ST72E121 datasheet(14/92 Pages) STMICROELECTRONICS

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ST72E121 ST72T121

CENTRAL PROCESSING UNIT (Cont’d)

Stack Pointer (SP)

Read/Write

Reset Value: 01FFh

The Stack Pointer is a 16-bit register which is al-

ways 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 7).

Since the stack is 256 bytes deep, the 8th most

significant bits are forced by hardware. Following

an MCU Reset, or after a Reset Stack Pointer in-

struction (RSP), the Stack Pointer contains its re-

set 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 a LD in-

struction.

Note: When the lower limit is exceeded, the Stack

Pointer wraps around to the stack upper limit, with-

out indicating the stack overflow. The previously

stored information is then overwritten and there-

fore lost. The stack also wraps in case of an under-

flow.

The stack is used to save the return address dur-

ing 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 instruc-

tions. 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 7.

– When an interrupt is received, the SP is decre-

mented 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 in-

terrupt five locations in the stack area.

Figure 7. Stack Manipulation Example

00000

SP7

SP6

SP5

SP4

SP3

SP2

SP1

SP0

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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