PIC18F44J10 datasheet(56/368 Pages) MICROCHIP

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PIC18F45J10 FAMILY

DS39682E-page 54

6.1.4.2

Return Stack Pointer (STKPTR)

The STKPTR register (Register 6-1) contains the Stack

Pointer value, the STKFUL (Stack Overflow) status bit

and the STKUNF (Stack Underflow) status bits. The

value of the Stack Pointer can be 0 through 31. The

Stack Pointer increments before values are pushed

onto the stack and decrements after values are popped

off the stack. On Reset, the Stack Pointer value will be

zero. The user may read and write the Stack Pointer

value. This feature can be used by a Real-Time

Operating

System

(RTOS)

for

return

stack

maintenance.

After the PC is pushed onto the stack 31 times (without

popping any values off the stack), the STKFUL bit is

set. The STKFUL bit is cleared by software or by a

POR.

The action that takes place when the stack becomes

full depends on the state of the STVREN (Stack Over-

flow Reset Enable) Configuration bit. (Refer to

Section 21.1 “Configuration Bits”

for a description of

the device Configuration bits.) If STVREN is set

(default), the 31st push will push the (PC + 2) value

onto the stack, set the STKFUL bit and reset the

device. The STKFUL bit will remain set and the Stack

Pointer will be set to zero.

If STVREN is cleared, the STKFUL bit will be set on the

31st push and the Stack Pointer will increment to 31.

Any additional pushes will not overwrite the 31st push

and the STKPTR will remain at 31.

When the stack has been popped enough times to

unload the stack, the next pop will return a value of zero

to the PC and sets the STKUNF bit, while the Stack

Pointer remains at zero. The STKUNF bit will remain

set until cleared by software or until a POR occurs.

6.1.4.3

PUSH

and POP Instructions

Since the Top-of-Stack is readable and writable, the

ability to push values onto the stack and pull values off

the stack without disturbing normal program execution

is a desirable feature. The PIC18 instruction set

includes two instructions, PUSH and POP, that permit

the TOS to be manipulated under software control.

TOSU, TOSH and TOSL can be modified to place data

or a return address on the stack.

The PUSH instruction places the current PC value onto

the stack. This increments the Stack Pointer and loads

the current PC value onto the stack.

The POP instruction discards the current TOS by decre-

menting the Stack Pointer. The previous value pushed

onto the stack then becomes the TOS value.

Note:

Returning a value of zero to the PC on an

underflow has the effect of vectoring the

program to the Reset vector, where the

stack conditions can be verified and

appropriate actions can be taken. This is

not the same as a Reset, as the contents

of the SFRs are not affected.

STKPTR: STACK POINTER REGISTER

R/C-0

U-0

R/W-0

STKFUL(1)

STKUNF(1)

—

SP4

SP3

SP2

SP1

SP0

bit 7

bit 0

Legend:

C = Clearable bit

R = Readable bit

W = Writable bit

U = Unimplemented bit, read as ‘0’

-n = Value at POR

‘1’ = Bit is set

‘0’ = Bit is cleared

x = Bit is unknown

bit 7

STKFUL:

Stack Full Flag bit(1)

= Stack became full or overflowed

= Stack has not become full or overflowed

bit 6

STKUNF:

Stack Underflow Flag bit(1)

= Stack underflow occurred

= Stack underflow did not occur

bit 5

Unimplemented:

Read as ‘0’

bit 4-0

SP<4:0>:

Stack Pointer Location bits

Note 1:

Bit 7 and bit 6 are cleared by user software or by a POR.

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