Unit 1.2 Slide, is this correct?

[andybendy]

I'm not totally sure if this image link works, so I will explain the question through text as well.

On the slide in the unit 1.2 lecture, where the title is: Arithmetic / Logical commands, it says the eq command has a return value of x==0, should this not be x==y?

[dolomiti7]

You are right, this is definitely a mistake on the slide. The eq VM command works similar to the LT and GT by comparing 2 operands on the stack. So x==y would be the correct description of the command.

A typical implementation would more look like (x-y)==0 and then use a ...;JEQ ASM instruction, maybe that's where the mixup came from.

[andybendy]

Oh okay, thank you for clarifying!

Why would the implementation likely be x-y==0 instead of x==y? Is that because all of the jump commands are centered on 0? Like there's no jump command comparing two values, they all compare a singular value to 0 (except for the unconditionals of course). Would it be possible to implement it using x==y but it'd just be more obtuse/unecessary, or it isn't even possible to logically implement?

[andybendy]

Oh okay, thank you for clarifying!

Why would the implementation likely be x-y==0 instead of x==y? Is that because all of the jump commands are centered on 0? Like there's no jump command comparing two values, they all compare a singular value to 0 (except for the unconditionals of course). Would it be possible to implement it using x==y but it'd just be more obtuse/unnecessary, or it isn't even possible to logically implement?

edit: spelling errors.

[pm100]

All compare inst auctions on almost all CPUs are implemented ,either explicitly or implicitly, as subtract followed by testing the result flags.