Datasheet

 2013-2015 Microchip Technology Inc. DS40001723D-page 17
PIC12(L)F1571/2
3.2.1 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.
3.2.1.1 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
simple 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.
3.2.1.2 Indirect Read with FSR
The program memory can be accessed as data by
setting bit 7 of the FSRnH register and reading the
matching INDFn register. The MOVIW instruction will
place the lower eight bits of the addressed word in the
W register. Writes to the program memory cannot be
performed via the INDFn registers. Instructions that
access the program memory via the FSR require one
extra instruction cycle to complete. Example 3-2
demonstrates accessing the program memory via an
FSR.
The HIGH operator will set bit<7> if a label points to a
location in program memory.
EXAMPLE 3-2: ACCESSING PROGRAM
MEMORY VIA FSR
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
constants
DW DATA0 ;First constant
DW DATA1 ;Second constant
DW DATA2
DW DATA3
my_function
;… LOTS OF CODE…
MOVLW DATA_INDEX
ADDLW LOW constants
MOVWF FSR1L
MOVLW
HIGH constants ;MSb is set
automatically
MOVWF FSR1H
BTFSC STATUS,C ;carry from ADDLW?
INCF FSR1H,f ;yes
MOVIW 0[FSR1]
;THE PROGRAM MEMORY IS IN W