Problem: ! bell ! help ! ! 1 ! 2 ! ! Bal help ! ! ! ! ! 1 ! 2 ! ! Bal help ! ! ! ! Bal bell ! 3 Jr $31 ! ! ! Jr $31 ! What happend here? What will happen here?

The activation concept Main 1. Executing program, wants “bell” 2. Creates son, Tells him “bell”, Falls asleep

The activation concept Son 1. Wakes up, starts executing “bell”, wants “help” Main 1. Executing program, wants “bell” 2. Creates son, Tells him “bell”, Falls asleep 3. Asleep

The activation concept Grandson Son 1. Wakes up, starts executing “bell”, wants “help” 2. Creates son, tells him help, falls asleep Main 1. Executing program, wants “bell” 2. Creates son, Tells him “bell”, Falls asleep 3. Asleep

The activation concept Grandson 1. Wakes up. starts executing “help” Son 1. Wakes up, starts executing “bell”, wants “help” 2. Creates son, tells him help, falls asleep 3. Asleep Main 1. Executing program, wants “bell” 2. Creates son, Tells him “bell”, Falls asleep 3. Asleep

The activation concept Grandson 1. Wakes up. starts executing “help” 2. Finished. Vanishes, Wakes up his parent Son 1. Wakes up, starts executing “bell”, wants “help” 2. Creates son, tells him help, falls asleep 3. Asleep Main 1. Executing program, wants “bell” 2. Creates son, Tells him “bell”, Falls asleep 3. Asleep

The activation concept Son 1. Wakes up, starts executing “bell”, wants “help” 2. Creates son, tells him help, falls asleep 3. Asleep 4. Finished, Vanishes, wakes up his parent Main 1. Executing program, wants “bell” 2. Creates son, Tells him “bell”, Falls asleep 3. Asleep

The activation concept Main 1. Executing program, wants “bell” 2. Creates son, Tells him “bell”, Falls asleep 3. Asleep 4. Executing

The main points: We talk about activations, not the code being executed. Last in - first out implies a stack OK for different activations to share the same instructions

A parents responsibilities to his unborn son: Be prepared for the son to trash $tx-registers. Put input arguments into $a0 - $a3. Give him a return address (by Bal- instr.). Tell him which code to execute (also by Bal).

The son’s responsibility to his sleeping parent: Leave the stack like he found it. Return results (if any) into registers $v0 - $v1. Leave the $ax-registers like he found them. OK to change $tx-registers and not restore them.

The activations responsibility to himself Need space for local variables? Create space on stack. Use $tx registers as scratchpads. Want to change $ax-register? Save the incoming values into local variables. Want to create a son? Protect the $tx-registers. Protect the return address ($31). by saving them on the stack

What does one activation own? Return address to his parent. His incoming parameters. A place to put his results. Some local variables. Which code to execute (his PC). Called his “activation record”

Stack the activation records: Grandson used activation record “grandson” Son (asleep) The concept gives unique context. saved activation record “son” Main (asleep) saved activation record “main”

The user stack $sp points to top- of- stack User stack (part of the data memory) $sp

The user stack Stack grows toward lower addresses. Basic stack operations: Push $t5 onto stack Pop from stack into $a2 Addi $sp $sp -4 Lw $a2 0($sp) Sw $t5 0($sp) Addi $sp $sp 4

Wrong!!!!! reason: interrupts. Push $t5 onto stack Pop from stack into $a2 Sw $t5 -4($sp) Addi $sp $sp 4 Addi $sp $sp -4 Lw $a2 -4($sp) This method writes Never use a negative beyond T.O.S first, displacement with then fixes $sp respect to T.O.S. reason: interrupts.

Example Compute  i Define as recursive definition: 1 if N = 1 sum(N) = N + sum(N-1) otherwise

Pseudocode Procedure sum(N:integer):integer; if (N=1) then return (1) else T := N + sum(N-1); return (T) end;

Where do we start? Parameter comes in in $a0. Return address in $31. This activation will probably need a son (because of the recursion) so: Local variables. save $31. save $a0. the variable T?

The code sum: addi $sp $sp -4 #store return address on stack sw $31 0($sp) addi $sp $sp -4 #store incoming parameter on stack sw $a0 0($sp) if: ori $t0 $r0 1 #test: fixed point? bne $a0 $t0 else then: ori $v0 $r0 1 b exit else: addi $a0 $a0 -1 #sum(N-1) in $v0, original $a0 lost bal sum #recursive call to sum lw $t0 0($sp) #retrive the icoming parameter value add $v0 $v0 $t0 #$v0 has N + T exit: lw $31 4($sp) #restore return address addi $sp $sp 8 #Stack cleaned jr $31 #end activation