I'm starting to learn x86 assembly and tried to write a piece of code that will take input as i/o redirection and write it back as uppercase letters by i/o redirection to a file, but it seems to always cause segmentation fault although I tried to End it by a an exit system call and No output is written to the file.
READ: mov eax, 3 ; choose sys_read
mov ebx, 0 ; file descriptor stdin
mov ecx, Buffer ; pass address of buffer
mov edx, BUFFER_LENGTH ; set buffer length
int 80h ; call read()
cmp eax, 0 ; check if there is error reading
jb EXIT ; if -1 exit
je EXIT ; if 0 exit
mov esi, eax ; safe keeping number of bytes_read
dec esi ; adjust offset
mov ebp, Buffer ; Store address of Buffer in ebp
add ebp, esi ; ebp Now point to it's buffer end
; Now Start The Loop and Change Characters Needed
LOOP: cmp byte[ebp], 61h ; Check if it's equal to 'a'
jb NEXT ; GO to Next Character
cmp byte[ebp], 7Ah ; Check if it's equal to 'z'
ja NEXT ; Go to next Character
sub byte[ebp], 20h ; ELSE convert to uppercase then go to next automatically
NEXT: dec esi ; Decrement esi 'Counter'
dec ebp ; Decryment ebp "to point to previous Character"
jnz LOOP ; Go To the loop again to check Next Character until ZERO
; Once Reached Zero The ZF flag is set and write complete as Normal
WRITE: mov edx, eax ; pass how many bytes to be written
mov eax, 4 ; specify sys_write call
mov ebx, 1 ; specify file descriptor
mov ecx, Buffer ; pass buffer address "changed letter"
int 80h ; make write call
jmp READ ; Go to read again to read NEXT Chunk
EXIT: mov eax, 1 ; specify sys_exit
mov ebx, 0 ; specify return value
int 80h ; make sys_exit call
Related
THE PROGRAM IS USED TO ACCEPT CHARACTERS AND DISPLAY THEM IN REVERSE ORDER
The code is included here:
section .bss
num resb 1
section .text
global _start
_start:
call inputkey
call outputkey
;Output the number entered
mov eax, 1
mov ebx, 0
int 80h
inputkey:
;Read and store the user input
mov eax, 3
mov ebx, 2
mov ecx, num
mov edx, 1
int 80h
cmp ecx, 1Ch
je .sub2
push ecx
jmp inputkey
.sub2:
push ecx
ret
outputkey:
pop ecx
;Output the message
mov eax, 4
mov ebx, 1
;mov ecx, num
mov edx, 1
int 80h
cmp ecx, 1Ch
je .sub1
jmp outputkey
.sub1:
ret
The code to compile and run the program
logic.asm
is given here:
nasm -f elf logic.asm
ld -m elf_i386 -s -o logic logic.o
./logic
There are a few problems with the code. Firstly, for the sys_read syscall (eax = 3) you supplied 2 as the file descriptor, however 2 refers to stderr, but in this case you'd want stdin, which is 0 (I like to remember it as the non-zero numbers 1 and 2 being the output).
Next, an important thing to realize about the ret instruction is that it pops the value off the top of the stack and returns to it (treating it as an address). Meaning that even if you got to the .sub2 label, you'd likely get a segfault. With this in mind, the stack also tends to not be permanent storage, as in it is not preserved throughout procedures, so I'd recommend just making your buffer larger to e.g. 256 bytes and increment a value to point to an index in the buffer. (Using a fixed-size buffer will keep you from getting into the complications of memory allocation early, though if you want to go down that route you could do an external malloc call or just an mmap syscall.)
To demonstrate what I mean by an index into the reserved buffer:
section .bss
buf resb 256
; ...
inputkey:
xor esi, esi ; clear esi register, we'll use it as the index
mov eax, 3
mov ebx, 0 ; stdin file descriptor
mov edx, 1 ; read one byte
.l1: ; loop can start here instead of earlier, since the values eax, ebx and edx remain unchanged
lea ecx, [buf+esi] ; load the address of buf + esi
int 80h
cmp [buf+esi], 0x0a ; check for a \n character, meaning the user hit enter
je .e1
inc esi
jmp .l1
.e1:
ret
In this case, we also get to preserve esi up until the output, meaning that to reverse the input, we just print in descending order.
outputkey:
mov eax, 4
mov ebx, 1 ; stdout
mov edx, 1
.l2:
lea ecx, [buf+esi]
int 80h
test esi, esi ; if esi is zero it will set the ZF flag
jz .e2:
jmp .l2
.e2:
ret
Note: I haven't tested this code, so if there are any issues with it let me know.
I am studying Assembly language using this nasm tutorial. Here is the code that prints a string:
SECTION .data
msg db 'Hello!', 0Ah
SECTION .text
global _start
_start:
mov ebx, msg
mov eax, ebx
; calculate number of bytes in string
nextchar:
cmp byte [eax], 0
jz finished
inc eax
jmp nextchar
finished:
sub eax, ebx ; number of bytes in eax now
mov edx, eax ; number of bytes to write - one for each letter plus 0Ah (line feed character)
mov ecx, ebx ; move the memory address of our message string into ecx
mov ebx, 1 ; write to the STDOUT file
mov eax, 4 ; invoke sys_write (kernel opcode 4)
int 80h
mov ebx, 0 ; no errors
mov eax, 1 ; invoke sys_exit (kernel opcode 1)
int 80h
It works and successfully prints "Hello!\n" to STDOUT. One thing I don't understand: it searches for \0 byte in msg, but we didn't define it. Ideally, the correct message definition should be
msg db 'Hello!', 0Ah, 0h
How does it successfully get the zero byte at the end of the string?
The similar case is in exercise 7:
; String printing with line feed function
sprintLF:
call sprint
push eax ; push eax onto the stack to preserve it while we use the eax register in this function
mov eax, 0Ah ; move 0Ah into eax - 0Ah is the ascii character for a linefeed
push eax ; push the linefeed onto the stack so we can get the address
mov eax, esp ; move the address of the current stack pointer into eax for sprint
call sprint ; call our sprint function
pop eax ; remove our linefeed character from the stack
pop eax ; restore the original value of eax before our function was called
ret ; return to our program
It puts just 1 byte: 0Ah into eax without terminating 0h, but the string length is calculated correctly inside sprint. What is the cause?
I am getting a segmentation fault from this simple starting program.
I am using Ubuntu 16.10 and kdbg for debugging. Affter reaching starting __int 80h__, it stops moving to the next line.
section .bss ; section containing uninitialized data
BUFFLEN equ 1024 ; length of buffer
Buff: resb BUFFLEN ; text buffer itself
section .data ; section containing initialzed data
section .text ; secttion containing code
global _start ; linker needs to find the entry point!
_start:
nop ; this no-op keeps gdb happy
; read buffer full of text form stdin:
read:
mov eax, 3 ; specify sys_read call
mov ebx, 0 ; specify file descriptor 0 : standard input
mov ecx, Buff ; pass offset of the buffer to read to
mov edx, BUFFLEN ; pass number of bytes to be read at one pass
int 80h ; call sys_read to fill the buffer
mov esi,eax ; copy sys_read return value for safekeeping
cmp eax, 0 ; if eax = 0 , sys_read reached EOF on stdin
je Done ; jump if Equal ( to o, form compare)
; set up the register for the process buffer step:
mov ecx, esi ; place the number of bytes read into ecx
mov ebp, Buff ; pace address of buffer into ebp
dec ebp ; adjust the count to offset
; go through the buffer and cnvert lowercase to uppercase characters:
Scan:
cmp byte [ebp+ecx], 61h ; test input char agaisnst lowercase 'a'
jb Next ; if Below 'a' in ASCII, not lowercase
cmp byte [ebp+ecx], 7Ah ; test against lowercase 'z'
ja Next
sub byte [ebx+ecx], 20h ; subtract 20h to give uppercase..
Next:
dec ecx ; Decrement counter
jnz Scan ; if characters reamin, loop back
; Write the buffer full of processed text to stdout:
Write:
mov eax,4 ; Specify sys_write call
mov ebx, 1 ; Specify file descriptor 1 : stdout
mov ecx, Buff ; pass the offset of the buffer
mov edx, esi ; pass the # of bytes of data in the buffer
int 80h ; make sys_write kernel call
jmp read ; loop back and load another buffer full
Done:
mov eax, 1 ; Code for Exit sys_call
mov ebx, 0 ; return code of Zero
int 80h
I used these commands:
nasm -f elf -g -F stabs uppercaser1.asm
ld -m elf_i386 -o uppercaser1 uppercaser1.o
./uppercaser < inputflie
I think this code is generally public so I am posting with that in mind.
You should use the code as a guide to understanding what may be wrong in your code; however, it does not conform to any coding standard so it is pointless just to copy and paste and turn it in as an assignment; assuming that is the case.
You will never increase your assembly programming skills by merely doing a copy/paste.
lsb_release -a
...
Description: Ubuntu 16.04.3 LTS
nasm -f elf32 -g uppercase1.s -o uppercase1.o && ld -m elf_i386 uppercase1.o -o uppercase1
section .bss
Buff resb 1
section .data
section .text
global _start
_start:
nop
Read:
mov eax, 3 ; read syscall
mov ebx, 0 ; stdin
mov ecx, Buff ; pass address of the buffer to read to
mov edx, 1 ; read one char or one byte
int 0x80 ;
cmp eax, 0 ; if syscall returns returns 0
je Exit ;
cmp byte [Buff], 0x61 ; lower case a
jb Write ; jump if byte is below a in ASCII chart
cmp byte [Buff], 0x7a ; lower case z
ja Write ; jump if byte is above z in ASCII chart
sub byte [Buff], 0x20 ; changes the value in the buffer to an uppercase char
Write:
mov eax, 4 ; write syscall
mov ebx, 1 ; stdout
mov ecx, Buff ; what to print
mov edx, 1 ; length is one byte - each char is a byte
int 0x80
jmp Read ; go back to Read
Exit:
mov eax, 1
mov ebx, 0
int 0x80
Sample output:
david#ubuntuserver00A:~/asm$ ./uppercase1 < uppercase1.s
SECTION .BSS
BUFF RESB 1
SECTION .DATA
SECTION .TEXT
GLOBAL _START
_START:
NOP
READ:
MOV EAX, 3 ; READ SYSCALL
MOV EBX, 0 ; STDIN
MOV ECX, BUFF ; PASS ADDRESS OF THE BUFFER TO READ TO
MOV EDX, 1 ; READ ONE CHAR OR ONE BYTE
INT 0X80 ;
CMP EAX, 0 ; IF SYSCALL RETURNS RETURNS 0
JE EXIT ;
CMP BYTE [BUFF], 0X61 ; LOWER CASE A
JB WRITE ; JUMP IF BYTE IS BELOW A IN ASCII CHART
CMP BYTE [BUFF], 0X7A ; LOWER CASE Z
JA WRITE ; JUMP IF BYTE IS ABOVE Z IN ASCII CHART
SUB BYTE [BUFF], 0X20 ; CHANGES THE VALUE IN THE BUFFER TO AN UPPERCASE CHAR
WRITE:
MOV EAX, 4 ; WRITE SYSCALL
MOV EBX, 1 ; STDOUT
MOV ECX, BUFF ; WHAT TO PRINT
MOV EDX, 1 ; LENGTH IS ONE BYTE - EACH CHAR IS A BYTE
INT 0X80
JMP READ ; GO BACK TO READ
EXIT:
MOV EAX, 1
MOV EBX, 0
INT 0X80
code is as follows
getstr:
; get a LF terminated string from stdin
; in: EAX = dest buffer
; out: ax = bytes read
; EAX NOT preserved, all other registers preserved
;op mod opr1 opr2 comment
;--------------------------------------------------------
push ebx
push ecx
push edx
sub esp, 2 ; allocate memory
mov word [esp], 0x0000 ; zero memory
mov ecx, eax ; set the correct buffer
mov ebx, 0 ; stdin = 0
mov edx, 1 ; 1 byte reads
mov eax, 3 ; syscall read
.loop:
int 0x80 ; do read
test byte [ecx], 0xA
je .done
inc ecx
add word [esp], 1 ; increment the count
jmp .loop
.done:
mov byte [ecx],0x0
pop ax
pop edx
pop ecx
pop ebx
ret
gdb dump shows that 0 bytes were read
(gdb) info registers
eax 0x0 0
does anybody know what is going on here?
Two errors (assuming you use NASM):
First, int 80h / eax=3 changes eax. Thus, the next call to that function has not the wished eax, but the code 1 for exit. Move the label .loop just before the mov eax, 3 ; syscall read.
Second, test byte [ecx], 0xA doesn't compare the values. It performs an AND and sets the flags accordingly. The zero flag indicates that the result of the AND was zero. Change the line to cmp byte [ecx], 0xA.
Hi every one I am trying to do a tower of Hanoi in assembly x86 but I am trying to use arrays. So this code gets a number from user as a parameter in Linux, then error checks a bunch of stuff. So now I just want to make the algorithm which use the three arrays i made (start, end, temp) and output them step by step. If someone can help it would be greatly appreciated. `
%include "asm_io.inc"
segment .data ; where all the predefined variables are stored
aofr: db "Argument out of Range", 0 ; define aofr as a String "Argument out of Range"
ia: db "Incorrect Argument", 0 ; define ia as a String "Incorrect Argument"
tma: db "Too many Arguments", 0 ; define tma as a String "Too many Arguments"
hantowNumber dd 0 ; define hantowNumber this is what the size of the tower will be stored in
start: dd 0,0,0,0,0,0,0,0,9 ; this array is where all the rings start at
end: dd 0,0,0,0,0,0,0,0,9 ; this array is where all the rings end up at
temp: dd 0,0,0,0,0,0,0,0,9 ; this array is used to help move the rings
test: dd 0,0,0,0,0,0,0,0,9
; The next couple lines define the strings to show the pegs and what they look like
towerName: db " Tower 1 Tower 2 Tower 3 ", 10, 0
lastLineRow: db "XXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXXXXX ", 10, 0
buffer: db " ", 0
fmt:db "%d",10,0
segment .bss ; where all the input variables are stored
segment .text
global asm_main ; run the main function
extern printf
asm_main:
enter 0,0 ; setup routine
pusha
mov edx, dword 0 ; set edx to zero this is where the hantowNumber is saved for now
mov ecx, dword[ebp+8] ; ecx has how many arguments are given
mov eax, dword[ebp+12] ; save the first argument in eax
add eax, 4 ; move the pointer to the main argument
mov ebx, dword[eax] ; save the number into ebx
push ebx ; reserve ebx
push ecx ; reserve ecx
cmp ecx, dword 2 ; compare if there are more the one argument given
jg TmA ; if more then one argument is given then jump Too many Argument (TmA)
mov ecx, 0 ; ecx = 0
movzx eax, byte[ebx+ecx] ; eax is the first character number from the inputed number
sub eax, 48 ; subtract 48 to get the actual number/letter/symbol
cmp eax, 10 ; check if eax is less then 10
jg IA ; if eax is greater then 10 then it is a letter or symbol
string_To_int: ; change String to int procedure
add edx, eax ; put the number in edx
inc ecx ; increase counter (ecx)
movzx eax, byte[ebx+ecx] ; move the next number in eax
cmp eax, 0 ; if eax = 0 then there are no more numbers
mov [hantowNumber], edx ; change hantowNumber to what ever is in edx
je rangeCheck ; go to rangeCheck to check if between 2-8
sub eax, 48 ; subtract 48 to get the actual number/letter/symbol
cmp eax, 10 ; check if eax is less then 10
jg IA ; if eax is greater then 10 then it is a letter or symbol
imul edx, 10 ; multiply edx by 10 so the next number can be added to the end
jmp string_To_int ; jump back to string_To_int if not done
rangeCheck: ; check the range of the number
cmp edx, dword 2 ; compare edx with 2
jl AofR ; if hantowNumber (edx) < 2 then go to Argument out of Range (AofR)
cmp edx, dword 8 ; compare edx with 8
jg AofR ; if hantowNumber (edx) > 8 then go to Argument out of Range (AofR)
mov ecx, [hantowNumber] ; move the number enterd by user in ecx
mov esi, 28 ; esi == 28 for stack pointer counter
setStart: ; set the first array the starting peg
mov [start+esi], ecx ; put ecx into the array
sub esi, 4 ; take one away from stack pointer conter
dec ecx ; take one away from ecx so the next number can go in to the array
cmp ecx, 0 ; compare ecx with 0
jne setStart ; if ecx != 0 then go to setStart loop
; This is the section where the algoritham should go for tower of hanoi
mov ecx, [hantowNumber]
towerAlgorithm:
cmp ecx, 0
jg Exit ; jump to Exit at the end of the program
dec ecx
IA:
mov eax, ia ; put the string in eax
push eax ; reserve eax
call print_string ; output the string that is in eax
call print_nl ; print a new line after the output
pop eax ; put eax back to normal
add esp, 4 ; takes 4 from stack
jmp Exit ; jump to Exit at the end of the program
AofR:
mov eax, aofr ; put the string in eax
push eax ; reserve eax
call print_string ; output the string that is in eax
call print_nl ; print a new line after the output
pop eax ; put eax back to normal
add esp, 4 ; takes 4 from stack
jmp Exit ; jump to Exit at the end of the program
TmA:
mov eax, tma ; put the string in eax
push eax ; reserve eax
call print_string ; output the string that is in eax
call print_nl ; print a new line after the output
pop eax ; put eax back to normal
add esp, 4 ; takes 4 from stack
jmp Exit ; jump to Exit at the end of the program
Exit: ; ends the program when it jumps to here
add esp, 9 ; takes 8 from stack
popa
mov eax, 0 ; return back to C
leave
ret
haha I'm doing the exact same assignment and stuck on the algorithm however though when running your code it seems to identify "too many arguments" even though only one argument is provided, consider this algorithm when dealing with arguments(don't forget ./ is considered the "first argument" since it is the zeroth argument provided):
enter 0,0
pusha
; address of 1st argument is on stack at address ebp+12
; address of 2nd arg = address of 1st arg + 4
mov eax, dword [ebp+12] ;eax = address of 1st arg
add eax, 4 ;eax = address of 2nd arg
mov ebx, dword [eax] ;ebx = 2nd arg, it is pointer to string
mov eax, 0 ;clear the register
mov al, [ebx] ;it moves only 1 byte
sub eax, '0' ;now eax contains the numeric value of the firstcharacter of string