CSCI2021 Lecture 18 Summary

Announcements

• Project 3 posted

C Loop to Assembly

• General structure:

  • Loop: while(<condition>) { <loop body> }

  • Assembly conversion:

  • .LOOP: cmpX

  • jXX .END

  • <asm loop body>

  • jmp .LOOP

  • .END:

  • <asm following code>

C If to Assembly

• General structure:

  • If statement: if(<condition>) { <if body> }

  • Assembly conversion:

  • cmpX

  • jXX .AFTER

  • <asm if body>

  • .AFTER:

  • <asm following code>

Assembly Control Flow

• Generalizations not always accurately translate to assembly.

• Compilers may perform optimizations causing unexpected behavior.

Assembly: Procedures

• Procedures are defined as blocks of code with arguments and return values.

• Caller: procedure making the call.

• Callee: procedure being called.

Procedure Call Mechanics:

• Control flow managed via the instruction pointer %rip.

• Passing data involves arguments through registers and returning values.

• Local variables reside in the stack frame, which is allocated during the call.

Assembly Call and Return Operations

1. Call: - Push return address onto stack.

   • Jump to the callee address.

2. Return: - Restore return address from the stack.

Assembly Procedures: Passing Data

• Data passed via registers.

• If > 6 args, push onto caller's stack frame for callee retrieval.

• Return values in %rax.

Local Variables

• Stored in registers, handled via callee-saved and caller-saved conventions.

• Callee saves register values onto the stack before use if the registers are in use.

C vs Assembly Procedure Calls

• Comparisons made using control flow in recursive examples.

• Real scenarios require thorough understanding of stack and frame allocations.