Skipping and Stopping

Part of the Arithmetic and Control Flow section of Coddy's Assembly journey. Lesson 14 of 28.

A branch can skip selected work without ending the loop. A branch to the final label stops it. When skipping, still advance the counter; otherwise the same value can be tested forever.

mov rax, 0
mov rcx, 1
.loop:
cmp rcx, 4
jg .done
cmp rcx, 2
je .advance
add rax, rcx
.advance:
inc rcx
jmp .loop
.done:

The code skips 2 but advances afterward, so it adds 1 + 3 + 4.

A skip path still reaches the counter update; a stop path leaves the loop.

Use these instruction excerpts inside the supplied solve routine; the challenge provides the input and output code.

Challenge

Easy

Sum integers from 1 through r8, except the value equal to r9. The limit is between 0 and 30; the excluded value is between 0 and 40 and may be outside the range.

Edit solution.asm between solve: and the supplied ret. The locked main.asm reads the test numbers into r8, r9, r10, and r11 in order, and also stores them as four consecutive 8-byte integers at values. Unused inputs are zero. Leave your answer in rax; the harness prints it as one signed decimal integer followed by a newline. Keep the supplied wrapper and do not print anything yourself.

Try it yourself

bits 64
section .bss
    input_buffer resb 256
    output_buffer resb 32
    values resq 4
section .text
    global _start
    global values
    extern solve
_start:
    xor eax, eax
    xor edi, edi
    mov rsi, input_buffer
    mov edx, 255
    syscall
    test rax, rax
    jle .loaded
    mov rsi, input_buffer
    lea rdi, [input_buffer + rax]
    xor ecx, ecx
.scan:
    cmp rsi, rdi
    jae .loaded
    cmp ecx, 4
    jae .loaded
    movzx eax, byte [rsi]
    cmp al, 32
    jbe .space
    mov r10, 1
    cmp al, '-'
    jne .number
    mov r10, -1
    inc rsi
.number:
    xor rax, rax
.digit:
    cmp rsi, rdi
    jae .store
    movzx edx, byte [rsi]
    cmp dl, '0'
    jb .store
    cmp dl, '9'
    ja .store
    imul rax, rax, 10
    sub edx, '0'
    add rax, rdx
    inc rsi
    jmp .digit
.store:
    imul rax, r10
    mov [values + rcx * 8], rax
    inc ecx
.space:
    inc rsi
    jmp .scan
.loaded:
    mov r8, [values]
    mov r9, [values + 8]
    mov r10, [values + 16]
    mov r11, [values + 24]
    xor eax, eax
    call solve
    lea rsi, [output_buffer + 31]
    mov byte [rsi], 10
    mov ecx, 1
    xor r8d, r8d
    test rax, rax
    jns .positive
    mov r8d, 1
    neg rax
.positive:
    mov ebx, 10
.convert:
    xor edx, edx
    div rbx
    add dl, '0'
    dec rsi
    mov [rsi], dl
    inc ecx
    test rax, rax
    jnz .convert
    test r8d, r8d
    jz .write
    dec rsi
    mov byte [rsi], '-'
    inc ecx
.write:
    mov edx, ecx
    mov eax, 1
    mov edi, 1
    syscall
    mov eax, 60
    xor edi, edi
    syscall