PIC18F2331/2431/4331/4431
6.5.1
BANK SELECT REGISTER (BSR)
6.5.2
ACCESS BANK
Large areas of data memory require an efficient
addressing scheme to make rapid access to any
address possible. Ideally, this means that an entire
address does not need to be provided for each read or
write operation. For PIC18 devices, this is accom-
plished with a RAM banking scheme. This divides the
memory space into 16 contiguous banks of 256 bytes.
While the use of the BSR with an embedded 8-bit
address allows users to address the entire range of
data memory, it also means that the user must always
ensure that the correct bank is selected; otherwise,
data may be read from or written to the wrong location.
This can be disastrous if a GPR is the intended target
of an operation, but an SFR is written to instead.
Verifying and/or changing the BSR for each read or
write to data memory can become very inefficient.
Depending on the instruction, each location can be
addressed directly by its full 12-bit address, or an 8-bit
low-order address and a four-bit Bank Pointer. Most
instructions in the PIC18 instruction set make use of
the Bank Pointer, known as the Bank Select Register
(BSR). This SFR holds the four Most Significant bits of
a location’s address; the instruction itself includes the
eight Least Significant bits. Only the four lower bits of
the BSR are implemented (BSR<3:0>). The upper four
bits are unused; they will always read ‘0’ and cannot be
written to. The BSR can be loaded directly by using the
MOVLBinstruction.
To streamline access for the most commonly used data
memory locations, the data memory is configured with
an Access Bank, which allows users to access a
mapped block of memory without specifying a BSR.
The Access Bank consists of the first 128 bytes of
memory (00h-7Fh) in Bank 0 and the last 128 bytes of
memory (80h-FFh) in Block 15. The lower half is known
as the “Access RAM” and is composed of GPRs. This
upper half is also where the device’s SFRs are
mapped. These two areas are mapped contiguously in
the Access Bank and can be addressed in a linear
fashion by an 8-bit address (Figure 6-6).
The value of the BSR indicates the bank in data mem-
ory. The eight bits in the instruction show the location in
the bank and can be thought of as an offset from the
bank’s lower boundary. The relationship between the
BSR’s value and the bank division in data memory is
shown in Figure 6-6.
The Access Bank is used by core PIC18 instructions
that include the Access RAM bit (the ‘a’ parameter in
the instruction). When ‘a’ is equal to ‘1’, the instruction
uses the BSR and the 8-bit address included in the
opcode for the data memory address. When ‘a’ is ‘0’,
however, the instruction is forced to use the Access
Bank address map; the current value of the BSR is
ignored entirely.
Since up to 16 registers may share the same low-order
address, the user must always be careful to ensure that
the proper bank is selected before performing a data
read or write. For example, writing what should be pro-
gram data to the eight-bit address of F9h, while the
BSR is 0Fh, will end up resetting the program counter.
Using this “forced” addressing allows the instruction to
operate on a data address in a single cycle, without
updating the BSR first. For 8-bit addresses of 80h and
above, this means that users can evaluate and operate
on SFRs more efficiently. The Access RAM below 80h
is a good place for data values that the user might need
to access rapidly, such as immediate computational
results or common program variables. Access RAM
also allows for faster and more code efficient context
saving and switching of variables.
While any bank can be selected, only those banks that
are actually implemented can be read or written to.
Writes to unimplemented banks are ignored, while
reads from unimplemented banks will return ‘0’s. Even
so, the STATUS register will still be affected as if the
operation was successful. The data memory map in
Figure 6-5 indicates which banks are implemented.
In the core PIC18 instruction set, only the MOVFF
instruction fully specifies the 12-bit address of the
source and target registers. This instruction ignores the
BSR completely when it executes. All other instructions
include only the low-order address as an operand and
must use either the BSR or the Access Bank to locate
their target registers.
6.5.3
GENERAL PURPOSE REGISTER
(GPR) FILE
PIC18 devices may have banked memory in the GPR
area. This is data RAM, which is available for use by all
instructions. GPRs start at the bottom of Bank 0
(address 000h) and grow upwards towards the bottom of
the SFR area. GPRs are not initialized by a Power-on
Reset and are unchanged on all other Resets.
DS39616D-page 68
2010 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
