IA-32 assembly: Effect of 8 bit operations on 32 bit registers

Tag: assembly,ia-32

During a course in computer science one gave me a binary file and the task to find out what it does, using the technique of reverse engineering (i.e. disassemble the file and analyze it). I stumbled upon one line of IA-32 instruction:

add    %esi,%ecx

I learned that commands with suffix l operate on 32 bit registers, those with suffix w operate on 16 bit registers and I think (but don't know), that operators without suffix operate on 8 bit registers. Edit: See Ross Ridge's comment which falsifies my assumption.

So my questions are:

Is my assumption above correct?
If so, what is the effect (in contrast to addl) of the instruction above – and what effects have other operations like sub, mov and so on?

Thanks in advance!

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Normally the suffix is ignored/not important if proper register names are given. So the instruction above is a 32 Bit instruction because esi and ecx are 32 Bit registers.

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