Microchip Technology MA160014 Data Sheet
PIC18(L)F2X/4XK22
DS41412F-page 90
2010-2012 Microchip Technology Inc.
5.6
Data Addressing Modes
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.
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.
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
is available when the extended instruction set is
enabled (XINST Configuration bit = 1). Its operation is
discussed in greater detail in
.
5.6.1
INHERENT AND LITERAL
ADDRESSING
ADDRESSING
Many PIC18 control instructions do not need any argu-
ment at all; they either perform an operation that glob-
ally affects the device or they operate implicitly on one
register. This addressing mode is known as Inherent
Addressing. Examples include SLEEP, RESET and DAW.
ment at all; they either perform an operation that glob-
ally 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.
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.
5.6.2
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.
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 Least Significant
Byte. This address specifies either a register address in
one of the banks of data RAM (
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 (
) or a location in the 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
(
interpreted. When ‘a’ is ‘1’, the contents of the 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.
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.
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.
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.
5.6.3
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 which are to be read
or written. Since the FSRs are themselves located in
RAM as Special File Registers, they can also be
directly manipulated under program control. This
makes FSRs very useful in implementing data struc-
tures, such as tables and arrays in data memory.
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 which are to be read
or written. Since the FSRs are themselves located in
RAM as Special File Registers, they can also be
directly manipulated under program control. This
makes FSRs very useful in implementing data struc-
tures, 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
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 5-5:
HOW TO CLEAR RAM
(BANK 1) USING
INDIRECT ADDRESSING
(BANK 1) USING
INDIRECT ADDRESSING
Note:
The execution of some instructions in the
core PIC18 instruction set are changed
when the PIC18 extended instruction set is
enabled. See
core PIC18 instruction set are changed
when the PIC18 extended instruction set is
enabled. See
more information.
LFSR
FSR0, 100h ;
NEXT
CLRF
POSTINC0
; Clear INDF
; register then
; inc pointer
BTFSS
FSR0H, 1
; All done with
; Bank1?
BRA
NEXT
; NO, clear next
CONTINUE
; YES, continue