ST72324BJ2 datasheet(25/188 Pages) STMICROELECTRONICS

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ST72324B

Central processing unit (CPU)

25/188

5.3.5

Stack Pointer register (SP)

The Stack Pointer is a 16-bit register which is always pointing to the next free location in the

stack. It is then decremented after data has been pushed onto the stack and incremented

before data is popped from the stack (see Figure 10).

Since the stack is 256 bytes deep, the 8 most significant bits are forced by hardware.

Following an MCU reset, or after a Reset Stack Pointer instruction (RSP), the Stack Pointer

contains its reset value (the SP7 to SP0 bits are set) which is the stack higher address.

The least significant byte of the Stack Pointer (called S) can be directly accessed by an LD

instruction.

Note:

When the lower limit is exceeded, the Stack Pointer wraps around to the stack upper limit,

without indicating the stack overflow. The previously stored information is then overwritten

and therefore lost. The stack also wraps in case of an underflow.

The stack is used to save the return address during a subroutine call and the CPU context

during an interrupt. The user may also directly manipulate the stack by means of the PUSH

and POP instructions. In the case of an interrupt, the PCL is stored at the first location

pointed to by the SP. Then the other registers are stored in the next locations as shown in

Figure 10.

●

When an interrupt is received, the SP is decremented and the context is pushed on the

stack.

●

On return from interrupt, the SP is incremented and the context is popped from the

stack.

A subroutine call occupies two locations and an interrupt five locations in the stack area.

Figure 10.

Stack manipulation example

Reset value: 01 FFh

987

65432

00000

001

SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0

R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W R/W

PCH

PCL

PCH

PCL

PCH

PCL

PCH

PCL

PCH

PCL

Call

subroutine

Interrupt

event

Push Y

Pop Y

IRET

RET

or RSP

@ 01FFh

@ 0100h

Stack Higher Address = 01FFh

Stack Lower Address = 0100h

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