Datasheet

 1997 Microchip Technology Inc. DS30390E-page 41
PIC16C7X
Example 4-1 shows the calling of a subroutine in
page 1 of the program memory. This example assumes
that PCLATH is saved and restored by the interrupt ser-
vice routine (if interrupts are used).
EXAMPLE 4-1: CALL OF A SUBROUTINE IN
PAGE 1 FROM PAGE 0
ORG 0x500
BSF PCLATH,3 ;Select page 1 (800h-FFFh)
BCF PCLATH,4 ;Only on >4K devices
CALL SUB1_P1 ;Call subroutine in
: ;page 1 (800h-FFFh)
:
:
ORG 0x900
SUB1_P1: ;called subroutine
: ;page 1 (800h-FFFh)
:
RETURN ;return to Call subroutine
;in page 0 (000h-7FFh)
4.5 Indirect Addressing, INDF and FSR
Registers
The INDF register is not a physical register. Addressing
the INDF register will cause indirect addressing.
Indirect addressing is possible by using the INDF reg-
ister. Any instruction using the INDF register actually
accesses the register pointed to by the File Select Reg-
ister, FSR. Reading the INDF register itself indirectly
(FSR = '0') will read 00h. Writing to the INDF register
indirectly results in a no-operation (although status bits
may be affected). An effective 9-bit address is obtained
by concatenating the 8-bit FSR register and the IRP bit
(STATUS<7>), as shown in Figure 4-18.
A simple program to clear RAM locations 20h-2Fh
using indirect addressing is shown in Example 4-2.
EXAMPLE 4-2: INDIRECT ADDRESSING
movlw 0x20 ;initialize pointer
movwf FSR ;to RAM
NEXT clrf INDF ;clear INDF register
incf FSR,F ;inc pointer
btfss FSR,4 ;all done?
goto NEXT ;no clear next
CONTINUE
: ;yes continue
Applicable Devices
72
73 73A 74 74A 76 77
FIGURE 4-18: DIRECT/INDIRECT ADDRESSING
For register file map detail see Figure 4-4, and Figure 4-5.
Data
Memory
Indirect AddressingDirect Addressing
bank select location select
RP1:RP0 6
0
from opcode
IRP FSR register
7
0
bank select
location select
00 01 10 11
Bank 0 Bank 1 Bank 2 Bank 3
not used
FFh
80h
7Fh
00h
17Fh
100h
1FFh
180h