National Semi COP8
Stack Pointer The stack pointer holds the address of the return point from a calling routine, or data which has been pushed onto the stack.  If you watch the current stack pointer location in RAM it actually tracks, cycle by cycle, the current program counter.  By doing this, a call does not need extra processing time for pushing its address, it is already there.  The stack pointer is always the next Available location, the next two bytes holding a copy of the program counter.

A Push instruction actually places the "Pushed" data at the second of the two locations holding the copy of the program counter.  This copy then shifts down by one location.
Opcodes Quick reference to COP8TAC9 opcodes

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 This page last updated on September 04, 2011