Question

Thinking in Assembly language

What values will be written to the array when the following code executes?

.data

array DWORD 4 DUP(0)

.code

main PROC

mov eax,10

mov esi,0

call proc_1

add esi,4

add eax,10

mov array[esi],eax

INVOKE ExitProcess,0

main ENDP

proc_1 PROC

call proc_2

add esi,4

add eax,10

mov array[esi],eax

ret

proc_1 ENDP

proc_2 PROC

call proc_3

add esi,4

add eax,10

mov array[esi],eax

ret

proc_2 ENDP

proc_3 PROC

mov array[esi],eax

ret

proc_3 ENDP

Answer 1

Note: for simplicity purpose , instead of assigning the address in mov command, I have moved the value.

As you are well aware program execution starts from main. Let's debug step by step.

main PROC

mov eax,10 ( implies adrress of 10 is moved into eax, eax=10)

mov esi,0 (implies adrress of 0 is moved into esi , esi=0)

call proc_1 / proc_1 is called /

In proc_1 , proc_2 is called and in return which calls proc_3

proc_3 assigns address of eax to array[0] ( mov array[esi],eax) which implies array[0]=10) and we retrun to proc_2

In proc_2

add esi,4 ( add 0 and 4 and assign to esi , esi=4)

add eax,10 ( add 10 and 10 and assign to eax, eax=20)

mov array[esi],eax ( eax value 20 is moved into array[4])

ret ( return to procedure 1)

In proc_1 ( as of now we have following data, esi=4, eax=20 and array[0]=10,array[4]=20)

add esi,4 ( now esi+4=4+4=8)

add eax,10 ( eax+10=20+10=30)

mov array[esi],eax ( implies array[8]=30)

ret(return to main)

In main after call proc_1 statement we have below values, esi=8,eax=30 , array[0]=10,array[4]=20 and array[8]=30

add esi,4 ( esi+4=8+4=12)

add eax,10 (eax+10=30+10=40)

mov array[esi],esx ( 40 is assigned array to array[12])

Then by invoking exit process, we exit it and values of array is as follows array[0]=10,array[4]=20,array[8]=30 and array[12]=40

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Please give me a UPVOTE. Thank you :)