Chapter 3.6, Problem 3.21PP

Explanation of Solution

Given assembly code:

long test(long x, long y)

x in %rdi, y in %rsi

test:

leaq 0(,%rdi,8), %rax

testq %rsi, %rsi

jle .L2

movq %rsi, %rax

subq %rdi, %rax

movq %rdi, %rdx

andq %rsi, %rdx

cmpq %rsi, %rdi

cmovge %rdx, %rax

ret

.L2:

addq %rsi, %rdi

cmpq $-2, %rsi

cmovle %rdi, %rax

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; it copies effective address 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 serves as both source as well as destination.
  • It can either be a register or a memory location.
  • 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...