Day 21 - The Stack

The Stack

	I hope you know what a stack is. It's exactly what it sounds like, you
put things on top (push) and then you take things off the top (pop). The stack
in the NES starts at $01FF and grows down in memory. To handle where the next 
item pushed or poped from the stack comes from, there a register called S which
is an index in page 1 ($0100 to $01FF). The S register can modified by a TSX or
TXS instruction (Transfer S to X and transfer X to S respectively). The current
location on the stack always is $0100+S.

Push Instructions
	There are 2 instructions that explicitly push things on the stack, they
are PHA and PHP. PHA pushes the A register onto the stack and then decrements the
S register. PHP pushes the flag register (that stores Zero,Carry,Overflow, etc..)
onto the stack (and S-=1;). Note that there are no instructions to push the X
or Y registers, to accomplish that. You'll have to do something like:

; assuming you don't care about A's value
	txa ; transfer X to A
	pha ; push A
; or
	tya ; transfer Y to A
	pha ; push A

Pop Instructions
	The counter-parts to the push instructions are PLA and PLP. PLA pops 
A from the stack and PLP pops the flags from the stack (both do S+=1; afterwards).

; example:
	lda #3
	pha
	lda #4
	pha
	lda #5
	pla ; note that this is a POP
	pla

After that is finished A's value will be 3.

NOTE: PHP and PLP would be usefull in preserving flag status across subroutine calls (JSR).
NOTE2: I don't think JSR saves the flags, does it?

This Day In Review

	The stack is a good thing to know and is probably something
I should have mentioned earlier.

	Greetings and Salutations!,
		- GbaGuy

P.S. Could someone email me to tell me if this works or not, please?

Intro - Day 22