PIC18F44J50-I Datasheet, PDF (97/562 Pages) Microchip Technology – 28/44-Pin, Low-Power, High-Performance USB Microcontrollers

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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.
While the program memory can be addressed in only
one way, through the PC, information in the data mem-
ory 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.
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 more detail in Section 6.6.1 “Indexed
Addressing with Literal Offset”.
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.
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 ADDLW and MOVLW, which 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.
6.4.2 DIRECT ADDRESSING
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.
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 LSB. This address
specifies either a register address in one of the banks
of data RAM (Section 6.3.4 “General Purpose
Register File”), or a location in the Access Bank
(Section 6.3.3 “Access Bank”) as the data source for
the instruction.
The Access RAM bit, ‘a’, determines how the address
is interpreted. When ‘a’ is ‘1’, the contents of the BSR
(Section 6.3.2 “Bank Select Register”) 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.
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
original 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.
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
SFRs, they can also be directly manipulated under
program control. This makes FSRs very useful in
implementing data structures such as tables and arrays
in data memory.
The registers for Indirect Addressing are also
implemented 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. It also enables users to perform
Indexed Addressing and other Stack Pointer
operations for program memory in data memory.
EXAMPLE 6-5:
HOW TO CLEAR RAM
(BANK 1) USING INDIRECT
ADDRESSING
NEXT
LFSR
CLRF
BTFSS
BRA
CONTINUE
FSR0, 0x100 ;
POSTINC0 ; Clear INDF
; register then
; inc pointer
FSR0H, 1 ; All done with
; Bank1?
NEXT
; NO, clear next
; YES, continue
 2011 Microchip Technology Inc.
DS39931D-page 97