PIC18F2480/2580/4480/4580
Purpose Register File”) or a location in the Access
Bank (Section 6.3.2 “Access Bank”) as the data
source for the instruction.
6.4
Data Addressing Modes
Note:
The execution of some instructions in the
core PIC18 instruction set are changed
when the PIC18 extended instruction
set is enabled. See Section 6.6 “Data
Memory and the Extended Instruction
Set” for more information.
The Access RAM bit ‘a’ determines how the address is
interpreted. When ‘a’ is ‘1’, the contents of the BSR
(Section 6.3.1 “Bank Select Register (BSR)”) are
used with the address to determine the complete 12-bit
address of the register. When ‘a’ is ‘0’, the address is
interpreted as being a register in the Access Bank.
Addressing that uses the Access RAM is sometimes
also known as Direct Forced Addressing mode.
While the program memory can be addressed in only
one way – through the program counter – information
in the data memory space can be addressed in several
ways. For most instructions, the addressing mode is
fixed. Other instructions may use up to three modes,
depending on which operands are used and whether or
not the extended instruction set is enabled.
A few instructions, such as MOVFF, include the entire
12-bit address (either source or destination) in their
opcodes. In these cases, the BSR is ignored entirely.
The destination of the operation’s results is determined
by the destination bit, ‘d’. When ‘d’ is ‘1’, the results are
stored back in the source register, overwriting its origi-
nal contents. When ‘d’ is ‘0’, the results are stored in
the W register. Instructions without the ‘d’ argument
have a destination that is implicit in the instruction; their
destination is either the target register being operated
on or the W register.
The addressing modes are:
• Inherent
• Literal
• Direct
• Indirect
An additional addressing mode, Indexed Literal Offset,
is available when the extended instruction set is
enabled (XINST Configuration bit = 1). Its operation is
discussed in greater detail in Section 6.6.1 “Indexed
Addressing with Literal Offset”.
6.4.3
INDIRECT ADDRESSING
Indirect Addressing allows the user to access a location
in data memory without giving a fixed address in the
instruction. This is done by using File Select Registers
(FSRs) as pointers to the locations to be read or written
to. Since the FSRs are themselves located in RAM as
Special File Registers, they can also be directly manip-
ulated under program control. This makes FSRs very
useful in implementing data structures, such as tables
and arrays in data memory.
6.4.1
INHERENT AND LITERAL
ADDRESSING
Many PIC18 control instructions do not need any
argument at all; they either perform an operation that
globally affects the device or they operate implicitly on
one register. This addressing mode is known as
Inherent Addressing. Examples include SLEEP, RESET
and DAW.
The registers for Indirect Addressing are also imple-
mented with Indirect File Operands (INDFs) that permit
automatic manipulation of the pointer value with
auto-incrementing, auto-decrementing or offsetting
with another value. This allows for efficient code, using
loops, such as the example of clearing an entire RAM
bank in Example 6-5.
Other instructions work in a similar way but require an
additional explicit argument in the opcode. This is
known as Literal Addressing mode because they
require some literal value as an argument. Examples
include ADDLWand MOVLWwhich, respectively, add or
move a literal value to the W register. Other examples
include CALL and GOTO, which include a 20-bit
program memory address.
EXAMPLE 6-5:
HOW TO CLEAR RAM
(BANK 1) USING
INDIRECT ADDRESSING
6.4.2
DIRECT ADDRESSING
LFSR
CLRF
FSR0, 100h
POSTINC0
;
NEXT
; Clear INDF
; register then
; inc pointer
; All done with
; Bank1?
Direct Addressing specifies all or part of the source
and/or destination address of the operation within the
opcode itself. The options are specified by the
arguments accompanying the instruction.
BTFSS FSR0H,1
In the core PIC18 instruction set, bit-oriented and
byte-oriented instructions use some version of Direct
Addressing by default. All of these instructions include
some 8-bit literal address as their Least Significant
Byte. This address specifies either a register address in
one of the banks of data RAM (Section 6.3.3 “General
BRA
CONTINUE
NEXT
; NO, clear next
; YES, continue
© 2009 Microchip Technology Inc.
DS39637D-page 95
MICROCHIP [ MICROCHIP ]
