PIC18F1220ESLASHSOSQTP datasheet(44/308 Pages) MICROCHIP

44 / 308 page

PIC18F1220/1320

DS39605F-page 42

5.2

Return Address Stack

The return address stack allows any combination of up

to 31 program calls and interrupts to occur. The PC

(Program Counter) is pushed onto the stack when a

CALL

or RCALL instruction is executed, or an interrupt

is Acknowledged. The PC value is pulled off the stack

on a RETURN, RETLW or a RETFIE instruction. PCLATU

and PCLATH are not affected by any of the RETURN or

CALL

instructions.

The stack operates as a 31-word by 21-bit RAM and a

5-bit stack pointer, with the Stack Pointer initialized to

00000B

after all Resets. There is no RAM associated

with Stack Pointer, 00000B. This is only a Reset value.

During a CALL type instruction, causing a push onto the

stack, the Stack Pointer is first incremented and the

RAM location pointed to by the Stack Pointer

(STKPTR) register is written with the contents of the PC

(already pointing to the instruction following the CALL).

During a RETURN type instruction, causing a pop from

the stack, the contents of the RAM location pointed to

by the STKPTR are transferred to the PC and then the

Stack Pointer is decremented.

The stack space is not part of either program or data

space. The Stack Pointer is readable and writable and

the address on the top of the stack is readable and

writable

through

the

top-of-stack

Special

File

Registers. Data can also be pushed to or popped from

the stack using the top-of-stack SFRs. Status bits

indicate if the stack is full, has overflowed or

underflowed.

5.2.1

TOP-OF-STACK ACCESS

The top of the stack is readable and writable. Three

contents of the stack location pointed to by the

STKPTR register (Figure 5-3). This allows users to

implement a software stack if necessary. After a CALL,

RCALL

or interrupt, the software can read the pushed

value by reading the TOSU, TOSH and TOSL registers.

These values can be placed on a user defined software

stack. At return time, the software can replace the

TOSU, TOSH and TOSL and do a return.

The user must disable the global interrupt enable bits

while accessing the stack to prevent inadvertent stack

corruption.

5.2.2

RETURN STACK POINTER

(STKPTR)

The STKPTR register (Register 5-1) contains the stack

pointer value, the STKFUL (Stack Full) 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. At 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 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 STVR (Stack Overflow

Reset Enable) configuration bit. (Refer to Section 19.1

“Configuration Bits” for a description of the device

configuration bits.) If STVR 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 STVR 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 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 a POR occurs.

FIGURE 5-3:

RETURN ADDRESS STACK AND ASSOCIATED REGISTERS

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.

00011

001A34h

11111

11110

11101

00010

00001

00000

00010

Return Address Stack

Top-of-Stack

000D58h

TOSL

TOSH

TOSU

34h

1Ah

00h

STKPTR<4:0>

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