PIC16F1938 Datasheet, PDF (26/452 Pages) Microchip Technology – 28/40/44-Pin Flash-Based, 8-Bit CMOS Microcontrollers

PIC16(L)F1938/9
FIGURE 3-1:
PROGRAM MEMORY MAP
AND STACK FOR
16KW PARTS
PC<14:0>
CALL, CALLW
15
RETURN, RETLW
Interrupt, RETFIE
Stack Level 0
Stack Level 1
Stack Level 15
Reset Vector
0000h
On-chip
Program
Memory
Interrupt Vector
Page 0
Page 1
Page 2
Page 3
Page 4
0004h
0005h
07FFh
0800h
0FFFh
1000h
17FFh
1800h
1FFFh
2000h
Page 7
Rollover to Page 0
3FFFh
4000h
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:
constants
BRW
RETLW DATA0
RETLW DATA1
RETLW DATA2
RETLW DATA3
RETLW INSTRUCTION
;Add Index in W to
;program counter to
;select data
;Index0 data
;Index1 data
my_function
;… LOTS OF CODE…
MOVLW
DATA_INDEX
call constants
;… THE CONSTANT IS IN W
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.
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7FFFh
DS41574A-page 26
Preliminary
 2011 Microchip Technology Inc.