ST72734 datasheet(17/144 Pages) STMICROELECTRONICS

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ST72774/ST727754/ST72734

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CPU REGISTERS (Cont’d)

Stack Pointer (SP)

Read/Write

Reset Value: 01 FFh

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 6).

Since the stack is 256 bytes deep, the most

significant byte is 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 a 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 6.

– 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

interrupt five locations in the stack area.

Figure 6. Stack Manipulation Example

000

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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