Loops and Conditional Branches in ARM

cmp Sets Flags, b.cond Jumps

AArch64 compares with cmp (subtract, discard result) and branches with b.eq, b.lt, b.ge, etc. SUBS both subtracts and updates flags, which pairs cleanly with b.ne for decrement loops. Mispredicted branches still cost 15+ cycles on modern cores, so tight loop shape matters on every phone frame.

asmCopy
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Signed compares use b.lt / b.gt; unsigned sizes use b.lo / b.hi.

Summing 1 Through N

The counter walks down to zero; the accumulator picks up each value. N=0 should skip the body and return zero in x0.

For this exercise, you will implement the sum loop with cmp/b only (no C helpers). This task asks you to return the total in x0 per AAPCS64, because every Android JNI stub and iOS assembly thunk ends with that register holding the scalar result.

Keep the relevant man page, ABI doc, or Rust reference chapter open while you work. When your output disagrees with the reference implementation on the same machine, the mismatch is usually an alignment rule, an off-by-one terminator, or a register slot you misread in GDB. Skim the official documentation for the tool or ABI named in the exercise; the prose changes, but register roles, syscall numbers, and ownership rules stay stable across releases.