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ST9 USER GUIDE

3.2 STACK MODES

The ST9 allows you to have two separate stacks: a system stack and a user stack. The core

uses the system stack in interrupt routines and subroutines to save return addresses, the flag

ister). You can also use it under program control to save data, using the push and pop instruc-

tions.

The user stack works exactly the same way, using the pushu and popu instructions but only

under program control, which means that the user stack is not changed by the system. You

may choose to use a separate space for your data, or to store them in the same stack as the

return addresses.

Both stacks can independently be located either in RAM or in the register file. You select this

using the SSP and USP bits in the MODER register (R235) for the system stack and the user

stack, respectively. A low bit value selects a RAM stack, and a high bit value selects a Reg-

ister File stack.

Since the stacks grow towards low addresses, the stack pointers must be initialized to the

highest location plus one(3) of the space reserved to it. This location becomes the “bottom” of

the stack. When the stack is located in the register file, take care that it does not overwrite

other data, in particular the registers located in groups 14 (0EH) and 15 (0FH). For this reason

it is advisable to set the system stack pointer to the end of group 13 (0DH).

The last register of this group being R223, the instruction that sets the stack pointer will be:

sspr, #224 ; set stack pointer to one above end of group 13

Note 1:

Using two separate stacks in the same kind of storage (memory or register) area is likely to con-

sume more space than if a single stack is used. So most of the time, only one stack will hold

both return addresses and arguments for functions. You can then use pushu and popu instruc-

tions to manipulate data with the convenience of auto incrementing or decrementing the pointer

after each access.

As an example, refer to the C language start-up files which initialize both the user and system

stacks.

The following diagram illustrates the two options for locating the stack: in the register file or in

memory.

(3) The push instruction decrements the stack pointer before writing the data, so this location

would never be used if set to the top location.

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