Magic Sum

; z1_sum_target: Magic Sum. No per-character compare: the routine sums all four
; input bytes and checks the 16-bit total against a magic constant 279 (0x117).
; Find four uppercase letters whose ASCII codes sum to the target.
; r0 = 1 on accept (nonzero rule).
;
; The target 279 exceeds one byte, so the running total is kept by hand as a
; high/low pair: r4 = high byte (carry count), r3 = low byte. Each add that wraps
; (new sum < the digit just added) bumps the carry. Expected: high=1, low=0x17.
; "HACK": 72+65+67+75 = 279. Length must be 4. Answer: HACK

        len   r1
        cmp   r1, 4
        jnz   bad           ; length must be exactly 4

        mov   r3, 0         ; low byte of the running total
        mov   r4, 0         ; high byte (carry count)
        mov   r7, 0         ; index 0..3
loop:   cmp   r7, 4
        jge   final
        ldb   r0, [r7]      ; r0 = input[r7]
        add   r3, r0        ; low += byte (wraps mod 256)
        cmp   r3, r0        ; if the new low is below the byte we added, it wrapped
        jge   nocar
        inc   r4            ; carry into the high byte
nocar:  inc   r7
        jmp   loop

final:  cmp   r4, 1         ; high byte of 279 is 1
        jnz   bad
        cmp   r3, 0x17      ; low byte of 279 is 0x17 (23)
        jnz   bad

        mov   r0, 1
        ret
bad:    mov   r0, 0
        ret