- Just like "creating variables", a stack is created in
memory by marking some memory locations as
reserved
- A stack in defined using an address that
indicates the location of the top of the stack.
Every memory location starting at the "top of stack" address until the last byte in memory forms the system stack (and these locations are reserved)
- M68000 uses the addresss register A7 to
store the address of the "top of the system stack":
- The address register A7 is called the system stack pointer (or stack pointer for short).
- This is accomplished by decrementing the stack pointer
to reserve memory space to store a new value, and then
- Moving the value into the newly reserved memory location.
- The following M68000 assembler instructions can be used
to push the integer value 12345 onto the system stack:
SUBA.L #4, A7 // Move stack point up 4 bytes // - it will reserves 4 more bytes on the stack MOVE.L #12345, (A7) // Move the value 12345 into the newly // reserved location
- DEMO: click here
- Because pushing values onto a stack is a commonly used
operation in programs, M68000 has a special
pre-decrement indirect addressing mode
that assembler program can use to push values on the stack
- MOVE.s SRC, -(A7)
is equivalent to
- Subtract A7 by the size of the operand first
- Then move the SRC operand with the indirect mode
- Example:
MOVE.L #12345, -(A7) <===> SUBA.L #4, A7 MOVE.L #12345, (A7)
- DEMO: click here
- When you do not need the value on the top of the stack,
you "destroy" the value by unmarking the reservation.
- Because the stack is implemented by using A7 to mark the
end of the reserve area, we can remove the reservation
by simply increasing A7
- Example:
ADDA.L #4, A7 // will destroy the integer on the top // of the stack ADDA.L #8, A7 // will destroy the first 2 integers on the top // of the stack
- DEMO: click here