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The stack is a temporary
store for data.
The CPU may PUSH
important data onto the
stack, while it is
processing other data.
When it finishes that
task, it PULLS the saved
data off the stack.
Its like a pile of
plates.
The bottom plate is the
first bit of data that
was pushed onto the
stack.
The top plate is the
last data to be pushed.
The top plate is pulled
first and the bottom
plate is the last data
to be pulled.
It is a LAST IN, FIRST
OUT stack.
In the diagrams, X is
the first to be pushed,
then Y and lastly A.
The CPU goes away to
process other data.
Upon completion of that
task it returns to pull
the saved data.
First A is pulled, then
Y and lastly X.
The instruction for
pushing data is PHA.
Only data in the
accumulator can be
pushed onto the stack.
Other data can be pushed
if it is transferred to
the accumulator first.
The instruction for
pulling data from the
stack is PLA.
Data on the stack is
transferred to the
accumulator.
The 6502 stack consists
of 256 bytes and
occupies page 1,
addresses 256 to 511.
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