PIC18LF4331 datasheet(64/392 Pages) MICROCHIP

64 / 392 page

PIC18F2331/2431/4331/4431

DS39616D-page 64

 2010 Microchip Technology Inc.

6.1.2.3

PUSH

and POP Instructions

Since the Top-of-Stack (TOS) is readable and writable,

the ability to push values onto the stack and pull values

off the stack without disturbing normal program execu-

tion is a desirable option. To push the current PC value

onto the stack, a PUSH instruction can be executed.

This will increment the Stack Pointer and load the

current PC value onto the stack. TOSU, TOSH and

TOSL can then 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 instruc-

tion discards the current TOS by decrementing the

Stack Pointer. The previous value pushed onto the

stack then becomes the TOS value.

6.1.2.4

Stack Full/Underflow Resets

These Resets are enabled by programming the

STVREN bit in Configuration Register 4L. When the

STVREN bit is cleared, a full or underflow condition will

set the appropriate STKFUL or STKUNF bit, but not

cause a device Reset. When the STVREN bit is set, a

full or underflow condition will set the appropriate

STKFUL or STKUNF bit and then cause a device

Reset. The STKFUL or STKUNF bits are cleared by the

user software or a Power-on Reset.

6.1.3

FAST REGISTER STACK

A Fast Register Stack is provided for the STATUS,

WREG and BSR registers, to provide a “fast return”

option for interrupts. The stack for each register is only

one level deep and is neither readable nor writable. It is

loaded with the current value of the corresponding

interrupt sources will push values into the stack

registers.

The values in the registers are then loaded back into

their associated registers if the RETFIE, FAST instruc-

tion is used to return from the interrupt. If both low and

high-priority interrupts are enabled, the stack registers

cannot be used reliably to return from low-priority inter-

rupts. If a high-priority interrupt occurs while servicing a

low-priority interrupt, the stack register values stored by

the low-priority interrupt will be overwritten. In these

cases, users must save the key registers in software

during a low-priority interrupt.

If interrupt priority is not used, all interrupts may use the

Fast Register Stack for returns from interrupt. If no

interrupts are used, the Fast Register Stack can be

used to restore the STATUS, WREG and BSR registers

at the end of a subroutine call. To use the Fast Register

Stack for a subroutine call, a CALL label, FAST

instruction must be executed to save the STATUS,

WREG and BSR registers to the Fast Register Stack. A

RETURN

, FAST instruction is then executed to restore

these registers from the Fast Register Stack.

Example 6-1 shows a source code example that uses

the Fast Register Stack during a subroutine call and

return.

EXAMPLE 6-1:

FAST REGISTER STACK

CODE EXAMPLE

6.1.4

LOOK-UP TABLES IN PROGRAM

MEMORY

There may be programming situations that require the

creation of data structures, or look-up tables, in

program memory. For PIC18 devices, look-up tables

can be implemented two ways:

• Computed GOTO

• Table Reads

6.1.4.1

Computed GOTO

A computed GOTO is accomplished by adding an offset

to the program counter. An example is shown in

Example 6-2.

A look-up table can be formed with an ADDWF PCL

instruction and a group of RETLW nn instructions. The

W register is loaded with an offset into the table before

executing a call to that table. The first instruction of the

called routine is the ADDWF PCL instruction. The next

instruction executed will be one of the RETLW nn

instructions that returns the value “nn” to the calling

function.

The offset value (in WREG) specifies the number of

bytes that the program counter should advance and

should be multiples of 2 (LSb = 0).

In this method, only one data byte can be stored in

each instruction location and room on the return

address stack is required.

EXAMPLE 6-2:

COMPUTED GOTO USING

AN OFFSET VALUE

CALL

SUB1, FAST

;STATUS, WREG, BSR

;SAVED IN FAST REGISTER

;STACK



SUB1



RETURN FAST

;RESTORE VALUES SAVED

;IN FAST REGISTER STACK

MOVFW

OFFSET

CALL

TABLE

ORG

0xnn00

TABLE

ADDWF

PCL

RETLW

0xnn

RETLW

0xnn

RETLW

0xnn

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