PIC18F47J53 Datasheet, PDF (89/586 Pages) Microchip Technology – 28/44-Pin, High-Performance USB Microcontrollers with nanoWatt XLP Technology

PIC18F47J53 FAMILY
6.3.2 BANK SELECT REGISTER
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.
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 4-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 4 MSbs of a location’s
address; the instruction itself includes the 8 LSbs. 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 MOVLB instruction.
The value of the BSR indicates the bank in data
memory. The 8 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
illustrated in Figure 6-7.
Because up to 16 registers can share the same
low-order address, the user must always be careful to
ensure that the proper bank is selected before perform-
ing a data read or write. For example, writing what
should be program data to an 8-bit address of F9h
while the BSR is 0Fh, will end up resetting the PC.
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 6-6 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.
 2010 Microchip Technology Inc.
Preliminary
DS39964B-page 89