Day 21 - 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.
Patater GBAGuy Mirror
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
tya ; transfer Y to A
pha ; push A
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).
pla ; note that this is a POP
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!,
P.S. Could someone email me to tell me if this works or not, please?
Intro - Day 22