Microchip Technology MA160014 Hoja De Datos

 2010-2012 Microchip Technology Inc.

DS41412F-page 73

5.2.2

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

5.2.2.1

Computed GOTO

A computed GOTO is accomplished by adding an offset
to the program counter. An example is shown in

.

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 two (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. 

5.2.2.2

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 

.

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

.

.

.