Day 11 - The JSR Instruction

Published: 2005-Jul-22, Updated: 2010-Dec-19
Author: Mike Huber

JSR?

JSR stands for Jump to SubRoutine and is used to, well... jump to a subroutine.

How do we use it?

We use it by giving it the name of the label that we want to jump to. Example:

jsr OurSub ; blah, some code OurSub: ; blah, some more code ; return to instruction after the JSR.

Return? How?

With the RTS (ReTurn from Subroutine) instruction of course! So the full version of the above stuff is:

jsr OurSub ; blah, some code OurSub: ; blah, some code rts ; returns and resumes execution at the first "blah, some code".

Something more

One thing to remember though, if you just want to separate your code out. then what is described above should be enough. If you want to jump to a subroutine based on a comparison, you would have to do something like the following:

cmp #5 ; compare A with 5 bne DONTcall ; if A isn't 5 we branch over the subroutine call. jsr OurSub DONTcall: ;somewhere... possibly far away: OurSub: ;blah, code. rts ; return

This Day In Review

I hope you get that, if you don't.. well.. I just won't think like that. ;) Seriously though, if you ever need help with something from these tutorials or find an error, don't hesitate to tell me.

Happy coding!,
-Mike H a.k.a GbaGuy