How to interpret BLO in this example

Question

In this example im trying to figure out what the BLO is comparing. I know it branches if the carry flag is not set. Is it comparing r1 with old r1 (the instruction above it?) As the code doesn't branch at BGE. Thanks so much.

        LDR     r0,=0X3
        LDR     r1, =0X8F
        CMP     r0,r1
        BGE     a_label
        SUBS    r1,r1,#0XC9

a_label BLO     stop
        SUBS        r1,r1,#0X7D

stop      

Answer 1

It can be tricky to interpret condition codes in the absence of a CMP instruction, because the mnemonics were written to make interpretation easy when there's a CMP. But in this example, it's not so bad: remember that CMP is just SUBS with the result discarded, so

SUBS   r1, r1, #0xC9
BLO    stop

represents the same condition as

CMP    r1, #0xC9
BLO    stop

but leaves a different result in r1. The condition therefore is that it will branch when r1 (old value) is lower than 0xC9, in an unsigned sense -- in other words when the SUBS results in a wrap of the unsigned value of r1.