41594B datasheet(16/372 Pages) MICROCHIP

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PIC12LF1840T48A

DS41594B-page 16

Preliminary

 2011-2012 Microchip Technology Inc.

FIGURE 3-1:

PROGRAM MEMORY MAP

AND STACK FOR

PIC12LF1840T48A

3.1.1

READING PROGRAM MEMORY AS

DATA

There are two methods of accessing constants in pro-

gram memory. The first method is to use tables of

RETLW instructions. The second method is to set an

FSR to point to the program memory.

3.1.1.1

RETLW Instruction

The RETLW instruction can be used to provide access

to tables of constants. The recommended way to create

such a table is shown in Example 3-1.

EXAMPLE 3-1:

RETLW INSTRUCTION

The BRW instruction makes this type of table very sim-

ple to implement. If your code must remain portable

with previous generations of microcontrollers, then the

BRW instruction is not available so the older table read

method must be used.

PC<14:0>

0000h

0004h

Stack Level 0

Stack Level 15

Reset Vector

Interrupt Vector

CALL, CALLW

RETURN, RETLW

Stack Level 1

0005h

On-chip

Program

Memory

Page 0

07FFh

Rollover to Page 0

0800h

0FFFh

1000h

7FFFh

Page 1

Rollover to Page 1

Interrupt, RETFIE

constants

BRW

;Add Index in W to

;program counter to

;select data

RETLW DATA0

;Index0 data

RETLW DATA1

;Index1 data

RETLW DATA2

RETLW DATA3

my_function

;… LOTS OF CODE…

MOVLW

DATA_INDEX

call constants

;… THE CONSTANT IS IN W

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