RM0312 datasheet(19/275 Pages) STMICROELECTRONICS

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RM0312

Central processing unit (CPU)

Figure 1. Programming model

Stack pointer (SP)

The stack pointer is a 16-bit register. It contains the address of the next free location of the

stack. Depending on the product, the most significant bits can be forced to a preset value.

The stack is used to save the CPU context on subroutine calls or interrupts. The user can

also directly use it through the POP and PUSH instructions.

The stack pointer can be initialized by the startup function provided with the C compiler. For

applications written in C language, the initialization is then performed according to the

address specified in the linker file for C users. If you use your own linker file or startup file,

make sure the stack pointer is initialized properly (with the address given in the datasheets).

For applications written in assembler, you can use either the startup function provided by ST

or write your own by initializing the stack pointer with the correct address.

The stack pointer is decremented after data has been pushed onto the stack and

incremented after data is popped from the stack. It is up to the application to ensure that the

lower limit is not exceeded.

A subroutine call occupies two or three locations. An interrupt occupies nine locations to

store all the internal registers (except SP). For more details refer to Figure 2.

Note:

The WFI/HALT instructions save the context in advance. If an interrupt occurs while the

CPU is in one of these modes, the latency is reduced.

A ACCUMULATOR

SP STACK POINTER

X INDEX

Y INDEX

PC PROGRAM COUNTER

PCH

PCL

CC CODE CONDITION

VI1 H I0 N Z C

PCE

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