Computer Architecture & Operations I

1. Instructor: Yaohang Li Computer Architecture & Operations I

2. Review • Last Class • Procedure Call • Steps of procedure call • caller and callee • Branch Addressing • This Class • Leaf procedure • Quiz • Next Class • Non-leaf procedure

3. Procedure • Procedure (function) • A stored subroutine that performs a specific task based on the parameters with which it is provided • Important when writing a large program • Allow a programmer to focus on a specific task

4. Procedure Calling • Steps required • Place parameters in registers • Transfer control to procedure • Acquire storage for procedure • Perform procedure’s operations • Place result in register for caller • Return to place of call

5. Caller and Callee • Caller • The program that instigates a procedure and provides the necessary parameter values • Callee • A procedure that executes a series of stored instructions based on parameters provided by the caller and then returns control to the caller

6. Register Usage • $a0 – $a3: arguments (reg’s 4 – 7) • $v0, $v1: result values (reg’s 2 and 3) • $t0 – $t9: temporaries • Can be overwritten by callee • $s0 – $s7: saved • Must be saved/restored by callee • $gp: global pointer for static data (reg 28) • $sp: stack pointer (reg 29) • $fp: frame pointer (reg 30) • $ra: return address (reg 31)

7. Program Counter (PC) • Program Counter • A register in CPU • Containing the address of the instruction in the program being executed

8. Stack • Stack • A last-in-first-out queue • Stack pointer • $sp • Point to the address of the most recent element in the stack • Push • Add element onto the stack • Pop • Remove element from the stack

9. Procedure Call Instructions • Procedure call: jump and link jal ProcedureLabel • Address of following instruction put in $ra • Jumps to target address • Procedure return: jump register jr $ra • Copies $ra to program counter • Can also be used for computed jumps • e.g., for case/switch statements

10. Leaf Procedure and non-Leaf Procedure • Leaf Procedure • Procedures that do not call other procedures • Non-leaf Procedure • Procedures that call other procedures

11. Leaf Procedure Example • C code: int leaf_example (int g, int h, int i, int j){ int f; f = (g + h) - (i + j); return f;} • Arguments g, …, j in $a0, …, $a3 • f in $s0 (hence, need to save $s0 on stack) • Result in $v0

12. Leaf Procedure Example • MIPS code: (leaf example)addi $sp, $sp, -12 sw $t1, 8($sp) sw $t0, 4($sp) sw $s0, 0($sp) add $t0, $a0, $a1 add $t1, $a2, $a3 sub $s0, $t0, $t1 add $v0, $s0, $zero lw $s0, 0($sp) lw $t0, 4($sp) lw $t1, 8($sp) addi $sp, $sp, 12 jr $ra Save $s0, $t1, $t0 on stack Procedure body Result Restore $s0, $t1, $t0 from the stack Return

13. Status of Stack

14. Temporary Registers • MIPS Assumption $t0 – $t9: temporary registers that are not preserved by the callee on a procedure call $s0 – $s7: saved registers Must be preserved by callee on a procedure call If used, the callees saves and restores them

15. Simplified Leaf Procedure Example • MIPS code: (leaf example)addi $sp, $sp, -4 sw $s0, 0($sp) add $t0, $a0, $a1 add $t1, $a2, $a3 sub $s0, $t0, $t1 add $v0, $s0, $zero lw $s0, 0($sp) addi $sp, $sp, 4 jr $ra Save $s0 on stack Procedure body Result Restore $s0 from the stack Return

16. Summary • Procedure Call • Registers used • Stack • jal and jr • leaf and no-leaf procedure • Allocating space for new data on the heap

17. What I want you to do • Review Chapter 2