PIC16F1933-ESLASHP datasheet(25/418 Pages) MICROCHIP

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Preliminary

DS41364A-page 23

PIC16F193X/LF193X

FIGURE 2-3:

PROGRAM MEMORY MAP

AND STACK FOR THE

PIC16F1938/PIC16LF1938/

PIC16F1939/PIC16LF1939

2.1.1

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.

2.1.1.1

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 2-1.

EXAMPLE 2-1:

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.

2.1.1.2

Indirect Read with FSR

The program memory can be accessed as data by set-

ting bit 7 of the FSRxH register and reading the match-

ing INDFx register. The MOVIW instruction will place the

lower 8 bits of the addressed word in the W register.

Writes to the program memory cannot be performed via

the INDF registers. Instructions that access the pro-

gram memory via the FSR require one extra instruction

cycle to complete. Example 2-2 demonstrates access-

ing the program memory via an FSR.

EXAMPLE 2-2:

ACCESSING PROGRAM

MEMORY VIA FSR

PC<14:0>

0000h

0004h

Stack Level 0

Stack Level 15

Reset Vector

Interrupt Vector

Stack Level 1

0005h

On-chip

Program

Memory

Page 0

07FFh

Rollover to Page 0

0800h

0FFFh

1000h

7FFFh

Page 1

Rollover to Page 7

Page 2

Page 3

17FFh

1800h

1FFFh

2000h

Page 4

Page 7

3FFFh

4000h

CALL, CALLW

RETURN, RETLW

INTERRUPT, RETFIE

constants

brw

retlw DATA1

retlw DATA2

retlw DATA3

retlw DATA4

my_function

;… LOTS OF CODE…

movlw

DATA_INDEX

call constants

;… THE CONSTANT IS IN W

bsf FSR1H,7

moviw 0[INDF1]

;THE PROGRAM MEMORY IS IN W

PIC16F1933-E/P Datasheet(PDF) 25 Page - Microchip Technology

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