PM0044 datasheet(14/162 Pages) STMICROELECTRONICS

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STM8 core description

PM0044

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Doc ID 13590 Rev 3

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 subroutines calls or interrupts. The user can

also directly use it through the POP and PUSH instructions.

After an MCU reset the Stack Pointer is set to its upper limit value. It is then decremented

after data has been pushed onto the stack and incremented after data is popped from the

stack. When the lower limit is exceeded, the stack pointer wraps around to the stack upper

limit. The previously stored information is then overwritten, and therefore lost.

A subroutine call occupies two or three locations.

When an interrupt occurs, the CPU registers (CC, X, Y, A, PC) are pushed onto the stack.

This operation takes 9 CPU cycles and uses 9 bytes in RAM.

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.

Figure 2.

Context save/restore for interrupts

PM0044 Datasheet(PDF) 14 Page - STMicroelectronics