I’m puzzled by the code from the Pintos OS, named loader.S. I don’t get how the string .string “PiLo” is used by puts when there’s no label involved.

    sub %dx, %dx            # Serial port 0.
    mov $0xe3, %al          # 9600 bps, N-8-1.
                    # AH is already 0 (Initialize Port).
    int $0x14           # Destroys AX.

    call puts
    .string "PiLo"

puts:
    xchg %si, %ss:(%esp)
    push %ax
next_char:
    mov %cs:(%si), %al
    inc %si
    test %al, %al
    jz 1f
    call putc
    jmp next_char
1:  pop %ax
    xchg %si, %ss:(%esp)
    ret

putc:
    pusha

1:  sub %bh, %bh            # Page 0.
    mov $0x0e, %ah          # Teletype output service.
    int $0x10

    mov $0x01, %ah          # Serial port output service.
    sub %dx, %dx            # Serial port 0.
2:  int $0x14           # Destroys AH.
    test $0x80, %ah         # Output timed out?
    jz 3f
    movw $0x9090, 2b        # Turn "int $0x14" above into NOPs.

3:
    cmp $'r', %al
    jne popa_ret
    mov $'n', %al
    jmp 1b

if someone can help to understand the code.