The branch jumps when the condition is false
An if in C says "run this when the condition holds". The ARM7TDMI has no instruction for that. It has a compare, which performs a subtraction, keeps the four condition flags and throws the difference away, and it has branches that read those flags. So the compiler builds an if out of the only material available: the guarded code goes in a straight line and a branch goes in front of it, jumping past it when the condition fails.
That inversion is the first thing to internalise. For a plain guarded statement, the mnemonic in the listing is the negation of the operator in the source.
0 cmp r0, r1
2 bne 6 ~>
4 mov r0, #0
6 ~>bx lr
cmp r0, r1 sets the flags from r0 - r1. bne 6 skips the instruction at 4 whenever the two registers differ, so the mov runs exactly when they are equal — the source condition was == and the listing shows you bne. The ~> in the gutter at address 6 marks where a branch lands; everything between the branch and its target is the body of the if.
Here is the same skeleton with a constant and a different relation:
0 cmp r0, #3
2 ble 6 ~>
4 sub r0, r1
6 ~>bx lr
ble skips the subtraction when the first argument is less than or equal to 3, so the subtraction runs when it is greater than 3. Source >, listing ble. Read every branch this way: flip the mnemonic and you have the condition that was written.
There is no push and no pop in either listing. A function that calls nothing keeps its return address in lr for its whole life and leaves through a single bx lr, so the guarded body sits between the branch and that one return.
Your target is the same four-instruction skeleton: a compare, a branch over one instruction, that instruction, and the return.
Your task
Write func_0811a638 to reproduce the target assembly.