Technology Zone

[RobotGymnast]

Ths has bothered me for a while, how does a CPU make a decision? It's essentially pumping a string of electrons, so how does that work on an IF statement? My only thinking could be you evaluate the condition, multiply the result by 64 or something, and then jump to [current address+(condition*0x40)], because then it would jump ahead 64 on TRUE, and jump ahead 0 on FALSE.. but that doesn't sound right. Oh and yes, this is an ASM representation vs a CPU representation.

How does a computer evaluate a conditional?

[Slugsnack]

Old thread but has no reply yet..

A computer evaluates conditions essentially by comparing and if equal, it sets a flag in the EFLAGs register named the zero flag. There are then JMPs that are conditional on the state of various flags.

So in an IF statement, the condition is evaluated and the code execution jumps conditionally as a result of the comparison.

Is that what you wanted to know ?

[RobotGymnast]

yes it is, but since then I've figured it out.. I was actually looking for a much lower-level explanation though. I understand conditional jumps, but what I wanted to know was how it evaluates the condition. Either way, it's all just boolean logic (greater than, less than, greater than or equal, etc.).. they can all be evaluated with the right boolean function. I understand it. Thanks though

[Slugsnack]

At a hardware level, it's just logic gates. For example, to compare a 1 and a 0 state, they could put it through a OR gate and if the result is one then that could clear the zero flag. I am speculating about the exact gate they use but OR would certainly work for comparing 1 and 0.

[RobotGymnast]

yeah I know. again, I figured it out. For those still wondering, reading the Art of Assembly really helps. Thanks though