My assignment for school is to loop through a sequence of characters in a string and swap them such that the end result is the original string in reverse.
I have written 3 assembly functions and one cpp function but on the function below I am getting a few errors when I try to run the program and I'm not sure how to fix it. I will post both the cpp code and assembly code below with the errors pointed out, if anyone could point out what my mistake is I would appreciate it a lot!
My c++ code is below
#include<iostream>
#include <string>
using namespace std;
extern"C"
char reverse(char*, int);
int main()
{
char str[64] = {NULL};
int lenght;
cout << " Please Enter the text you want to reverse:";
cin >> str;
lenght = strlen(str);
reverse(str, lenght);
cout << " the reversed of the input is: " << str << endl;
}
Below is my assembly code
.model flat
.code
_reverse PROC ;named _test because C automatically prepends an underscode, it is needed to interoperate
push ebp
mov ebp,esp ;stack pointer to ebp
mov ebx,[ebp+8] ; address of first array element
mov ecx,[ebp+12] ; the number of elemets in array
mov eax,ebx
mov ebp,0 ;move 0 to base pointer
mov edx,0 ; set data register to 0
mov edi,0
Setup:
mov esi , ecx
shr ecx,1
add ecx,edx
dec esi
reverse:
cmp ebp , ecx
je allDone
mov edx, eax
add eax , edi
add edx , esi
LoopMe:
mov bl, [edx]
mov bh, [eax]
mov [edx],bh
mov [eax],bl
inc edi
dec esi
cmp edi, esi
je allDone
inc ebp
jmp reverse
allDone:
pop ebp ; pop ebp out of stack
ret ; retunr the value of eax
_reverse ENDP
END
On the line close to the beginning where it reads push ebp I'm getting an error that says
invalid instruction operands
and towards the end where it reads pop ebp I'm getting an error where it says the same thing.
Not sure if this is big but I'm also getting a syntax error on the very first line of code that reads .model flat.
Based on reproducing the symptoms, I diagnose the problem as: this is 32-bit x86 assembly (clearly), but it was treated as x64 assembly, and that didn't work.
the .model directive is not valid for x64, so there is a syntax error there.
pushing and popping 32-bit registers is not encodeable in x64, so there are invalid operand errors there.
If this is in a project in Visual Studio, set the "platform" for either the whole solution or this individual project to x86/win32 (it has different names in different places, but set it to 32-bit).
Related
My assignment is to Implement a function in assembly that would do the following:
loop through a sequence of characters and swap them such that the end result is the original string in reverse ( 100 points )
Hint: collect the string from user as a C-string then pass it to the assembly function along with the number of characters entered by the user. To find out the number of characters use strlen() function.
i have written both c++ and assembly programs and it works fine for extent: for example if i input 12345 the out put is correctly shown as 54321 , but if go more than 5 characters : the out put starts to be incorrect: for example if i input 123456 the output is :653241. i will greatly appreciate anyone who can point where my mistake is:
.code
_reverse PROC
push ebp
mov ebp,esp ;stack pointer to ebp
mov ebx,[ebp+8] ; address of first array element
mov ecx,[ebp+12] ; the number of elemets in array
mov eax,ebx
mov ebp,0 ;move 0 to base pointer
mov edx,0 ; set data register to 0
mov edi,0
Setup:
mov esi , ecx
shr ecx,1
add ecx,edx
dec esi
reverse:
cmp ebp , ecx
je allDone
mov edx, eax
add eax , edi
add edx , esi
Swap:
mov bl, [edx]
mov bh, [eax]
mov [edx],bh
mov [eax],bl
inc edi
dec esi
cmp edi, esi
je allDone
inc ebp
jmp reverse
allDone:
pop ebp ; pop ebp out of stack
ret ; retunr the value of eax
_reverse ENDP
END
and here is my c++ code:
#include<iostream>
#include <string>
using namespace std;
extern"C"
char reverse(char*, int);
int main()
{
char str[64] = {NULL};
int lenght;
cout << " Please Enter the text you want to reverse:";
cin >> str;
lenght = strlen(str);
reverse(str, lenght);
cout << " the reversed of the input is: " << str << endl;
}
You didn't comment your code, so IDK what exactly you're trying to do, but it looks like you are manually doing the array indexing with MOV / ADD instead of using an addressing mode like [eax + edi].
However, it looks like you're modifying your original value and then using it in a way that would make sense if it was unmodified.
mov edx, eax ; EAX holds a pointer to the start of array, read every iter
add eax , edi ; modify the start of the array!!!
add edx , esi
Swap:
inc edi
dec esi
EAX grows by EDI every step, and EDI increases linearly. So EAX increases geometrically (integral(x * dx) = x^2).
Single-stepping this in a debugger should have found this easily.
BTW, the normal way to do this is to walk one pointer up, one pointer down, and fall out of the loop when they cross. Then you don't need a separate counter, just cmp / ja. (Don't check for JNE or JE, because they can cross each other without ever being equal.)
Overall you the right idea to start at both ends of the string and swap elements until you get to the middle. Implementation is horrible though.
mov ebp,0 ;move 0 to base pointer
This seems to be loop counter (comment is useless or even worse); I guess idea was to swap length/2 elements which is perfectly fine. HINT I'd just compare pointers/indexes and exit once they collide.
mov edx,0 ; set data register to 0
...
add ecx,edx
mov edx, eax
Useless and misleading.
mov edi,0
mov esi , ecx
dec esi
Looks like indexes to start/end of the string. OK. HINT I'd go with pointers to start/end of the string; but indexes work too
cmp ebp , ecx
je allDone
Exit if did length/2 iterations. OK.
mov edx, eax
add eax , edi
add edx , esi
eax and edx point to current symbols to be swapped. Almost OK but this clobbers eax! Each loop iteration after second will use wrong pointers! This is what caused your problem in the first place. This wouldn't have happened if you used pointers instead indexes, or if you'd used offset addressing [eax+edi]/[eax+esi]
...
Swap part is OK
cmp edi, esi
je allDone
Second exit condition, this time comparing for index collision! Generally one exit condition should be enough; several exit conditions usually either superfluous or hint at some flaw in the algorithm. Also equality comparison is not enough - indexes can go from edi<esi to edi>esi during single iteration.
I'm using Visual Studio to write an assembly program using C++. I'm getting two errors:
A2071, initializer magnitude too large for specified size. It states this in my assembly file at Line 1, but that's where I have the processor (.586) at. Even if I move it to line 2, it says the issue is at line 1 (even when line 1 is empty?
I have an A2044 code. Invalid character in file. However, I don't believe anything non-standard is in the code.
I went to a professor at my university and he couldn't figure it out either.
.586
.MODEL FLAT, C ;Use flat memory model and C calling conventions
.STACK ;Define a stack segment of 1KB
.DATA
string DB "ATest This String?.,/[}", '$'
strSize DD 23
.CODE
strToLower PROC
LEA EAX, string
PUSH EAX
CALL toLower2 ; write toLower2
POP EAX
LEA EAX, string ; return char* to C++
RET
strToLower ENDP
;---------------------------------------------
;Procedure: Convert to LowerCase
;Input: Address in EBX
; unsigned in AL for each letter
;Output: EAX will contain new string
;---------------------------------------------
toLower2 PROC ;65-90 is upper, 97-122 is lower (XOR 32?)
LEA EBX, string
MOV ECX, strSize
PUSH AX ; PUSH AL before manipulating it
loop1: MOV AL, [EBX] ; Put char into AL to manipulate
XOR BL, BL ;?????????????
MOV BL, AL ;Set condition here???
SUB BL, 65 ;Capital A (65)
CMP BL, 25 ;if(i > 64 && < 90) i += 32;
JA NoCap ;
ADD AL, 32 ;Adds 32 to ASCII value, making lower
NoCap: MOV [EBX], AL
INC EBX
LOOP loop1
POP AX ;Replace/POP AL
LEA EAX, string
toLower2 ENDP
END
My C++ looks like this:
extern "C" { char* strToLower(); }
#include <iostream>
using namespace std;
int main()
{
char* pChar = strToLower(); // prints atest this string?.,/[}$
cout << pChar << endl;
system("Pause");
return 1;
}
this is the c++ file:
#include <iostream>
using namespace std;
//This is the C prototype of the assembly function, it requires extern"C" to
//show the name is going to be decorated as _test and not the C++ way of
//doing things
extern"C"
{
//char arrayReverse(char*, int);
int numChars(char *, int);
char swapChars(char *, int);
}
int main()
{
const int SIZE = 7;
char arr[SIZE] = { 'h', 'e', 'l', 'l', 'o', '1', '2' };
int val1 = numChars(arr, SIZE);
cout << "The number of elements is: " << val1 << endl;
char val2 = swapChars(arr, SIZE);
cout << "Swapped! " << endl;
return 0;
}
and my swapChars() file:
.686
.model flat
.code
_swapChars PROC ; named _test because C automatically prepends an underscode, it is needed to interoperate
push ebp
mov ebp,esp ; stack pointer to ebp
mov ebx,[ebp+8] ; address of first array element
mov ecx,[ebp+12] ; number of elements in array
mov ebp,0
mov eax,0
mov edx,ebx ;move first to edx
add edx, 7 ;last element in the array
loopMe:
cmp ebp, ecx ;comparing iterator to total elements
jge nextLoopMe
mov eax, [ebx] ;move 1st element into tmp eax
mov ebx, [edx] ;move last element into first
mov edx, eax ;move tmp into last
push ebx ;push first element onto stack
add ebx, 1 ;first + 1
sub edx, 1 ;last - 1
add ebp, 1 ;increment
jmp loopMe
nextLoopMe:
mov ebx,[ebp+8] ;find first element again USING AS FFRAME POINTER AGAIN
cmp ebx, ecx ;comparing first element to number of elements
je allDone
pop ebx
add ebx, 1
jmp nextLoopMe
allDone:
pop ebp
ret
_swapChars ENDP
END
This is supposed to take the value in arr[0] and swap that with arr[6], arr[1] with arr[5] etc. until the entire array is swapped and then display it. I don't know if any of the code I wrote does anything I want it to, but I'm looking for a way to see what is going on.
Is there a way I can make the asm code print something to the console while iterating through the loop?
Do brackets around the register ( [ebx] ) mean the value for the register?
When in loopMe:, the third line
mov eax, [ebx]
I get an exception "Exception thrown at 0x012125FC in assignment4.exe: 0xC0000005: Access violation reading location 0xCCCCCCCD."
Am I handling the swaps correctly?
Thanks for your time.
You do really need to learn to use the debugger to step thru this. That said, here are some problems I see.
add edx,7
will point edx past the end of your array. Just like arr[7] would in the C code. It should be add edx,6 to point edx to the last character.
It's error prone to change ebp in your the middle of your proc and I think you have an error there. You change it's value, but then expect [ebp+8] to reference the same data later.
You are not modifying the list correctly either. To move a char from one element to another you would do something like:
mov al, [ebx] ; copy byte from address ebx to register al
mov [edx], al ; copy byte in register al into address edx
The eax register is 32-bits and will copy 4 bytes at a time, not 1.
First of all, your code is not safe since you forget to add \0 at the end of the char array. When you use function to process your char, or char string, it will initiate a memory leak. The size should be 8 and the last one in the array should be \0.
I am working on a project where we need to pass an array of type char as a parameter and reverse the array. I feel like I am very close to getting it done, but I am stuck on the actual swapping process.
For my swapping function in my .asm, I used the same method I would in c++ (use an unused register as a temp, then swap the front and the back.) What I am not understanding is how would I go about changing the actual content at that address. I assumed performing the following would "change" the content at the destination address:
mov eax,[edx]
However, this did not work as planned. After I ran a for loop to iterate through the array again, everything stayed the same.
If anyone can point me in the right direction, it would be great. I have provided the code below with as much comments as I could provide.
Also, I am doing all this in a single .asm file; however, my professor wants me to have 3 separate .asm document for each of the following functions: swap, reverse, and getLength. I tried to include the other 2 .asm document in the reverse.asm, but it kept giving me an error.
Assembly Code Starts:
.686
.model flat
.code
_reverse PROC
push ebp
mov ebp,esp ;Have ebp point to esp
mov ebx,[ebp+8] ;Point to beginning of array
mov eax,ebx
mov edx,1
mov ecx,0
mov edi,0
jmp getLength
getLength:
cmp ebp, 0 ;Counter to iterate until needed to stop
je setup
add ecx,1
mov ebp,[ebx+edx]
add edx,1
jmp getLength
setup: ;This is to set up the numbers correctly and get array length divided by 2
mov esi,ecx
mov edx,0
mov eax,ecx
mov ecx,2
div ecx
mov ecx,eax
add ecx,edx ;Set up ecx(Length of string) correctly by adding modulo if odd length string
mov eax,ebx
dec esi
jmp reverse
reverse: ;I started the reverse function by using a counter to iterate through length / 2
cmp edi, ecx
je allDone
mov ebx,eax ;Set ebx to the beginning of array
mov edx,eax ;Set edx to the beginning of array
add ebx,edi ;Move ebx to correct index to perform swap
add edx,esi ;Move edx to the back at the correct index
jmp swap ;Invoke swap function
swap:
mov ebp,ebx ;Move value to temp
mov ebx,[edx] ;Swap the back end value to the front
mov edx,[edx] ;Move temp to back
inc edi ;Increment to move up one index to set up next swap
dec esi ;Decrement to move back one index to set up for next swap
jmp reverse ;Jump back to reverse to setup next index swapping
allDone:
pop ebp
ret
_reverse ENDP
END
C++ Code starts:
#include <iostream>
#include <string>
using namespace std;
extern "C" char reverse(char*);
int main()
{
const int SIZE = 20;
char str1[SIZE] = { NULL };
cout << "Please enter a string: ";
cin >> str1;
cout << "Your string is: ";
for (int i = 0; str1[i] != NULL; i++)
{
cout << str1[i];
}
cout << "." << endl;
reverse(str1);
cout << "Your string in reverse is: ";
for (int i = 0; str1[i] != NULL; i++)
{
cout << str1[i];
}
cout << "." << endl;
system("PAUSE");
return 0;
}
So after many more hours of tinkering and looking around, I was finally able to figure out how to properly copy over a byte. I will post my .asm code below with comments if anybody needs it for future reference.
I was actually moving the content of the current address into a 32 bit registers. After I changed it from mov ebx,[eax] to mov bl,[eax], it copied the value correctly.
I will only post the code that I was having difficulty with so I do not give away the entire project for other students.
ASM Code Below:
swap:
mov bl,[edx] ;Uses bl since we are trying to copy a 1 byte char value
mov bh,[eax] ;Uses bh since we are trying to copy a 1 byte char value
mov [edx],bh ;Passing the value to the end of the array
mov [eax],bl ;Passing the value to the beginning of the array
inc eax ;Moving the array one index forward
dec edx ;Moving the array one index backwards
dec ecx ;Decreasing the counter by one to continue loop as needed
jmp reverse ;Jump back to reverse to check if additional swap is needed
Thanks for everyone that helped.
mov eax,[edx] (assuming intel syntax) places the 32 bits found in memory at address edx into eax. I.e, this code retrieves data from a memory location. If you'd like to write to a mem location, you need to reverse this, i.e mov [edx], eax
After playing with some 16 bit code overnight for sorting, I've the following two functions that may be of use. Obviously, you can't copy/paste them - you'll have to study it. However, you'll notice that it is able to swap items of arbitrary size. Perfect for swapping elements that are structures of some type.
; copies cx bytes from ds:si to es:di
copyBytes:
shr cx, 1
jnc .swapCopy1Loop
movsb
.swapCopy1Loop:
shr cx, 1
jnc .swapCopy2Loop
movsw
.swapCopy2Loop:
rep movsd
ret
; bp+0 bp+2 bp+4
;void swap(void *ptr1, void *ptr2, int dataSizeBytes)
swapElems:
push bp
mov bp, sp
add bp, 4
push di
push si
push es
mov ax, ds
mov es, ax
sub sp, [bp+4] ; allocate dataSizeBytes on the stack, starting at bp-6 - dataSizeBytes
mov di, sp
mov si, [bp+0]
mov cx, [bp+4]
call copyBytes
mov si, [bp+2]
mov di, [bp+0]
mov cx, [bp+4]
call copyBytes
mov si, sp
mov di, [bp+2]
mov cx, [bp+4]
call copyBytes
add sp, [bp+4]
pop es
pop si
pop di
pop bp
ret 2 * 3
I am currently learning assembly programming as part of one of my university modules. I have a program written in C++ with inline x86 assembly which takes a string of 6 characters and encrypts them based on the encryption key.
Here's the full program: https://gist.github.com/anonymous/1bb0c3be77566d9b791d
My code fo the encrypt_chars function:
void encrypt_chars (int length, char EKey)
{ char temp_char; // char temporary store
for (int i = 0; i < length; i++) // encrypt characters one at a time
{
temp_char = OChars [i]; // temp_char now contains the address values of the individual character
__asm
{
push eax // Save values contained within register to stack
push ecx
movzx ecx, temp_char
push ecx // Push argument #2
lea eax, EKey
push eax // Push argument #1
call encrypt
add esp, 8 // Clean parameters of stack
mov temp_char, al // Move the temp character into a register
pop ecx
pop eax
}
EChars [i] = temp_char; // Store encrypted char in the encrypted chars array
}
return;
// Inputs: register EAX = 32-bit address of Ekey,
// ECX = the character to be encrypted (in the low 8-bit field, CL).
// Output: register EAX = the encrypted value of the source character (in the low 8-bit field, AL).
__asm
{
encrypt:
push ebp // Set stack
mov ebp, esp // Set up the base pointer
mov eax, [ebp + 8] // Move value of parameter 1 into EAX
mov ecx, [ebp + 12] // Move value of parameter 2 into ECX
push edi // Used for string and memory array copying
push ecx // Loop counter for pushing character onto stack
not byte ptr[eax] // Negation
add byte ptr[eax], 0x04 // Adds hex 4 to EKey
movzx edi, byte ptr[eax] // Moves value of EKey into EDI using zeroes
pop eax // Pop the character value from stack
xor eax, edi // XOR character to give encrypted value of source
pop edi // Pop original address of EDI from the stack
rol al, 1 // Rotates the encrypted value of source by 1 bit (left)
rol al, 1 // Rotates the encrypted value of source by 1 bit (left) again
add al, 0x04 // Adds hex 4 to encrypted value of source
mov esp, ebp // Deallocate values
pop ebp // Restore the base pointer
ret
}
//--- End of Assembly code
}
My questions are:
What is the best/ most efficient way to convert this for loop into assembly?
Is there a way to remove the call for encrypt and place the code directly in its place?
How can I optimise/minimise the use of registers and instructions to make the code smaller and potentially faster?
Is there a way for me to convert the OChars and EChars arrays into assembly?
If possible, would you be able to provide me with an explanation of how the solution works as I am eager to learn.
I can't help with optimization or the cryptography but i can show you a way to go about making a loop, if you look at the loop in this function:
void f()
{
int a, b ;
for(a = 10, b = 1; a != 0; --a)
{
b = b << 2 ;
}
}
The loop is essentially:
for(/*initialize*/; /*condition*/; /*modify*/)
{
// run code
}
So the function in assembly would look something along these lines:
_f:
push ebp
mov ebp, esp
sub esp, 8 ; int a,b
initialize: ; for
mov dword ptr [ebp-4], 10 ; a = 10,
mov dword ptr [ebp-8], 1 ; b = 1
mov eax, [ebp-4]
condition:
test eax, eax ; tests if a == 0
je exit
runCode:
mov eax, [ebp-8]
shl eax, 2 ; b = b << 2
mov dword ptr [ebp-8], eax
modify:
mov eax, [ebp-4]
sub eax, 1 ; --a
mov dword ptr [ebp-4], eax
jmp condition
exit:
mov esp, ebp
pop ebp
ret
Plus I show in the source how you make local variables;
subtract the space from the stack pointer.
and access them through the base pointer.
I tried to make the source as generic intel x86 assembly syntax as i could so my apologies if anything needs changing for your specific environment i was more aiming to give a general idea about how to construct a loop in assembly then giving you something you can copy, paste and run.
I would suggest to look into assembly code which is generated by compiler. You can change and optimize it later.
How do you get assembler output from C/C++ source in gcc?