.intel_syntax noprefix

# syscall numbers
.equ SYS_READ,      0
.equ SYS_WRITE,     1
.equ SYS_OPEN,      2
.equ SYS_CLOSE,     3
.equ SYS_POLL,      7
.equ SYS_IOCTL,     16
.equ SYS_EXIT,      60

# ioctl / termios
.equ TCGETS,        0x5401
.equ TCSETS,        0x5402
.equ TERMIOS_SIZE,  60      # sizeof(struct termios) on x86_64

# termios c_lflag bits to clear
.equ ICANON,        0x2
.equ ECHO,          0x8
.equ ISIG,          0x1

# termios c_iflag bits to clear
.equ IGNBRK,        0x1
.equ BRKINT,        0x2
.equ PARMRK,        0x8
.equ ISTRIP,        0x20
.equ INLCR,         0x40
.equ IGNCR,         0x80
.equ ICRNL,         0x100
.equ IXON,          0x400

# termios c_oflag bits to clear
.equ OPOST,         0x1

# termios c_cflag
.equ CSIZE,         0x30
.equ PARENB,        0x100
.equ CS8,           0x30
.equ CLOCAL,        0x800
.equ CREAD,         0x80

# baud
.equ B115200,       0x1002

# open flags
.equ O_RDWR,        0x2
.equ O_NOCTTY,      0x100

# poll
.equ POLLIN,        0x1
.equ POLLFD_SIZE,   8

# termios field offsets
.equ OFLAG_OFF,     4
.equ CFLAG_OFF,     8
.equ LFLAG_OFF,     12
.equ CC_OFF,        17      # c_cc array offset
.equ VMIN,          6       # c_cc index
.equ VTIME,         5       # c_cc index
.equ ISPEED_OFF,    52
.equ OSPEED_OFF,    56

.data
devpath: .asciz "/dev/ttyUSB0"
banner: .ascii "Holck's minitty - ctrl-d to exit!\r\n"
.equ BANNER_LEN, . - banner

.bss
.align 8
serial_fd:   .quad 0
old_termios: .space TERMIOS_SIZE
raw_termios: .space TERMIOS_SIZE
ser_termios: .space TERMIOS_SIZE
pollfds:     .space POLLFD_SIZE * 2     # two pollfd structs
buf:         .space 256

.text
.globl _start

_start:
    # Open serial port
    mov rdi, offset devpath
    mov rsi, O_RDWR | O_NOCTTY
    xor rdx, rdx
    mov rax, SYS_OPEN
    syscall
    test rax, rax
    js exit_fail
    mov [serial_fd], rax

    # Get serial termios & configure
    mov rdi, rax
    mov rsi, TCGETS
    lea rdx, [ser_termios]
    mov rax, SYS_IOCTL
    syscall

    # cfmakeraw equivalent for serial
    lea rbx, [ser_termios]
    # iflag: clear everything raw clears
    and dword ptr [rbx], ~(IGNBRK|BRKINT|PARMRK|ISTRIP|INLCR|IGNCR|ICRNL|IXON)
    # oflag: clear OPOST
    and dword ptr [rbx + OFLAG_OFF], ~OPOST
    # cflag: CS8, CLOCAL, CREAD, no parity
    mov eax, [rbx + CFLAG_OFF]
    and eax, ~(CSIZE | PARENB)
    or  eax, CS8 | CLOCAL | CREAD
    mov [rbx + CFLAG_OFF], eax
    # lflag: clear ECHO, ICANON, ISIG
    and dword ptr [rbx + LFLAG_OFF], ~(ECHO|ICANON|ISIG)
    # VMIN=0, VTIME=1
    mov byte ptr [rbx + CC_OFF + VMIN], 0
    mov byte ptr [rbx + CC_OFF + VTIME], 1
    # baud 115200
    mov dword ptr [rbx + ISPEED_OFF], B115200
    mov dword ptr [rbx + OSPEED_OFF], B115200

    mov rdi, [serial_fd]
    mov rsi, TCSETS
    lea rdx, [ser_termios]
    mov rax, SYS_IOCTL
    syscall

    # Save stdin termios
    xor rdi, rdi
    mov rsi, TCGETS
    lea rdx, [old_termios]
    mov rax, SYS_IOCTL
    syscall

    # Copy old to raw and make raw
    lea rsi, [old_termios]
    lea rdi, [raw_termios]
    mov rcx, TERMIOS_SIZE
    rep movsb

    lea rbx, [raw_termios]
    and dword ptr [rbx], ~(IGNBRK|BRKINT|PARMRK|ISTRIP|INLCR|IGNCR|ICRNL|IXON)
    and dword ptr [rbx + OFLAG_OFF], ~OPOST
    mov eax, [rbx + CFLAG_OFF]
    and eax, ~(CSIZE | PARENB)
    or  eax, CS8
    mov [rbx + CFLAG_OFF], eax
    and dword ptr [rbx + LFLAG_OFF], ~(ECHO|ICANON|ISIG)
    mov byte ptr [rbx + CC_OFF + VMIN], 1
    mov byte ptr [rbx + CC_OFF + VTIME], 0

    # Set stdin raw
    xor rdi, rdi
    mov rsi, TCSETS
    lea rdx, [raw_termios]
    mov rax, SYS_IOCTL
    syscall

    # Print banner
    mov rdi, 1
    lea rsi, [banner]
    mov rdx, BANNER_LEN
    mov rax, SYS_WRITE
    syscall

    # Setup pollfds: [0]=stdin, [1]=serial
    lea rbx, [pollfds]
    mov dword ptr [rbx], 0              # fd = stdin
    mov word ptr [rbx + 4], POLLIN      # events
    mov word ptr [rbx + 6], 0           # revents

    mov eax, [serial_fd]
    mov dword ptr [rbx + 8], eax        # fd = serial
    mov word ptr [rbx + 12], POLLIN
    mov word ptr [rbx + 14], 0

poll_loop:
    lea rdi, [pollfds]
    mov rsi, 2
    mov rdx, -1                         # infinite timeout
    mov rax, SYS_POLL
    syscall

    # Check stdin
    lea rbx, [pollfds]
    movzx eax, word ptr [rbx + 6]       # revents
    test eax, POLLIN
    jz check_serial

    # Read stdin
    xor rdi, rdi
    lea rsi, [buf]
    mov rdx, 256
    mov rax, SYS_READ
    syscall
    test rax, rax
    jle done

    # Scan for Ctrl-D (0x04)
    mov rcx, rax
    lea rsi, [buf]
scan:
    cmp byte ptr [rsi], 4
    je done
    inc rsi
    dec rcx
    jnz scan

    # Write to serial
    mov rdi, [serial_fd]
    lea rsi, [buf]
    mov rdx, rax
    mov rax, SYS_WRITE
    syscall

check_serial:
    lea rbx, [pollfds]
    movzx eax, word ptr [rbx + 14]      # revents
    test eax, POLLIN
    jz poll_loop

    # Read serial
    mov rdi, [serial_fd]
    lea rsi, [buf]
    mov rdx, 256
    mov rax, SYS_READ
    syscall
    test rax, rax
    jle poll_loop

    # Write to stdout
    mov rdx, rax
    mov rdi, 1
    lea rsi, [buf]
    mov rax, SYS_WRITE
    syscall
    jmp poll_loop

done:
    # Restore stdin termios
    xor rdi, rdi
    mov rsi, TCSETS
    lea rdx, [old_termios]
    mov rax, SYS_IOCTL
    syscall

    # Close serial
    mov rdi, [serial_fd]
    mov rax, SYS_CLOSE
    syscall

    xor rdi, rdi
    mov rax, SYS_EXIT
    syscall

exit_fail:
    mov rdi, 1
    mov rax, SYS_EXIT
    syscall