ARM Instruction

ARM goto Instruction

goto label (C) is the same as B label in ARM.

Conditional Branch

Set the condition bits (N, Z, V, C) in the program status register.
Then check on these condition bits to branch conditionally.

Conditional bits

N = 1 if result is Negative

Z = 1 if result is Zero

V = 1 if there’s overflow

C = 1 if there’s carry out

SUBS (with the ending S) sets conditional bits in cpsr register.

SUBS r1, r0, #0

r0 = r1 – #0, and set the condition bits NZVC based on the result.
If r1==r2, then N = 0, Z= 1, V = 0, C =1
If r < r2 (no overflow), then N =1, Z = 0, V=0

Another way to do that is to use a ‘Comparison Instruction’

CMP r1, r2
BEQ label  @if Z = 1, then go to label

Compare and set condition bits
Subtract a register or an immediate value from a register value and updates status register
Then check on these condition bits to branch conditionally

COM r0, #4
BEQ this_instr
ADD r4, r5, r6
B end
this_instr: SUB r0, r0, #1
end:

if(r0 == 4)
    r0 --;
r4 = r5 + r6;

if(r0 != 4)
    r4 = r5 + r6;
r0 --;

ARM Decision Instructions

BEQ label	==	BRANCH EQUAL
BNE label	!=	BRANCH NOT EQUAL
BLE label	<=	BRANCH LESS THAN EQUAL
BLT label	<	BRANCH LESS THAN
BGE label	>=	BRANCH GREATER THAN EQUAL
BGT label	>	BRANCH GREATER THAN

The condition is T/F based upon the fields in the Program Status Register.

If…else

if (x==y)
    x += y;
else
    x -= y;

CMP r01, r1
BNE else_aprt
ADD r0, r0, r1
B end
else_part: SUB r0, r0,r1
end:

Published on October 23, 2015