Read input from user and print it back on console in MIPS assembly

Vido · 03-28-2017, 04:17 PM

Im trying to make simple program which will read input from user and print it back to console here is part of mine program

Code:

LEAF(main)

#Print to user enter integer
    li    a0,1                # first argument fd 1
    la    a1,prompt           # second argument memory location of hello string
    li    a2,20               # lenght of string to print
    li    v0,__NR_write       # syscall write,they are defined in unistd.h
    syscall

#Read the integer and save it in s0
    li    v,__NR_read
    syscall
    move s0, v0

So mine program waits for input from user and after entering 1 or 9 the value of v0 and and S0 (inspected with gdb) are 0x2 not 0x1 and 0x9 respectively. So __NR_read require some paramiters in a a0,a1 .. registers. I found this so here are my questions

Code:

asmlinkage long sys_read(unsigned int fd, char __user *buf, size_t count);

1. firs argument is file descriptor so for stdin fd is 0
2. second argument __char *buf so its pointer to a bufer ? If I place memory address is that correct ?
3. third argument site_t count so its number of how many byts ? what to put here ? 10 ?

EDIT:

So I did tried to place memory location of var1 as second argument like so

Code:

#Read the integer and save it in s0
    nop
    nop
    li    a0,0		# firs argument, for stdin is 0 ISTR, see "man 2 read"
    la    a1,(var1)	# second argument load adress of var1 into a1
    li    a2,10 	# third argument is count of byts 
    li    v0,__NR_read
    syscall

    var1:	   .space	12

but Im getting SIGSEGV, Segmentation fault.

Vido · 03-29-2017, 10:50 AM

I loved it, I just have some leftowers which caused Segmentation fault, here is whole program

Code:

/*
 * hello-1.2/Makefile
 *
 * This file is subject to the terms and conditions of the GNU General Public
 * License.  See the file "COPYING" in the main directory of this archive
 * for more details.
 *
 * Copyright (C) 1995, 1997 by Ralf Baechle
 */
#include <regdef.h>
#include <sys/asm.h>
#include <sys/syscall.h>

EXPORT(__start)


    .set    noreorder
    LEAF(main)

#Prompt for the integer to enter
    li    a0,1		# first argument fd 1 for iutput
    la    a1,prompt	# second argument memory location of hello string
    li    a2,20		# lenght of string to print
    li    v0,__NR_write	# syscall write,they are defined in unistd.h
    syscall

#Read the integer and save it in s0
    nop
    nop
    li    a0,0		# firs argument, for stdin is 0 ISTR, see "man 2 read"
    la    a1,(var1)	# second argument load adress of var1 into a1
    li    a2,12 	# third argument is count of byts 
    li    v0,__NR_read
    syscall
    move t0, v0


#Print the message
    li    a0,1		# first argument fd 1
    la    a1,output	# second argument memory location of hello string
    li    a2,256		# lenght of string to print
    li    v0,__NR_write	# syscall write,they are defined in unistd.h
    syscall

#Output the number
    nop
    nop


quit:			# label quit
    li    v0,__NR_exit	# load system call for exit in v0
    syscall

    nop

    END(main)

    .data

prompt:    .asciiz    "Enter an intriger: "
output:    .asciiz    "\nYou typed the number "
var1:	   .space     256

sundialsvcs · 03-29-2017, 06:42 PM

How about write the thing as a one-liner in "C," and grab the assembly output from the compiler?

Vido · 03-30-2017, 10:29 AM

Quote:

Originally Posted by sundialsvcs

How about write the thing as a one-liner in "C," and grab the assembly output from the compiler?

I did tried that but not on one line and the problem is that gcc does not write registers as $t9 rather gcc write it as for example $25, and also I get stuff like

Code:

	lui	$28,%hi(__gnu_local_gp)
	addiu	$28,$28,%lo(__gnu_local_gp)

which I dont understand, so for first line it would be load upper immediate %hi would be register hi? and this thing (__gnu_local_gp) I dont know for what it stands for? Global Pointer ? And there is a lot things like so which is not match of help.