ST7FLCD1_05 datasheet(18/95 Pages) STMICROELECTRONICS

18 / 95 page

Central Processing Unit (CPU)

ST7FLCD1

18/95

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

pointer value increments when data is taken from the stack, it decrements once data is transferred

into 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 highest 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 the stack upper limit, without

indicating a 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. You can directly manipulate the stack using PUSH and POP instructions. In case of

interrupt, the PCL is stored at the first location pointed to by the SP. Other registers are then stored

in the next locations as shown in Figure 6.

When interrupt is received, the SP value decrements and the context is pushed to the stack.

On return from interrupt, the SP value increments and the context is popped from the stack.

A subroutine call and interrupt occupy two and five locations in the stack area respectively.

Figure 6: Stack Manipulation Example

Table 4: Instruction Set (Sheet 1 of 2)

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

Load and Transfer

CLR

Stack operation

PUSH

POP

RSP

Increment/Decrement

INC

DEC

Compare and Tests

TNZ

BCP

Logical operations

AND

XOR

CPL

NEG

Subroutine

Call

Interrupt

Event

PUSH Y

POP Y

IRET

RET

or RSP

Stack Higher Address = 01FFh

Stack Lower Address = 0100h

PCL

PCH

PCL

PCH

PCL

PCH

PCL

PCH

PCL

PCH

PCL

PCH

PCL

PCH

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