I am trying to take multiple inputs in a loop and display them after that. The problem is with the first iteration of the loop. The first time while going through the loop it doesn't wait for user input but instead it directly goes to the second iteration as shown on the picture:
My code is here for the loop
section .text
computer_info:
push rbp
mov rbp, rsp
sub rsp, 300
lea rbx, [computers]
add QWORD [counter], 0
.input_computers:
mov rdi, QWORD echo_computer_name
call print_string
call print_nl
call read_string
mov rsi, rax
mov rdi, rbx
mov rcx, 16
rep movsb
add rbx, 16
add QWORD [counter], 1
cmp QWORD [counter], 2
jne .input_computers
mov QWORD [counter], 0
sub rbx, rbx
.display_loop:
mov rdi, computers
add rdi, rbx
call print_string
call print_nl
add rbx, 16
add QWORD [counter], 1
cmp QWORD [counter], 2
jne .display_loop
add rsp, 300
pop rbp
ret
user_info:
push rbp
mov rbp, rsp
sub rsp, 32
call main
add rsp, 32
pop rbp
ret
search:
call main
delete:
call main
main:
; We have these three lines for compatability only
push rbp
mov rbp, rsp
sub rsp,32
mov rdi, QWORD echo_welcome
call print_string
call print_nl
mov rdi, QWORD echo_computer
call print_string
call print_nl
mov rdi, QWORD echo_user
call print_string
call print_nl
mov rdi, QWORD echo_search
call print_string
call print_nl
mov rdi, QWORD echo_delete
call print_string
call print_nl
call print_nl
mov rdi, QWORD echo_selection
call print_string
call read_int
mov rdi, rax
cmp rdi, 1
je computer_info
cmp rdi, 2
je user_info
cmp rdi, 3
je search
cmp rdi, 4
je delete
; and these lines are for compatability
add rsp, 32
pop rbp
ret
Related
On mac OS(which has intel inside), I tried to make a simple x86 hybrid program with main module written in C and a function written in x86 assembly language (NASM assembler).
Then, the following function is to reverse the string of the argument.
My C code is
#include <stdio.h>
char *revstring(char *s);
int main(int argc, char* argv[]){
for (int i=1; i<argc; i++){
printf("%s->", argv[i]);
printf("%s\n", revstring(argv[i]));
}
}
Then my assembly code
section .text
global _revstring
_revstring:
push rbp
mov rbp, rsp
mov rax, [rbp+8]
mov rcx, rax
_find_end:
mov dl, [rax]
inc rax
test dl, dl
jnz _find_end
sub rax, 2
_swap:
cmp rax, rcx
jbe _fin
mov dl, [rax]
xchg dl, [rcx]
mov [rax], dl
dec rax
inc rcx
jmp _swap
_fin:
mov rax, [rbp+8]
pop rbp
ret
Or
section .text
global _revstring
_revstring:
push rbp
mov rbp, rsp
mov rax, [rbp+8]
mov rcx, rax
find_end:
mov dl, [rax]
inc rax
test dl, dl
jnz find_end
sub rax, 2
swap:
cmp rax, rcx
jbe fin
mov dl, [rax]
xchg dl, [rcx]
mov [rax], dl
dec rax
inc rcx
jmp swap
fin:
mov rax, [rbp+8]
pop rbp
ret
Currnt MacOS cannot run 32 bit program, so I built the program by using these commands.
cc -m64 -std=c99 -c revs.c
nasm -f macho64 revstring.s
cc -m64 -o revs revs.o revstring.o
But When I enter
./revs abc123
the following error occured.
zsh: bus error ./revs abc123
I cannot find any solutions, so could anyone help me?
I am trying to understand how a variable sized static array work internally:
Following is a fixed size static array in C and its Assembly equivalent;
int main()
{
int arr[2] = {3};
}
================
main:
push rbp
mov rbp, rsp
mov QWORD PTR [rbp-8], 0
mov DWORD PTR [rbp-8], 2
mov eax, 0
pop rbp
ret
However a variable sized array is shown below
int main()
{
int varSize ;
int Arr[varSize];
}
=================
main:
push rbp
mov rbp, rsp
sub rsp, 32
mov rax, rsp
mov rcx, rax
mov eax, DWORD PTR [rbp-4]
movsx rdx, eax
sub rdx, 1
mov QWORD PTR [rbp-16], rdx
movsx rdx, eax
mov r8, rdx
mov r9d, 0
movsx rdx, eax
mov rsi, rdx
mov edi, 0
cdqe
lea rdx, [0+rax*4]
mov eax, 16
sub rax, 1
add rax, rdx
mov edi, 16
mov edx, 0
div rdi
imul rax, rax, 16
sub rsp, rax
mov rax, rsp
add rax, 3
shr rax, 2
sal rax, 2
mov QWORD PTR [rbp-24], rax
mov rsp, rcx
mov eax, 0
leave
ret
I am seeing a whole lot of assembly instructions if I declare a variable sized array. Can some one explain how is this flexibility of variable size achieved?
Same mechanism as alloca() - allocate memory by decreasing the stack pointer, with the assumption that the stack is big enough and/or the OS will grow it as needed.
There might be a bit of an issue when the requested size is over a memory page and the stack is near its end. Normally, the OS grows the stack by setting up a guard page at the stack top and watching for faults in that area, but that assumes that the stack grows more or less sequentially (by pushes and function calls). If the decreased stack pointer overshoots the guard page, it might end up pointing at a bogus location. I'm not sure what does the compiler do about that possibility.
I never learn C language so it makes me confuse. I just like to know if I did it correctly or where I need to improve. For this code I used assembly x86 32 bit. Thanks
This is what I supposed to do:
Write a procedure with the signature
char *strchar(char *s1, char c1)
that returns a pointer to the first occurrence of the character c1 within the string s1 or, if not found, returns a null.
This is what I came out with:
strchar (char*, char):
push ebp
mov ebp, esp
mov dword ptr [ebp-24], edi
mov EAX , esi
mov BYTE PTR [ebp-28], al
.L5:
mov EAX , dword ptr [ebp-24]
movzx EAX , byte ptr [ EAX ]
test AL, AL
je .L2
mov EAX , dword PTR [ebp-24]
movzx EAX , BYTE PTR [ EAX ]
cmp BYTE PTR [ebp-28], al
jne .L3
mov eax, dword PTR [ebp-24]
jmp .L6
.L3:
add dword PTR [ebp-24], 1
jmp .L5
.L2:
LEA eax, [ebp-9]
MOV DWORD PTR [EBP-8], eax
MOV EAX, DWORD PTR [ebp-8]
.L6:
POP EBP
RET
The lines:
mov dword ptr [ebp-24], edi
mov EAX , esi
mov BYTE PTR [ebp-28], al
assume that a stack frame has been allocated for this function which doesn’t appear true; I think you should have something like:
sub esp, 32
after the
mov ebp,esp
Also, the three lines after L2 seem confused. The only way to get to L2 is if the nil (0) byte is discovered in the string, at which point, the code should return a NULL pointer.
The exit path in the code (L6) leaves eax alone, so all that should be needed is:
L2:
mov eax, 0
It might make debugging easier if you kept the alias up to date; so:
L2:
mov eax, 0
mov [ebp-24], eax
Also, the calling convention used here is a bit odd: the string is passed in edi and the character in esi. Normally, in x86-32, these would both be passed on the stack. This looks like it might have been x86-64 code, converted to x86-32....
A final note; this assembly code looks like the output of a compiler with optimisations disabled. Often, generating the assembly with the optimisations enabled generates easier to understand code. This code, for example, could be much more concisely written as below, without even devolving into weird intel ops:
strchar:
mov edx, esi
mov eax, edi
L:
mov dh, [eax]
test dh, dh
jz null
cmp dh, dl
je done
inc eax
jmp L
null:
mov eax, 0
done:
ret
Here is one with stack overhead
[global strchar]
strchar:
push ebp
mov ebp, esp
mov dl, byte [ebp + 12]
mov ecx, dword [ebp + 8]
xor eax, eax
.loop: mov al, [ecx]
or al, al
jz .exit
cmp al, dl
jz .found
add ecx, 1
jmp .loop
.found: mov eax, ecx
.exit:
leave
ret
Here is one without stack overhead
[global strchar]
strchar:
mov dl, byte [esp + 8]
mov ecx, dword [esp + 4]
xor eax, eax
.loop: mov al, [ecx]
or al, al
jz .exit
cmp al, dl
jz .found
add ecx, 1
jmp .loop
.found: mov eax, ecx
.exit:
ret
These are using the 'cdecl' calling convention. For 'stdcall' change the last 'ret' to 'ret 8'.
I'm stuck at figuring out to copy the string source to target, which should be initialized to all zeroes. It appears as though I need to find the size of the string, start a counter register, and push stringitem[counter] to the stack, increment counter register. I can't figure out how to even get started, let alone search for a word in the string.
Thanks!
bits 64
global main
extern printf
section .text
main:
; function setup
push rbp
mov rbp, rsp
sub rsp, 32
;
lea rdi, [rel message]
mov al, 0
call printf
;
lea rdi, [rel source]
mov al, 0
call printf
;
;mov edi, source
;mov esi, target
;lea rdi, [esi]
;mov al, 0
;call printf
;mov ecx,sizeof source -1
; mov esi,0
;L1:
; mov eax,source[esi];
; push eax
; inc esi
; loop L1
; function return
mov eax, 0
add rsp, 32
pop rbp
ret
section .data
message: db 'Project',0x0D,0x0a,'Author',0x0D,0x0a,0
source: db 0x0D,0x0a,"I can't figure out how to copy this text to target.",0x0D,0x0a,0
target: db '0000000000000000000000000000000000000000000',0x0D,0x0a,0
For your data memory layout this will do
lea rdi, [rel target]
lea rsi, [rel source]
mov rcx, target-source
cld
rep movsb
Otherwise as Jester said, a simple byte-to-byte copy will also do
lea rdi, [rel target]
lea rsi, [rel source]
cld
.copy:
lodsb
stosb
test al, al
jnz .copy
I am trying to convert C to x86. I am using a struct...
struct person_record_struct
{
char last_name[128];
char first_name[128];
char year_of_birth[10];
int month_of_birth; // January => 1
int day_of_birth; // 1st Day of a Month => 1
char drivers_license_no[128];
};
typedef struct person_record_struct person_record;
I am having trouble getting my scanf to work. Here is the C..
result = scanf("%s\n%s\n%s\n%d\n%d\n%s\n", &records[counter].last_name[0],
&records[counter].first_name[0], &records[counter].year_of_birth[0],
&records[counter].month_of_birth, &records[counter].day_of_birth,
&records[counter].drivers_license_no[0]);
And my x86..
;counter # [ebp-4]
;records # [ebp-16]
; format_string_main_2 db '%s\n%s\n%s\n%d\n%d\n%s\n', 0
; read in info
; push drivers_license_no
mov ebx, [ebp-16] ;
mov eax, [ebp-4]
mov ecx, struct_size
mul ecx
add eax, ebx
lea eax, [eax+276]
push eax
; push day_of_birth
mov ebx, [ebp-16]
mov eax, [ebp-4]
mov ecx, struct_size
mul ecx
add eax, ebx
lea eax, [eax+272]
push eax
; push month_of_birth
mov ebx, [ebp-16]
mov eax, [ebp-4]
mov ecx, struct_size
mul ecx
add eax, ebx
lea eax, [eax+268]
push ax
; push year_of_birth
mov ebx, [ebp-16]
mov eax, [ebp-4]
mov ecx, struct_size
mul ecx
add eax, ebx
lea eax, [eax+256]
push eax
; push first_name
mov ebx, [ebp-16]
mov eax, [ebp-4]
mov ecx, struct_size
mul ecx
add eax, ebx
lea eax, [eax+128]
push eax
; push last_name
mov ebx, [ebp-16]
mov eax, [ebp-4]
mov ecx, struct_size
mul ecx
add eax, ebx
lea eax, [eax+0]
push eax
push format_string_main_2
call scanf
add esp, 28
mov [ebp-12], eax
I'm using a check to see if result is 6 and if it's not my program that prints an error and exits. It keeps having an error and I'm not sure what I am doing wrong. Any help would be much appreciated. Thank you.
This is my calloc call which appears to be correct...
; // allocate the buffer of all the records
; records = (person_record *)calloc(number_of_records, sizeof(person_record));
push struct_size
mov eax, [ebp-8]
push eax
call calloc
add esp, 8
mov [ebp-16], eax
Under month_of_birth you have push ax instead of push eax. This would push only the lower 16 bits of the address on the stack, virtually guaranteeing a crash in scanf. Fix that and it should be OK.
There are many weird/wrong things going on in your code. It will be easier to show a cleaner way. You have not mentioned the Assembler you are using, there are a few for x86 and each has its own syntax. Here is how you can do it using NASM:
extern printf, scanf, calloc, exit, free, puts
global main
struc person_record
.last_name resb 128
.first_name resb 128
.year_of_birth resb 10
.month_of_birth resd 1
.day_of_birth resd 1
.drivers_license_no resb 128
.size equ $ - person_record
endstruc
MAX_RECORDS equ 2
section .data
Space db 32, 0
input_format db "%s%s%s%d%d%s", 0
output_format db "%s %s %s %d %d %s", 0
section .text
main:
push person_record.size
push MAX_RECORDS
call calloc
add esp, 4 * 2
mov esi, eax
mov ebx, eax
mov edi, MAX_RECORDS - 1
.FillRecord:
lea eax, [ebx + person_record.drivers_license_no]
push eax
lea ecx, [ebx + person_record.day_of_birth]
push ecx
lea edx, [ebx + person_record.month_of_birth]
push edx
lea eax, [ebx + person_record.year_of_birth]
push eax
lea ecx, [ebx + person_record.first_name]
push ecx
lea edx, [ebx + person_record.last_name]
push edx
push input_format
call scanf
add esp, 4 * 7
push Space
call puts
add esp, 4 * 1
add ebx, person_record.size
dec edi
jns .FillRecord
mov ebx, esi
mov edi, MAX_RECORDS - 1
.ShowRecord:
lea eax, [ebx + person_record.drivers_license_no]
push eax
mov ecx, [ebx + person_record.day_of_birth]
push ecx
mov edx, [ebx + person_record.month_of_birth]
push edx
lea eax, [ebx + person_record.year_of_birth]
push eax
lea ecx, [ebx + person_record.first_name]
push ecx
lea edx, [ebx + person_record.last_name]
push edx
push output_format
call printf
add esp, 4 * 7
push Space
call puts
add esp, 4 * 1
add ebx, person_record.size
dec edi
jns .ShowRecord
push esi
call free
add esp, 4 * 1
push 0
call exit
And the input and output of 2 records: