PIC12(L)F1822/PIC16(L)F1823
DS41413C-page 22
2010-2012 Microchip Technology Inc.
FIGURE 3-1:
PROGRAM MEMORY MAP
AND STACK FOR
PIC12(L)F1822/16(L)F1823
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
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>
15
0000h
0004h
Stack Level 0
Stack Level 15
Reset Vector
Interrupt Vector
Stack Level 1
0005h
On-chip
Program
Memory
Page 0
07FFh
Wraps to Page 0
0800h
CALL, CALLW
RETURN, RETLW
Interrupt, RETFIE
Rollover to Page 0
7FFFh
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
