binprint.c

int write(int fd, const char*, unsigned long long length);
#define stderr_out(str) \
    write(2, str, sizeof(str))

int main(int argc, char* argv[]) {
  if (argc != 2)
  {
        stderr_out(
        "bin_print expects one argument exactly which is the number to convert in hex or dec\n");
        return -1;
    }

    unsigned long long number = 0;

    _Bool isHex = 0;
    for(char* p = argv[1]; *p; p++)
    {
        const char c = *p;
        if (!isHex)
        {
            number *= 10;
            number += (c - '0');
        }
        else
        {
            number *= 16;
            if (c >= 'a' && c <= 'f')
            {
                number += ((c - 'a') + 10);
            } else if (c >= 'A' && c <= 'F')
            {
                number += ((c - 'A') + 10);
            } else
            {
                number += (c - '0');
            }
        }

        if (!isHex && p == argv[1] + 1)
        {
            isHex = (c == 'x');
            if (isHex) number = 0;
        }
    }

    char format_buffer[66];
    {
        int i = 65;
        for(; i--;)
        {
            if (number)
            {
                format_buffer[i] = (number & 1 ? '1' : '0');
                number >>= 1;
                continue;
            }
            break;
        }
        format_buffer[65] = '\n';
        write(1, format_buffer + i + 1, 65 - i);
    }
    return 0;
}

minirt.s

.globl write
// parameters are just passed through
write:
    /* syscall write(int fd, const void *buf, size_t count) */
    mov     w8, #64     /* write is syscall #64 */
    svc     #0          /* invoke syscall */
    ret

.globl _start
_start:
    ldr w0, [sp]
    add x1, sp, #8 // this 8 would be a 4 in 32bit mode
    bl main
    b exit

.globl exit
exit:
    /* syscall exit(int status) */
    mov     w8, #93     /* exit is syscall #93 */
    svc     #0          /* invoke syscall */
    ret