Disch Explains Stack Basics

// This is a conversation I had with a friend of mine, Disch.  He is a
// well known ROM hacker, and I thought this conversation we had could
// help others understand how the Stack Pointer and Program Counter
// work.

[23:39] *** Now talking to Disch
[23:39] Disch is ~disch_@dpc67143199073.direcpc.com Disch
[23:39] Disch on @#rom-hacking @#rom-hacking-ops
[23:39] Disch using discworld.esper.net Fabricati deim, pvnc.
[23:39] Disch has identified for this nick
[23:39] <Spliff> Disch, I am having issues with the Stack Pointer and 
Program Counter
[23:40] <Spliff> I know you're not 24/7 encyclopedia, but I need some help 
here...
[23:40] <Spliff> How do they work?
[23:40] <Spliff> What do they do?
[23:40] <Disch> The Program Counter just tells where the CPU is executing 
from
[23:40] <Disch> like if the PC is at $8000 ... then the next instruction to 
be executed comes from $8000
[23:41] <Disch> JMP $C123  <--- all that does is set the PC to $C123 ... so 
that it's the next instruction executed
[23:41] <Disch> The Stack Pointer just tells where the top of the stack is
[23:41] <Spliff> What is the stack?
[23:41] <Disch> okay
[23:41] <Disch> imagine a stack of plates
[23:42] <Disch> or
[23:42] <Disch> before it's a stack
[23:42] <Disch> say I just put one plate on the table
[23:42] <Disch> and another plate on top of it
[23:42] <Disch> the second plate is now the top of the stack... and in order 
to take off any plates below it, I need to take off that top plate first
[23:43] <Disch> it's kind of hard to explain...
[23:43] <Disch> but okay
[23:43] <Disch> you know about PHA and PLA?
[23:43] <Spliff> I haven't began to delve into ALL the mnemonics yet
[23:43] <Spliff> I only know maybe 10 offhand at most
[23:43] <Disch> well I can run through them
[23:43] <Spliff> Maybe 20
[23:44] <Disch> PHA "pushes" a value onto the stack (like adding another 
plate to the stack)
[23:44] <Spliff> Onto the top?
[23:44] <Disch> PLA "pulls" a value off the stack (like taking a plate off)
[23:44] <Disch> so here's an example:
[23:44] <Disch> LDA #$00       ; stack:  empty
[23:45] <Disch> PHA       ; stack:  [top] $00 [bottom]
[23:45] <Disch> LDA #$65
[23:45] <Disch> PHA        ; stack:  [top] $65 $00 [bottom]
[23:45] <Disch> LDA #$23
[23:45] <Disch> PHA          ; stack:  [top] $23 $65 $00 [bottom]
[23:46] <Disch> LDA #$00
[23:46] <Disch> PLA       ; A is now $23, stack:  [top] $65 $00 [bottom]
[23:46] <Disch> PLA       ; A is now $65  stack:  [top] $00 [bottom]
[23:46] <Disch> make sense?
[23:47] <Spliff> Yeah, but whats the deal with the second $00?
[23:47] <Spliff> I get the rest, by the way
[23:47] <Disch> just to show that PLA changes A
[23:48] <Spliff> Oh
[23:48] <Disch> so anyway
[23:48] <Disch> the stack exists on the $01xx page
[23:48] <Disch> where the Stack Pointer (SP) determines the next place to be 
the top of the stack
[23:48] <Spliff> $100 - $1FF?
[23:48] <Disch> yeah
[23:49] <Disch> now the stack is actually backwards kinda
[23:49] <Disch> $01FF is the normal "bottom" of the stack
[23:49] <Disch> so when the stack is empty... SP is $FF
[23:49] <Disch> when a value is pushed... it gets written to $0100+SP   then 
SP is decremented
[23:50] <Disch> when a value is pulled, SP gets incremented... then the 
value returned is from $0100+SP
[23:50] <Disch> am I making sense?  lol
[23:50] <Spliff> Yeah, it made sense when I remembered to read "$01FF is the 
bottom"
[23:51] <Spliff> Until then, I was kinda confused
[23:51] <Disch> heh
[23:51] <Disch> so any more Q's?  ^^
[23:52] <Spliff> So when we push from $FF, incrementing by $01, we are now 
at $FE?
[23:52] <Disch> that's decrementing.. no incrementing ;P  but yeah that's 
right
[23:53] <Disch> LDA #$00   ; if SP is $FF here...
[23:53] <Spliff> Ah, decrementing
[23:53] <Disch> PHA   ; $00 gets written to $01FF here, SP is now $FE
[23:53] <Spliff> But we're pushing
[23:53] <Spliff> And pulling brings it back to FF
[23:53] <Disch> right
[23:53] <Spliff> Sweet
[23:53] <Spliff> Now what can that do for a game?
[23:53] <Disch> well
[23:53] <Spliff> Pushing and Pulling?
[23:53] <Disch> the stack is mostly used with JSR/RTS
[23:54] <Spliff> Bit shifts?
[23:54] <Disch> JSR is like JMP... only it "saves" the PC by pushing it to 
the stack
[23:54] <Disch> RTS pulls that back from the stack
[23:54] <Disch> so like
[23:54] <Disch> say a game has a subroutine to print some text
[23:55] <Disch> at address $C654
[23:55] <Disch> $C654:
[23:55] <Disch>   LDA blah
[23:55] <Disch>   STA blah
[23:55] <Disch>   ; more crap to print text
[23:55] <Disch>   RTS
[23:55] <Disch> then elsewhere in the game
[23:55] <Disch> say you want to print some text... move the player... then 
print some more text
[23:56] <Disch> say you're at $8000 or something
[23:56] <Disch> $8000:
[23:56] <Disch>   JSR $C654   ; jump to draw text routine
[23:56] <Disch>   ;  when the draw text routine hits that RTS, execution 
will continue from here
[23:56] <Disch>   JSR Move_player
[23:56] <Disch>   JSR $C654
[23:56] <Disch>   ; to print more text
[23:57] <Disch> make sense?
[23:57] <Spliff> I didn't know Move_player was a valid syntax...I always 
thought it was Hex numbers after a Mnemonic
[23:58] <Disch> well that was just an example
[23:58] <Disch> (if you're using an assembler you can use labels and stuff 
-- but whatever)
[23:58] <Disch> put a number there for Move_player then ;P
[23:59] <Spliff> Lol
[23:59] <Spliff> hmmm...
[00:00] <Spliff> So JSR jumps right to C654>
[00:00] <Spliff> ?*
[00:00] <Disch> yeah
[00:00] <Disch> but
[00:00] <Disch> it also pushes the return address ($8003) to the stack
[00:00] <Spliff> Ahhh
[00:00] <Disch> so when the game hits RTS.. it'll pull the address from the 
stack and jump to it
[00:01] <Spliff> "I'm gonna jump to C654 now, but first I'm gonna make sure 
the the of the Stack is set to where I am now first"
[00:01] <Spliff> Right?
[00:01] <Disch> actually... technically it would push $8002  ;P
[00:01] <Disch> yeah
[00:01] <Spliff> Aight :)
[00:01] <Disch> RTS is "go back to wherever called JSR last"
[00:01] <Spliff> Ahh :)
[00:02] <Spliff> "Now I've visited C654 and done my business, now lets go 
back to where the top of the Stack tells us to"?
[00:02] <Disch> yep
[00:02] <Spliff> :D
[00:03] <Spliff> This convo should be placed on Romhacking.net :p
[00:03] <Disch> haw
[00:03] <Spliff> "Understanding the Stack Pointer"
[00:03] <Disch> feel free to submit it or whatever