Question

Write ARM assembly language instructions to change the value of r0 as follows: • set the most significant byte of r0 to one’s (i.e., 0xFF) if the least significant byte of r1 is less than 90. • complement the least significant byte of r0 if r1 is odd.

Answer 1

Answer :- 1) The code to change MSB to 0xFF if LSB is less than 90 is-

AND R2, R1, #0xFF ; R2 = LSB of R1

CMP R2, #90 ; Compare R2 with 90

BGE End1 ; If value in R2 is greater than or equal to 90 then goto label named as End1

ORR R0, R0, #0xFF000000 ; If less than 90 then make MSB as 0xFF.

End1 : B End1 ; Infinite loop, end of program

Answer :- 2) To complement the LSB of R0 if R1 is odd.

AND R2, R1, #0x00000001 ; R2 = Zeroth bit of R1, for odd numbers zeroth bit is always 1. For even it will be zero.

BEQ R2, End1 ; If R2 = 0 then goto label named as End1

EOR R0, R0, #0x000000FF ; Doing xor of LSB of R0 and 0xFF to toggle the bits in lower byte

End1 : B End1 ; Infinite loop, end of program