Reading a single-key input on Linux (without waiting for return) using x86_64 sys_call - linux

Question

I want to make Linux just take 1 keystroke from keyboard using sys_read, but sys_read just wait until i pressed enter. How to read 1 keystroke ? this is my code:
Mov EAX,3
Mov EBX,0
Mov ECX,Nada
Mov EDX,1
Int 80h
Cmp ECX,49
Je Do_C
Jmp Error
I already tried using BIOS interrupt but it's failed (Segmentation fault), I want capture number 1 to 8 input from keyboard.

Answer 1

Syscalls in 64-bit linux
The tables from man syscall provide a good overview here:
arch/ABI instruction syscall # retval Notes
──────────────────────────────────────────────────────────────────
i386 int $0x80 eax eax
x86_64 syscall rax rax See below
arch/ABI arg1 arg2 arg3 arg4 arg5 arg6 arg7 Notes
──────────────────────────────────────────────────────────────────
i386 ebx ecx edx esi edi ebp -
x86_64 rdi rsi rdx r10 r8 r9 -
I have omitted the lines that are not relevant here. In 32-bit mode, the parameters were transferred in ebx, ecx, etc and the syscall number is in eax. In 64-bit mode it is a little different: All registers are now 64-bit wide and therefore have a different name. The syscall number is still in eax, which now becomes rax. But the parameters are now passed in rdi, rsi, etc. In addition, the instruction syscall is used here instead of int 0x80 to trigger a syscall.
The order of the parameters can also be read in the man pages, here man 2 ioctl and man 2 read:
int ioctl(int fd, unsigned long request, ...);
ssize_t read(int fd, void *buf, size_t count);
So here the value of int fd is in rdi, the second parameter in rsi etc.
How to get rid of waiting for a newline
Firstly create a termios structure in memory (in .bss section):
termios:
c_iflag resd 1 ; input mode flags
c_oflag resd 1 ; output mode flags
c_cflag resd 1 ; control mode flags
c_lflag resd 1 ; local mode flags
c_line resb 1 ; line discipline
c_cc resb 19 ; control characters
Then get the current terminal settings and disable canonical mode:
; Get current settings
mov eax, 16 ; syscall number: SYS_ioctl
mov edi, 0 ; fd: STDIN_FILENO
mov esi, 0x5401 ; request: TCGETS
mov rdx, termios ; request data
syscall
; Modify flags
and byte [c_lflag], 0FDh ; Clear ICANON to disable canonical mode
; Write termios structure back
mov eax, 16 ; syscall number: SYS_ioctl
mov edi, 0 ; fd: STDIN_FILENO
mov esi, 0x5402 ; request: TCSETS
mov rdx, termios ; request data
syscall
Now you can use sys_read to read in the keystroke:
mov eax, 0 ; syscall number: SYS_read
mov edi, 0 ; int fd: STDIN_FILENO
mov rsi, buf ; void* buf
mov rdx, len ; size_t count
syscall
Afterwards check the return value in rax: It contains the number of characters read.
(Or a -errno code on error, e.g. if you closed stdin by running ./a.out <&- in bash. Use strace to print a decoded trace of the system calls your program makes, so you don't need to actually write error handling in toy experiments.)
References:
What happens if you use the 32-bit int 0x80 Linux ABI in 64-bit code?
Why does the sys_read system call end when it detects a new line?
How do i read single character input from keyboard using nasm (assembly) under ubuntu?
Using the raw keyboard mode under Linux (external site with example in 32-bit assembly)