PIC18F23K22-ESLASHML datasheet(73/496 Pages) MICROCHIP

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Preliminary

DS41412C-page 73

PIC18(L)F2X/4XK22

EXAMPLE 5-1:

FAST REGISTER STACK

CODE EXAMPLE

5.1.4

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 in two ways:

• Computed GOTO

• Table Reads

5.1.4.1

Computed GOTO

A computed GOTO is accomplished by adding an offset

to the program counter. An example is shown in

Example 5-2.

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 may be stored in

each instruction location and room on the return

address stack is required.

EXAMPLE 5-2:

COMPUTED GOTO USING

AN OFFSET VALUE

5.1.4.2

Table Reads and Table Writes

A better method of storing data in program memory

allows two bytes of data to be stored in each instruction

location.

Look-up table data may be stored two bytes per

program word by using table reads and writes. The

Table Pointer (TBLPTR) register specifies the byte

address and the Table Latch (TABLAT) register

contains the data that is read from or written to program

memory. Data is transferred to or from program

memory one byte at a time.

Table read and table write operations are discussed

further in Section 6.1 “Table Reads and Table

Writes”.

CALL SUB1, FAST

;STATUS, WREG, BSR

;SAVED IN FAST REGISTER

;STACK



SUB1



RETURN, FAST

;RESTORE VALUES SAVED

;IN FAST REGISTER STACK

MOVF

OFFSET, W

CALL

TABLE

ORG

nn00h

TABLE

ADDWF

PCL

RETLW

nnh

RETLW

nnh

RETLW

nnh

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