Question

What are the values of the registers, as decimal integers, after executing the following code?

        mov   r0, #9
        mov   r1, #1
    top:
        cmp   r0, #0
        beq   finish
        lsl   r1, r1, #1
        sub   r0, r0, #1
        b     top
    finish:
        mov   r2, #47
    

R0:  R1:  R2:  R3:

Answer 1

Answer is as follows :

mov r0, #9 // move data 9 to register r0. So r0 contains 9

mov r1, #1 // move data 1 to register r1. So r1 contains 1

top:

cmp r0, #0 // Compare value of r0 with 0 i.e. 9 with 0 (not equal)

beq finish //if both values are equal, than jump to label finish. Both are not equal

lsl r1, r1, #1 // logical shift the contents of r1 i.e. 1 in decimal or 0001 in binary after performing shift left we get 0010 i.e. 2. So r1 contains 2 now

sub r0, r0, #1 // perform r0 = r0-1 i.e. r0 = 9-1 = 8

b top // branch to top

finish:

mov r2, #47

So at first r0 = 8 , r1 = 2

Again branch to label top when r0= 8:

Both values are not equal again, so perform the operations.

lsl r1, r1, #1 // logical shift the contents of r1 i.e. 2 in decimal or 0010 in binary after performing shift left we get 0100 i.e. 4. So r1 contains 4 now

sub r0, r0, #1 // perform r0 = r0-1 i.e. r0 = 8-1 = 7

b top // branch to top

So now r0 = 7 , r1 = 4

By doing these calculations

we get when r0 = 7 ,

r1 = 1000 (after shifting) i.e. 8, r0 = 6

we get when r0 = 6

r1 = 10000 (after shifting) i.e. 16, r0 = 5

we get when r0 = 5

r1 = 100000 (after shifting) i.e. 32, r0 = 4

we get when r0 = 4

r1 = 1000000 (after shifting) i.e. 64, r0 = 3

we get when r0 = 3

r1 = 10000000 (after shifting) i.e. 128, r0 = 2

we get when r0 = 2

r1 = 100000000 (after shifting) i.e. 257, r0 = 1

we get when r0 = 1

r1 = 1000000000 (after shifting) i.e. 513, r0 = 0

we get when r0 = 0

So values get equal and control jumps to label finish

finish:

mov r2, #47 //move data 47 to register r2. So r2 contains 47.

SO AT LAST WE GET :

r0 = 0 , r1 = 513 , r2 = 47

There is no update in register r3. So it contains default value

All the registers are assumed to be 16 bit.

if there is any query please ask in comments..