I have recently been working on a data decryption program in x86 Assembler language. This being my first project involving assembler language has really taught me the value efficiency.
The original program took in a standard cstring and applied an encryption routine to it, I was then asked to write an assembler routine that would decrypt the new string and return it to its former self, only all lower-case. I found the lessons learnt while messing around with the assembler code invaluable, as they lead to me learning some great optimisation techniques which could make all the difference when applied to a large complex program. The routine I was working on was not large enough to see massive gains in performance from the optimisation I was implementing, however knowing how to speed up code that may have to be called thousands of times a second is a vital skill to learn in game development. All though the code may be fairly elementary, I’ve found a real passion for this type of programming and would definitely like to develop my knowledge in this area.
void decrypt_chars (const int ARRAY_LENGTH, char EKey)
{
_asm{
////////////// For Loop - Setup /////////////////////
push eax ;//Save Registers
push ecx ;//...
mov eax, 0 ;//Initialise loop counter
//////////// For Loop - Jump //////////////////////////
forloop: ;//Label
cmp eax, ARRAY_LENGTH ;
jg endLoop ;//(EAX > ARRAY_LENGTH)
movsx ecx, EChars[eax] ;//Get and store encrypted character
//////////////// Setup Parameters/Stack ///////////////
push eax ;//Save regsiter values to the stack
push ecx ;//...
movsx eax, EKey ;//Store 'EKey' in EAX register
///////// Modify Encryption Key /////////////////////
and eax,0x3C ;//Bitwise AND 'EKey' with '0x3C'
ror eax,1 ;//Rotate right EAX, Halving 'EKey'
ror eax,1 ;//Rotate right EAX, Halving 'EKey'
add eax,0x01 ;//Increment 'EKey'
mov edx,eax ;//Store modified EKey in EDX register
pop eax ;//Restore character to be encrypted in EAX
///////////// Apply Decryption /////////////////////
rol al,1 ;//Undo ror al,1
xor eax,edx ;//Undo xor eax,edx
sub eax,edx ;//Undo add eax,edx
//////////////// To Lowercase //////////////////////
cmp al, 0x41 ;//ASCII values 0x41 - 0x5A represent Captial Letters, Upper Bound
jl done ;//Not in range
cmp al, 0x5A ;//Lower Bound
jge done ;//Not in range
add al, 0x20 ;//Lowercase equivalent is + 0x20 in ASCII table
done: ;//Label
mov bl,al; ;//Store encrypted character in bl
pop eax ;//...
/////////////// End For Loop - Setdown //////////////
movsx cl, bl ;//Store decrypted character in CL
mov DChars[eax], cl ;//Move decrypted character into DChars[index] array
add eax, 1 ;//Incremenet loop counter
//////////// For Loop - Jump //////////////////////////
cmp eax, ARRAY_LENGTH ;
jle forloop ;//( EAX <= ARRAY_LENGTH )
endLoop: ;//Label
///////////////// For Loop - Exit //////////////////////
pop ecx ;//Restore Registers
pop eax ;
}
return;
}
John Collinge 