2:   .asciiz   "x= "

   # Start .text segment (program code)
.text
.globl   main
  
main:
   # Print string msg1 "Enter x: "
   li   $v0,4       # print_string syscall code = 4
   la   $a0, msg1   # load the address of msg
   syscall

   # Get input x from user and save
   li   $v0,5       # read_int syscall code = 5
   syscall  
   move   $t0,$v0   # input x is stored in $t0
  
   # Print string msg2 "x= "
   li   $v0,4       # print_string syscall code = 4
   la   $a0, msg2   # load the address of msg
   syscall
  
   li $a0,3
   li $a1,0
  
   # 13 = 8 + 4 + 1
   # 13 = 2^3 + 2^2 + 2^0
   move $t1,$t0
   sll $t1,$t1,0 # x*2^0
   add $a1,$a1,$t1
  
   move $t1,$t0  
   sll $t1,$t1,2 # x*2^2
   add $a1,$a1,$t1
  
   move $t1,$t0
   sll $t1,$t1,3 # x*2^3
   add $a1,$a1,$t1
  
   sub $a0,$a0,$a1 #x=x-13*x
  
   li   $v0,1          
   syscall

   li   $v0,10       # exit
   syscall

#-----------------------------------------

Sample output:

Part 2. a[j-3] = a[2j]+j

.data
   a:   .word 3, 8, 9, 2, 6, 2, 4, 7, 3, 7, 3, 1, 4, 9, 8, 3 #array
   msg1:   .asciiz   "j="
  
   # Start .text segment (program code)
.text
.globl   main
  
main:
   # Print string msg1 "j="
   li   $v0,4       # print_string syscall code = 4
   la   $a0, msg1   # load the address of msg1
   syscall
  
   # Get input j from user and save
   li   $v0,5       # read_int syscall code = 5
   syscall  
   move   $t0,$v0 # $t0 stores j
  
   la $t1, a    #base address of the array a
   sub $t2,$t0,3 # $t2=j-3
   sll $t3,$t0,1 # $t3=2*j
  
   add $t3, $t3, $t3 # double the index
add $t3, $t3, $t3    # double the index again (now 4x)
   add $t4, $t1, $t3 # combine the two components of the address
lw $t5, 0($t4) # get the value from the array cell
  
   add $t5,$t5,$t0 # a[2j]+j
  
   add $t0, $t0, $t0 # double the index
   add $t0, $t0, $t0 # double the index again (now 4x)
   add $t4, $t1, $t2   # combine the two components of the address
   sw $t5, 0($t4) # a[j-3]=a[2j]+j
  
   li   $v0,10       # exit
   syscall

#---------------------------------------

Sample output:

Initial array:

Array after program execution: