Chapter 4, Problem 4.45HW

A.

Program Plan Intro

Given Assembly code:

subq $8, %rsp

movq REG, (%rsp)

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.

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.

Jump Instruction:

• The “jump” instruction causes execution to switch to an entirely new position in program.
• The “label” indicates jump destinations in assembly code.
• The “je” instruction denotes “jump if equal” or “jump if zero”.
  • The comparison operation is performed.
  • If result of comparison is either equal or zero, then jump operation takes place.
• The “ja” instruction denotes “jump if above”.
  • The comparison operation is performed.
  • If result of comparison is greater, then jump operation takes place.
• The “pop” instruction resumes execution of jump instruction.
• The “jmpq” instruction jumps to given address. It denotes a direct jump.

Explanation of Solution

Behavior of instruction pushq:

• No, the given code sequence is not correctly describes the behavior of given instruction.
• The instruction “subq $8, %rsp” subtracts 8 from value of “%rsp”.
• If “REG” denotes “%rsp” then it computes “%rsp-8”.
• The instruction “movq REG, (%rsp)” moves value of “REG” into location of “(%rsp)”.
• The value “%rsp-8” is been pushed into stack instead of “%rsp”.
• Hence, it does not describe behavior of instruction “pushq %rsp”.

B.

Program Plan Intro

Given Assembly code:

subq $8, %rsp

movq REG, (%rsp)

Processing stages:

• The processing of an instruction has number of operations.
• The operations are organized into particular sequence of stages.
• It attempts to follow a uniform sequence for all instructions.
• The description of stages are shown below:
  • Fetch:
    • It uses program counter “PC” as memory address to read instruction bytes from memory.
    • The 4-bit portions “icode” and “ifun” of specifier byte is extracted from instruction.
    • It fetches “valC” that denotes an 8-byte constant.
    • It computes “valP” that denotes value of “PC” plus length of fetched instruction.
  • Decode:
    • The register file is been read with two operands.
    • It gives values “valA” and “valB” for operands.
    • It reads registers with instruction fields “rA” and “rB”.
  • Execute:
    • In this stage the ALU either performs required operation or increments and decrements stack pointer.
    • The resulting value is termed as “valE”.
    • The condition codes are evaluated and destination register is updated based on condition.
    • It determines whether branch should be taken or not in a jump instruction.
  • Memory:
    • The data is been written to memory or read from memory in this stage.
    •  The value that is read is determined as “valM”.
  • Write back:
    • The results are been written to register file.
    • It can write up to two results.
  • PC update:
    • The program counter “PC” denotes memory address to read bytes of instruction from memory.
    • It is used to set next instruction’s address.

Explanation of Solution

Modified code:

movq REG, -8(%rsp)

subq $8, %rsp

Explanation:

• The instruction “movq REG, -8(%rsp)” moves value of “REG” into location of “(%rsp)-8”.
• The instruction “subq $8, %rsp” subtracts 8 from value of “%rsp”.
• If “REG” denotes “%rsp” then it computes “%rsp”.
• The value “%rsp” is now been pushed into stack.
• Hence, it describes behavior of instruction “pushq %rsp”.