activation block. usage example

8/9/2019 Activation Block. Usage Example

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Activation block

Usage example

Mariano Rico, UC3M 2010


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Statement

Write a program that uses the functioncounter. This function has twoparameters: (1)the address of a zero-

terminated string and (2) a character.This function returns the number oftimes (an integer) that the givencharacter appears in the string

In C terminology the funtion signaturewould be int counter (char* s, char c)


3/32

Calling the function (bigpicture)

The main program must

1) Reserve space in the stack to store theresult(s)

2) Load the function parameters in thestack (in the right order)

3) Call the function

4) Download the function parametersfrom the stack

5) Get the result from the stack


4/32

Calling the function (step bystep)

Initial state

The stack pointer (%esp) points to thetop of the stack

%esp Initial position


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1) Reserve space in the stack to store the result(s)2) Load the functions parameters in the stack (in the right

order)

3) Call the function- Execute the funcion

1) Download the functions parameters from the stack2) Get the result(s) from the stack

sub $4, %esp

push theletter

push thestring

call counter

add $8, %esp

pop %eax

%esp

Initial position

Results space

Paramsspace


6/32



order)



sub $4, %esp

push theletter

push thestring

call counter

add $8, %esp

pop %eax

thestringthestring

thelettertheletter

%esp

Initial position

Results space

Paramsspace


7/32



order)



sub $4, %esp

push theletter

push thestring

call counter

add $8, %esp

pop %eax

@RET

thestringthestring

thelettertheletter

%esp

Initial position

Results space

Paramsspace


8/32



order)



sub $4, %esp

push theletter

push thestring

call counter

add $8, %esp

pop %eax

@RET

thestringthestring

thelettertheletter

resultresult

%esp

Initial position

Results space

Paramsspace


9/32



order)



sub $4, %esp

push theletter

push thestring

call counter

add $8, %esp

pop %eax

@RET

thestringthestring

thelettertheletter

resultresult%esp

Initial position

Results space

Paramsspace


10/32



order)



sub $4, %esp

push theletter

push thestring

call counter

add $8, %esp

pop %eax

@RET

thestringthestring

thelettertheletter

resultresult

%esp Initial position

Results space

Paramsspace


11/32

The function (big picture)1) Save the value of%ebp (as a pointer to the

activation block)

2) Copy the value of%esp to %ebp

3) Allocate space in the stack for local variables

4) Save registries that will be used by thesubroutine

5) Do the subroutines job6) Store results in the space allocated

7) Restore the registries used by the subroutine

8) Move the top of the stack to the activationblock pointer

9) Restore the value of%ebp

10) Execute RET

Initialposition

Resultsspace

Paramsspace

@RET

thestringthestring

thelettertheletter

%esp%esp

Initial state

The stack islike this:(step 3 inthe callingprogram)


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counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eax

push %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx

loop:cmpb $0, (%eax, %ebx)

je res

cmpb %cl, (%eax, %ebx)

jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax

mov %ebp, %esppop %ebp

ret

The function (step by step)1) Save the value of%ebp (as a pointer to the

activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp%esp%esp

Initialposition

Resultsspace


13/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax


ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp%esp%esp

Initialposition

Resultsspace

%ebp%ebp


14/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax


ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace


15/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax


ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace


16/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax


ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

0x00000000

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspacelocal var = 0


17/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax


ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

0x00000000

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

%eax points tothestring


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counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax


ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

0x00000000

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

%eax points tothestring%cl points totheletter


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counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax


ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

0x00000000

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

%eax points tothestring%cl points totheletter%ebx is an index (ini

val=0)


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counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx

loop: cmpb $0, (%eax, %ebx)

je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax


ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

0x00000000

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

%eax points tothestring%cl points totheletter%ebx is an index (ini

val=0)Comparison of byte

h f i ( b )


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counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax


ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

0x00000000

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

If it reaches theend of the stringthen jump to res,otherwise

h f i ( b )


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counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax


ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

0x00000000

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

compares thebyte in %cl to thebyte pointed by theindex

Th f i ( b )


23/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax

mov %ebp, %esp

pop %ebp

ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

0x00000000

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

If equal (letterdetected) go toincr

Th f i ( b )


24/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax

mov %ebp, %esp

pop %ebp

ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

0x00000000

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

If equal (letterdetected) go toincr increase theindex

Th f ti ( t b t )


25/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax

mov %ebp, %esp

pop %ebp

ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

0x00000000

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

If equal (letterdetected) go toincr increase theindex and do theloop again

Th f ti ( t b t )


26/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax

mov %ebp, %esp

pop %ebp

ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

0x00000001

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

otherwiseincrease the localvar

Th f ti ( t b t )


27/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax

mov %ebp, %esp

pop %ebp

ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

%ebp

loc varvalue

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

Copies the value ofthe local var to%eax

Th f ti ( t b t )


28/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax

mov %ebp, %esp

pop %ebp

ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

resultresult

%ebp

loc varvalue

%eax

%ebx

%ecx%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

Copies the valuestored in %eax tothe results space

Th f ti ( t b t )


29/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax

mov %ebp, %esp

pop %ebp

ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

resultresult

%ebp

loc varvalue

%eax

%ebx

%ecx

%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

Th f ti ( t b t )


30/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx

loop:

cmpb $0, (%eax, %ebx)

je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax

mov %ebp, %esp

pop %ebp

ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

resultresult

%ebp

loc varvalue

%eax

%ebx

%ecx

%esp%esp

Initialposition

Resultsspace

%ebp%ebp

Local vaspace

Deletes localvariables

Th f ti ( t b t )


31/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx

loop:


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax

mov %ebp, %esp

pop %ebp

ret


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

resultresult

%ebp

loc varvalue

%eax

%ebx

%ecx

%esp%esp

Initialposition

Resultsspace

Local vaspace

Th f ti ( t b t )


32/32

counter:

push %ebp

mov %esp, %ebp

sub $4, %esp

push %eaxpush %ebx

push %ecx

movl $0, -4(%ebp)

mov 8(%ebp), %eax

mov 12(%ebp), %cl

mov $0, %ebx

loop:


je res


jne incr

incl -4(%ebp)

incr:

inc %ebx

jmp loopres:

mov -4(%ebp), %eax

mov %eax, 16(%ebp)

pop %ecx

pop %ebx

pop %eax

mov %ebp, %esp

pop %ebp


activation block)








10) Execute RET

Paramsspace

@RET

thestringthestring

thelettertheletter

resultresult

%ebp

loc varvalue

%eax

%ebx

%ecx

%esp%esp

Initialposition

Resultsspace

Local vaspace

activation block. usage example

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