x86 Assembly Language Part-IV

[Music] all right let's move forward let's look at the last part of this lecture set on x86 which is encoding the x86 Isa right how do we encode it in bits so since our we are looking at uh we you know we are looking at 32bit ISA so my main discussion would be

on 32 bits but definitely uh this part we'll also keep it more generic so we'll also talk about how to encode uh instructions in other uh sizes right so let us first take a look at the general overview of every x86 instruction because you know x86 has been around for a long time so there A

lot has been done to ensure compatibility in the sense that today's code runs on yesterday's processes and yesterday's code runs on today's process does not happen 100% of the time for example if today I take uh a code with MMX registers and it'll definitely not run on an older processor but but at least what we want

is that all the code which runs on let's say processors 10 years ago will still run on today's processors right so that compatibility needs to be there otherwise it's it's sort of a bad business decision otherwise so let's take a look at the overview of instruction encoding so the so since this is a cisk instruction set

instructions can have different lengths sorry this should be cisk CIS they instructions can have have different lengths and the lengths can really really vary right so lengths can uh be different different instructions can have very very different lengths and sizes so this will make processor design difficult but we'll uh decide this we'll

talk about this later Let's uh take a look at the broad overview of the instruction encoding scheme the first one to four bytes which are optional have a prefix so what is the prefix the 1 to 4 by prefix specifies uh you know this prefix field specifies the prefix so for example we have taken a