STM8S datasheet(32/467 Pages) STMICROELECTRONICS

32 / 467 page

Memory and register map

RM0016

32/467

DocID14587 Rev 14

Customized stack model

STM8S and STM8AF stack pointer handling allows a customized stack model to be

implemented. This permits a flexible stack size without restrictions due to the stack roll-over

limit. Implementing the customized stack also benefits portability of the software on products

with different memory configurations. Figure 5 shows the customized stack model.

Figure 5. Customized stack model

1. The stack roll-over limit is not implemented on all devices.

2. The guard cells are RAM locations that have to be continuously polled by the application program to detect

whether a stack overflow has taken place.

In this stack model, the initial stack pointer must be placed beyond the stack roll-over limit.

Consequently, the growing stack never reaches the stack roll-over limit. It is clear that in this

implementation the stack size is not limited by the roll-over mechanism. Nevertheless, the

user has to define the stack position and stack size in the link file, and he has to ensure that

the stack pointer does not exceed the defined stack area (stack overflow or under-run).

The RAM locations above and below the customized stack can be regularly used as RAM to

store variables or other information.

Guard cells can be implemented at the lower end of the stack to detect if the stack pointer

exceeds the defined limit. These cells are standard RAM locations, initialized with fixed

values that the stack overwrites if an overflow occurs. The user software can regularly poll

these cells, detect the overflow condition, and put the application in a fail safe state.

During the software validation phase hardware breakpoints can be set at both limits of the

stack to validate that neither a stack overflow nor an under-run happens.

ai15056v2

Customized stack model

End address

Stack roll-over limit (1)

Stack pointer initialization value

Start address

RAM

Optional guard cells (2)

Flexible stack size

STM8S Fiches technique(PDF) 32 Page - STMicroelectronics