Select in Assembly Language

Assembly language doesn’t have direct equivalents for many high-level concepts like channels or goroutines. However, we can demonstrate a similar concept using interrupts and polling to simulate concurrent operations.

section .data
    msg1 db 'received one', 0
    msg2 db 'received two', 0
    fmt db '%s', 10, 0   ; Format string for printf

section .text
    extern printf
    global main

main:
    ; Set up stack frame
    push rbp
    mov rbp, rsp

    ; Initialize counter
    mov r12, 0

.loop:
    ; Check if we've printed both messages
    cmp r12, 2
    je .done

    ; Simulate select by checking flags
    test byte [timer1], 1
    jnz .print_one
    test byte [timer2], 1
    jnz .print_two

    ; If neither flag is set, continue looping
    jmp .loop

.print_one:
    ; Print "received one"
    mov rdi, fmt
    mov rsi, msg1
    xor rax, rax
    call printf

    ; Clear the flag and increment counter
    mov byte [timer1], 0
    inc r12
    jmp .loop

.print_two:
    ; Print "received two"
    mov rdi, fmt
    mov rsi, msg2
    xor rax, rax
    call printf

    ; Clear the flag and increment counter
    mov byte [timer2], 0
    inc r12
    jmp .loop

.done:
    ; Clean up stack frame and return
    mov rsp, rbp
    pop rbp
    xor rax, rax
    ret

section .bss
    timer1 resb 1
    timer2 resb 1

section .text
    global TIMER1_ISR
    global TIMER2_ISR

TIMER1_ISR:
    mov byte [timer1], 1
    iretq

TIMER2_ISR:
    mov byte [timer2], 1
    iretq

This Assembly code simulates the behavior of the original example using a different approach:

1. We define two messages: “received one” and “received two”.

2. The main function contains a loop that checks two flags (timer1 and timer2) repeatedly.

3. When a flag is set, the corresponding message is printed, the flag is cleared, and a counter is incremented.

4. The loop continues until both messages have been printed.

5. We define two interrupt service routines (ISRs) that set the flags when called. These ISRs would be triggered by hardware timers in a real system.

To run this program, you would need to:

1. Assemble the code into an object file.
2. Link it with a C runtime that provides printf.
3. Set up the interrupt descriptor table to use the defined ISRs.
4. Configure hardware timers to trigger the interrupts after 1 and 2 seconds respectively.

This Assembly implementation doesn’t truly run concurrently like the original example. Instead, it uses interrupts to simulate concurrent events and polls flags in a loop to check for these events. This approach is more typical in low-level systems programming where true concurrency might not be available.

The output would be similar to the original, with “received one” printed after about 1 second and “received two” after about 2 seconds, although the exact timing would depend on the system’s timer resolution and interrupt latency.