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

Transcription

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

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

3 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 MicroComputer Engineering ActivationConcept page 3

4 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 MicroComputer Engineering ActivationConcept page 4

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

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

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

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

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 MicroComputer Engineering ActivationConcept page 9

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). MicroComputer Engineering ActivationConcept page 10

11 The son s responsibility to his sleeping parent: Leave the stack like he found it. Return results (if any) into registers $v0 - $v1. OK to change $tx/$ax-registers and not restore them. MicroComputer Engineering ActivationConcept page 11

12 The activations responsibility to himself Need space for local variables? Create space on stack. Use $tx registers as scratchpads. Want to create a son? Protect the $tx and $ax-registers if needed. Protect the return address ($31). by saving them on the stack MicroComputer Engineering ActivationConcept page 12

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 MicroComputer Engineering ActivationConcept page 13

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

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

16 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 MicroComputer Engineering ActivationConcept page 16

17 Wrong 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. MicroComputer Engineering ActivationConcept page 17

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

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

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

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? MicroComputer Engineering ActivationConcept page 21

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 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 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 MicroComputer Engineering ActivationConcept page 22

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) MicroComputer Engineering ActivationConcept page 23