Question

I’m using C and ASM{} in one of our classes in order to complete an assembler project where we have to encrypt an input string, transmit and decrypt it.

The key is loaded into an empty C char array (20 chars long) and is then used later on with the XOR statement to encrypt.

The code to load the address of the array is:

lea esi, key

which puts the address of ‘key’ into esi. The address here is the same as the address of the key array when we examine the register in debug mode, so we know that works.

mov edx, [esi]

we thought this would move the value of esi’s first index into edx, as we use “mov [esi], eax” to put the value of the a register into the esi array. The reverse seemed logical.

However, the value which edx actually gets is “875575655” (last run) or similar. Far too large for a char value.

I get the feeling we may be accessing the array wrong,
Any advice would be appreciated.

Note: Usually to increase the array index we’re simply using inc esi and then read like we did above, this is how we were planning on reading from the array as well.

Answer 1

With mov edx, [esi] you read DWORD (because edx size is double word). Use mov dl, [esi] to read byte.