Chapter 3, Problem 3.66HW

Program Plan Intro

Given assembly code:

# long sum_col(long n, long A[NR(n)][NC(n)], long j)

# n in %rdi, A in %rsi, j in %rdx

sum_col:

  leaq 1(,%rdi,4), %r8       

  leaq (%rdi,%rdi,2), %rax   

  movq %rax, %rdi            

  testq %rax, %rax           

  jle .L4                    

  salq $3, %r8               

  leaq (%rsi,%rdx,8), %rcx   

  movl $0, %eax             

  movl $0, %edx              

.L3:

  addq (%rcx), %rax          

  addq $1, %rdx              

  addq %r8, %rcx             

  cmpq %rdi, %rdx            

  jne .L3                    

  rep

  ret

.L4:

  movl $0, %eax              

  ret

Load Effective Address:

• The load effective address instruction “leaq” is a variant of “movq” instruction.
• The instruction form reads memory to a register, but memory is not been referenced at all.
• The first operand of instruction is a memory reference; the effective address is been copied to destination.
• The pointers could be generated for later references of memory.
• The common arithmetic operations could be described compactly using this instruction.
• The operand in destination should be a register.

Data movement instructions:

• The different instructions are been grouped as “instruction classes”.
• The instructions in a class performs same operation but with different sizes of operand.
• The “Mov” class denotes data movement instructions that copy data from a source location to a destination.
• The class has 4 instructions that includes:
  • movb:
    • It copies data from a source location to a destination.
    • It denotes an instruction that operates on 1 byte data size.
  • movw: 
    • It copies data from a source location to a destination.
    • It denotes an instruction that operates on 2 bytes data size.
  • movl:
    • It copies data from a source location to a destination.
    • It denotes an instruction that operates on 4 bytes data size.
  • movq:
    • It copies data from a source location to a destination.
    • It denotes an instruction that operates on 8 bytes data size.

Comparison Instruction:

• The “CMP” instruction sets condition code according to differences of their two operands.
• The working pattern is same as “SUB” instruction but it sets condition code without updating destinations.
• The zero flag is been set if two operands are equal.
• The ordering relations between operands could be determined using other flags.
• The “cmpl” instruction compares values that are double word.

Unary and Binary Operations:

• The details of unary operations includes:
  • The single operand functions as both source as well as destination.
  • It can either be a memory location or a register.
  • The instruction “incq” causes 8 byte element on stack top to be incremented.
  • The instruction “decq” causes 8 byte element on stack top to be decremented.
• The details of binary operations includes:
  • The first operand denotes the source.
  • The second operand works as both source as well as destination.
  • The first operand can either be an immediate value, memory location or register.
  • The second operand can either be a register or a memory location.