String Input and Output in Assembly

Constant Strings

The bottom line is a C string is just a region of memory with some ASCIIcharacters in it. One ASCII character is one byte, and a zero byte indicates the end of the string. So any way you can create bytes with known values, you can create strings too. Here we're using the handy C function puts to print the string onto the screen.

extern puts

mov rdi,the_secret_message
call puts ; write our string to the screen

ret

the_secret_message:
	db 0x59
	db 0x75
	db 0x70
	db 0 ;<- zero byte marks end of string

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This is a pretty atrocious way to write strings, so the assembler supports a bunch of other syntaxes. These are all equivalent:

	db 0x4D, 0x6F, 0x6F, 0x73, 0x65, 0  (Try this in NetRun now!)

	db 'M', 'o', 'o', 's', 'e', 0  (Try this in NetRun now!)

	db 'Moose', 0  (Try this in NetRun now!)

In the assembler, single and double quotes are interchangable. Unlike in C++, "\n" doesn't give a newline, it prints "\n"! To get an actual newline, you need to use 0xA. Don't forget the zero byte to end the string.

	db 'Moose',0xA
	db '... and squirrel.',0

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Keep in mind that puts adds a newline at the end of the string. Call printf to avoid the newline.

Variable Strings

There are these handy C functions gets and puts that read or write strings to the screen. They both take just one argument, a pointer to the string data to read or write. For example, I can store a modifiable string statically, in "section .data":

extern gets
extern puts
mov rdi,mystring
call gets
mov rdi,mystring
call puts
ret

section .data
mystring:
	times 100 db 'v'

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*Or* I can allocate space on the stack to store the string:

extern gets
extern puts

sub rsp,100 ; allocate 100 bytes of stack space

mov rdi,rsp
call gets ; read into our string
mov rdi,rsp
call puts ; write our string to the screen

add rsp,100; give back stack space
ret

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Or I can call "malloc" to allocate space for the string. I need to use a preserved register to hang onto the allocated pointer; here I'm using r12.

extern gets
extern puts
extern malloc, free

push r12 ; preserve main's copy on the stack

mov rdi,100
call malloc
mov r12,rax ; <- malloc returns the pointer in rax

mov rdi,r12
call gets ; read into our string
mov rdi,r12
call puts ; write our string to the screen

mov rdi,r12
call free ; dispose of our copy of the string

pop r12 ; restore main's copy of this register
ret

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