Assembly Code Example

1
Assembly Code Example

• Selection Sort

2
Swap Function in C

• Swap(int num1, int num2)
• // post values of num1 and num2 have
• // been swapped
• int temp
• temp num1
• num1 num2
• num2 temp

3
Swap Function in Assembly Code

• Swap lw t0, 0(a0) t0 gets num1
• lw t1, 0(a1) t1 gets num2
• sw t0, 0(a1) num2 gets t0
• sw t1, 0(a0) num1 gets t1
• jr ra

4
Selection Sort function in C

• void SelSort(int v, int length)
• int indexMin
• for(int s 0 s lt length-1 s)
• indexMin s
• for(int k s1 k lt length k)
• if(vk lt vindexMin)
• indexMin k
• swap(vs, vindexMin)

5
Selection Sort in Assembly Code

• Do we make a subsidiary function call?
• Yes, must save state, use s registers
• What must go into s registers?
• Variables whose values must be retained across
  function calls
• v base, length, length-1, s
• Other variables can use t registers
• k, minIndex, temps for address/branch
  computations
• Construct code by units (if, for, for)

6
Selection Sort in Assembly Code

• register assignments
• v base in s0 (move from a0)
• length in s1 (move from a1)
• length-1 in s2 (compute from n)
• s in s3 (initialize to 0)
• minIndex in t0
• k in t1

7
Selection Sort in Assembly Code

• if(vk lt vminIndex) minIndex k
• sll t3, t1, 2 t3 4 k
• add t3, s0, t3 t3 address of vk
• sll t4, t0, 2 t4 4 minIndex
• add t4, s0, t4 t4 addr of vminIndex
• lw t5, 0(t3) t5 vk
• lw t6, 0(t4) t6 vminIndex
• slt t2, t5, t6 vk lt vminIndex?
• beq t2, zero, endif if not skip if part
• add t0, t1, zero minIndex k
• endif

8
Selection Sort in Assembly Code

• for(k s1 k lt length k)
• if
• addi t1, s3, 1 k s 1
• fork slt t2, t1, s1 k lt length ?
• beq t2, zero, endfork
• if
• addi t1, t1, 1 k
• j fork
• endfork

9
Selection Sort in Assembly Code

• for(s 0 s lt length-1 s)
• minIndex s for k swap(vs,
  vminIndex)
• add s3, zero, zero s 0
• fors slt t2, s3, s2 s lt length-1 ?
• beq t2, zero, endfors
• add t0, s3, zero minIndex s
• for k ...
• sll t3, s3, 2 compute array addresses
• add a0, s0, t3 and store as parameters
• sll t3, t0, 2
• add a1, s0, t3
• jal Swap call swap function
• addi s3, s3, 1 s
• j fors
• endfors

10
Selection Sort in Assembly Code

• save state restore state
• addi sp, sp, -20
• sw ra, 16(sp) lw ra, 16(sp)
• sw s3, 12(sp) lw s3, 12(sp)
• sw s2, 8(sp) lw s2, 8(sp)
• sw s1, 4(sp) lw s1, 4(sp)
• sw s0, 0(sp) lw s0, 0(sp)
• addi sp, sp, 20
• initialize
• add s0, a0, zero return
• add s1, a1, zero jr ra
• addi s2, s1, -1

11
Assemble / Link / Load / Run

• Assemble
• translates assembly code to object code (machine
  lang)
• pseudo instructions are replaced by actual
  machine instructions
• e.g. move t0, s1 becomes add t0, s1, zero
• addresses are set relative to start of code
  segment
• relocation done by linker
• internal addresses which need to be resolved are
  kept in a symbol table
• external addresses which are needed are listed
• e.g. function calls not part of this segment

12
Assemble / Link / Load / Run

• Link puts together different object code
  segments
• resolves addresses with function calls across
  segments
• resolves final addresses of labels
• places data in data segment and resolves
  addresses
• Load place code into main memory
• Run start at label main