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Processor architecture
indicates its internal
construction.
The 6502 processor has
seven internal 8 bit
registers and an ALU
(arithmetic logic unit).
The X and Y registers
are called INDEX
registers and are used
in the manipulation of
data.
S is the stack register.
It contains an address
indicating where there
is a temporary store for
data called the STACK.
PROGRAM COUNTER HIGH AND
PROGRAM COUNTER LOW
together form a 16 bit
register, which contains
the address of the next
instruction to be
executed.
A is the ACCUMULATOR.
All data to be
manipulated is placed in
this register and worked
upon.
P is the PROCESSOR
STATUS REGISTER.
It contains several one
bit FLAGS.
These contain a one or a
zero to indicate that
something has happened
during manipulation of
data.
For example, one of the
flags is set if the
result of a calculation
is zero.
Another is set if the
result is negative.
By looking at these
flags the CPU is able to
make decisions.
This is why it is so
clever.
The ALU is not a
register. It is
discussed on another
page.
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