Input Output Functions
Here are some print functions for strings, integers and newline characters.
There is also a function for reading an integer. All the code is in Nasm's
syntax. The macros
epilogue are used to save space and
repetitiveness. Remember that the registers
R12-R15 need to be saved across function calls.
All of the print functions here use the C library
printf() function. Ideally we
should use the
write() system call, but I haven't written an implementation of
write() yet. Similary we use
scanf() instead of the
read() system call.
The below code will be referenced as the file
asm_io.asm for the rest of the
tutorial. Make a note that instead of using the data section, the read-only data
.rodata ) has been used.
%macro prologue 0 push rbp mov rbp,rsp push rbx push r12 push r13 push r14 push r15 %endmacro %macro epilogue 0 pop r15 pop r14 pop r13 pop r12 pop rbx leave ret %endmacro section .rodata int_format db "%i",0 string_format db "%s",0 section .text global print_string, print_nl, print_int, read_int extern printf, scanf, putchar print_string: prologue ; string address has to be passed in rdi mov rsi,rdi mov rdi,dword string_format xor rax,rax call printf epilogue print_nl: prologue mov rdi,0xA xor rax,rax call putchar epilogue print_int: prologue ;integer arg is in rdi mov rsi, rdi mov rdi, dword int_format xor rax,rax call printf epilogue read_int: prologue ;rdi is assumed to have the address of the int to be read in mov rsi, rdi mov rdi, dword int_format xor rax,rax call scanf epilogue
The arguments to these functions have been passed in the
RDI register so as to
follow the x86-64 ABI specifications. It
is not necessary, however. But yes, the
C library functions need to be passed their arguments according to the x86-64 ABI specifications. Remember that
buffered output functions, i.e. they buffer the output and then print it on
screen when the buffer is full or when explicitly flushed. This is unlike the
write() system call where the data is written onto the file (or
stdout ) pointed to by the file descriptor immediately.
The below code will be placed in the include file
asm_io.inc and will be
included in other programs if they call the above defined functions.
extern print_string, print_nl, print_int, read_int