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15 Appendix E: SX Data Sheet

SX-Key/Blitz Development System Manual 2.0

• Parallax, Inc. • Page 165

15.2.18 The Stack

The stack is an area of memory used to remember where to return to once a subroutine is complete. The

stack is eight levels deep with the Stack Extend (STACKX) option set and two levels deep by default

(On SX48/52 devices, the stack is always eight levels deep). That means it can remember the return addresses

for subroutines nested up to eight levels. The following explanation assumes that the SX has the Stack

Extend option selected. The stack is capable of two operations; push and pop. The stack behaves like a

plate holder in a salad buffet. A push is similar to placing a plate on the top of the stack and a pop is

similar to removing a plate from the top of the stack.

15.2.19 The Push

When a subroutine is called, the return address is pushed onto the stack. Specifically, each address in

the stack is moved to the next lower level in order to make room for the new address to be stored.

Stack 1 gets the value that was in the program counter. Stack 8 is overwritten with what was in Stack 7.

Consequently, the previous contents of Stack 8 are lost forever.

Figure 34 - The Push

PC<10:0>

Stack 1

Stack 2

Stack 3

Stack 4

Stack 5

Stack 6

Stack 7

Stack 8

45111 Arkusz danych(PDF) 165 Page - List of Unclassifed Manufacturers