Cheat Engine

[manc]

Say I had a binary number stored in a register, 1010101010111101, for example. If I wanted to clear bit 3 of any binary number(make it 0 if it isn't already), how could I do this in the least amount of instructions/opcodes? I'm not even sure what opcodes I could/should use to accomplish this (LDR,STR,AND,ADD,NOT,JMP, etc). This is theoretical LC-3 machine language (for understanding processor language/architecture), close to assembly I guess. Any input is appreciated.

EDIT: Updated / new question below

[Dark Byte]

OR the value with "NOT 8"

16 bit register: fff7
32 bit register: fffffff7

[manc]

Ah, masking, I see now. New question, if you happen to check this thread again:

I'm trying to take the value stored at x3030 and count how many of the bits are 1's, and then store that number at x3031. For example, 1111 0000 1010 0001 has 7 1's, so 0111 would be stored at x3031. I've figured out how to isolate each bit, check for 1's, and increment the counter if there is a 1. However, I'm having trouble stopping my loop.

My initial hope was that the mask would be incremented until it reached 1111 1111 1111 1111, and then when NOT'd, it would become 0. My hope was that this would end the loop....but the result isn't 0. It overflows and becomes negative...which ruins the logic of my program.....Any thoughts?

[Dark Byte]

easiest and most straightforward method:

Set counter (R6) to 0
Set Mask (r4) to 1

RepeatThis:
Check if Value (R7) AND Mask (R4) (binary compare) is bigger than 0
If so, increase Counter (r6)
Shift mask(R4) by 1 bit to the left
If Mask is <=1 (OverFlow) goto EndOfLoop (<=1 because some cpu's don't move the last bit back to the first bit, so it turns 0 then)
Goto repeatThis

EndOfLoop:
Counter now contains the number of bits

[manc]

[Dark Byte]

Let's assume 8 bits:
First iteration: Mask is 00000001
Second Iteration: Mask is 00000010
3: 00000100
...

7: 10000000
8: 00000001 (or 00000000 depending on what instruction you used, rotate or shift)


And no, <=1 does not include negatives. This is an unsigned compare

Another way to read: If 0 or 1

[manc]

[Dark Byte]

You must not combine the two scripts
One is for clearing a bit, and the other is to count bits

Do them after eachother. Not at the same time (First get the value, then count the bit of that value)

[manc]

I figured it out. Got it working. Thank you Dark Byte!

[SteveAndrew]

I got it working using AND against the unchanged value each time, on each pass of the mask shifting to the left.

when I tried:

[Dark Byte]

Yeah, it's a typo. I meant AND

[SteveAndrew]

Ah yes that's what I figured! Well manc got it working for him, and I got it working for me so yea thanks Dark Byte!

[manc]