
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.
- movb:
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.

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