Supporting Functions (procedures) What is needed? Functions: - - PowerPoint PPT Presentation

Supporting Functions (procedures)

What is needed?

• Functions: Analogy of a spy
• secret plan, acquire resources, perform task, cover tracks, return with result
• Program has to
• place params for function’s access
• transfer control to procedure
• acquire storage resources for the function
• perform function’s instructions
• place result for calling program’s access
• return control to point of origin

Using registers

• Registers are fast!
• $a0 – $a3: argument registers
• $v0-$v1: value registers
• $ra: return address register

Jump-Link and Program Counter

• Jump-and-link instruction
• jumps to addr, store next instruction’s addr in $ra: the return address
• jal ProcedureAddress
• Program Counter (PC)
• address of current instruction
• , jal stores PC + 4 in $ra to setup procedure return
• , another instruction: jr $ra
• jumps to address in $ra

Setup for Executing Functions

• Caller puts params in $a0 – $a3
• Uses jal X to jump to callee procedure X
• stores PC + 4 in $ra as well
• Callee performs its instructions
• Places results in $v0 – $v1
• Returns to caller by $jr $ra

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To summarize:

Procedure needs more registers. Overwrite?

• More than $a0 – $a4 & $v0,$v1 may be needed
• $t0 – $t9, $s0 – $s7
• Those needed by caller must be restored to their original value
• Use Stack for storing/restoring these registers

Stack

Last-In First-Out

Stack Pointer

$sp

Push and Pop operations

Stack

(in memory)

HIGH LOW Push

SP SP

(subtract) (grow)

Push

SP

(add) (shrink)

MIPS provides $sp

Compiling a C “leaf” procedure

int leaf_example (int g, int h, int i, int j) { int f; f = (g + h) - (i + j); return f; }

The Stack Pointer

s0 t0 t1 $sp $sp $sp

Before During After

Temporary registers

• $t0 - $t9 are not preserved by callee
• $s0 - $s7 are preserved by callee
• So only 8 registers need to be saved

Nested procedures

• A procedure may call another
• Also, recursive procedure
• May over-write arg. or return-address registers
• Solution: push them on the stack as well