Lecture 16

Arithmetic and Logic Operations

• addX SRC DST: Adds two values, resulting in DST = DST + SRC
• Operations Categories:
  • Immediate: Direct values
  • Register: Values from registers
  • Memory: Values stored in memory
• Binary Operations:
  • subX SRC DST: Subtract, DST = DST - SRC
  • imulX SRC DST: Integer multiply, DST = DST * SRC
  • xorX SRC DST: Bitwise XOR, DST = DST ^ SRC
  • orX SRC DST: Bitwise OR, DST = DST | SRC
  • andX SRC DST: Bitwise AND, DST = DST & SRC

Unary Operations

• Operations with one argument:
  • negX DST: Negates the value, DST = -DST
  • notX DST: Bitwise negate, DST = ~DST
  • incX DST: Increment, DST = DST + 1
  • decX DST: Decrement, DST = DST - 1

Shift Operations

• Shift Operations:
  • salX k, DST: Left shift by k, DST = DST << k
  • sarX k, DST: Arithmetic right shift by k, DST = DST >>A k
  • shrX k, DST: Logical right shift by k, DST = DST >>L k

Load Effective Address (LEA)

• lea SRC, DST: Loads the effective address of the memory operand SRC into DST without dereferencing. Example: leaq (%rsi, %rcx, 4), %rax computes R[%rsi] + R[%rcx] * 4.

Assembly vs C Comparison

• Example Function: long scale(long x, long y, long z) { return x + 4*y + 12*z; }
• Example Assembly Code:
  • leal (%rdi, %rsi, 4), %rax - calculates x + 4*y
  • leal (%rdx, %rdx, 2), %rdx - doubles z
  • leal (%rax, %rdx, 4), %rax - combines to calculate final result.

Control Flow

• Instructions: cmpX, testX (conditions) and jXX (jumps for conditional execution)
• Special register: %rip stores the address of the next executable instruction.

Comparisons in Control Flow

• cmpX B, A: Computes A - B and sets flags
• Flags set:
  • CF: Carry Flag (unsigned overflow)
  • ZF: Zero Flag (result is 0)
  • SF: Sign Flag (result is negative)
  • OF: Overflow Flag (signed overflow)

Example Tests

• Example: If %rbx = 0x3, cmpq $5, %rbx: sets flags accordingly based on operation result.
• Test Example: If %rcx = 0x8, testq %rcx, %rbx: performs bitwise AND and sets flags based on operation result.