1. MicroComputer Engineering ActivationConcept page 1 Problem: !bell!help! !1! 2! !Bal help! !!! Bal bell! 3Jr $31!!! !Jr $31 ! What happend here? What will happen here?

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

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

4. MicroComputer Engineering ActivationConcept page 4 The activation concept Main 1. Executing program, wants “bell” 2. Creates son, Tells him “bell”, Falls asleep 3. Asleep Son 1. Wakes up, starts executing “bell”, wants “help” 2. Creates son, tells him help, falls asleep Grandson

5. MicroComputer Engineering ActivationConcept page 5 The activation concept Main 1. Executing program, wants “bell” 2. Creates son, Tells him “bell”, Falls asleep 3. Asleep Son 1. Wakes up, starts executing “bell”, wants “help” 2. Creates son, tells him help, falls asleep 3. Asleep Grandson 1. Wakes up. starts executing “help”

6. MicroComputer Engineering ActivationConcept page 6 The activation concept Main 1. Executing program, wants “bell” 2. Creates son, Tells him “bell”, Falls asleep 3. Asleep Son 1. Wakes up, starts executing “bell”, wants “help” 2. Creates son, tells him help, falls asleep 3. Asleep Grandson 1. Wakes up. starts executing “help” 2. Finished. Vanishes, Wakes up his parent

7. MicroComputer Engineering ActivationConcept page 7 The activation concept Main 1. Executing program, wants “bell” 2. Creates son, Tells him “bell”, Falls asleep 3. Asleep 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

8. MicroComputer Engineering ActivationConcept page 8 The activation concept Main 1. Executing program, wants “bell” 2. Creates son, Tells him “bell”, Falls asleep 3. Asleep 4. Executing

9. MicroComputer Engineering ActivationConcept page 9 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

10. MicroComputer Engineering ActivationConcept page 10 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).

11. MicroComputer Engineering ActivationConcept page 11 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.

12. MicroComputer Engineering ActivationConcept page 12 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

13. MicroComputer Engineering ActivationConcept page 13 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”

14. MicroComputer Engineering ActivationConcept page 14 Stack the activation records: Main (asleep) Son (asleep) Grandson saved activation record “main” saved activation record “son” used activation record “grandson” The concept gives unique context.

15. MicroComputer Engineering ActivationConcept page 15 The user stack $sp points to top- of- stack User stack (part of the data memory) $sp

16. MicroComputer Engineering ActivationConcept page 16 The user stack Stack grows toward lower addresses. Basic stack operations: Push $t5 onto stackPop from stack into $a2 Addi $sp $sp -4Lw $a2 0($sp) Sw $t5 0($sp)Addi $sp $sp 4

17. MicroComputer Engineering ActivationConcept page 17 Wrong!!!!! Push $t5 onto stackPop from stack into $a2 Sw $t5 -4($sp)Addi $sp $sp 4 Addi $sp $sp -4Lw $a2 -4($sp) This method writesNever use a negative beyond T.O.S first,displacement with then fixes $sprespect to T.O.S. reason: interrupts.

18. MicroComputer Engineering ActivationConcept page 18 Framepointer Var2 Var1 Old fp Return addr Caller’s stack <= $fp points here <= $sp points here 0($fp) 4($fp) -4($fp) -8($fp) Low address High address

19. MicroComputer Engineering ActivationConcept page 19 Example N Compute    i i=1 Define as recursive definition: 1 if N = 1 sum(N) = N + sum(N-1) otherwise

20. MicroComputer Engineering ActivationConcept page 20 Pseudocode Procedure sum(N:integer):integer; if (N=1) then return (1) else T := N + sum(N-1); return (T) end;

21. MicroComputer Engineering ActivationConcept page 21 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?

22. MicroComputer Engineering ActivationConcept page 22 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 $r0 1 bexit else:addi$a0 $a0 -1#sum(N-1) in $v0, original $a0 lost bal sum#recursive call to sum lw$t0 0($sp)#retrive the incoming parameter value add$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

23. MicroComputer Engineering ActivationConcept page 23 Procedure Call 1. Place parameters in a place where the procedure can access them ($a0 - $a3) 2. Transfer control to the procedure (bal bell) 3. Acquire the storage resources needed for the procedure. 4. Perform the desired task. 5. Place the result value in a place where the calling program can access it. ($v0 - $v1) 6. Return control to the point of origin.(jr $31)