PIC18F97J60_11 Datasheet, PDF (86/492 Pages) Microchip Technology – 64/80/100-Pin, High-Performance, 1-Mbit Flash Microcontrollers with Ethernet

PIC18F97J60 FAMILY
6.3 Data Memory Organization
Note:
The operation of some aspects of data
memory is changed when the PIC18
extended instruction set is enabled. See
Section 6.6 “Data Memory and the
Extended Instruction Set” for more
information.
The data memory in PIC18 devices is implemented as
static RAM. Each register in the data memory has a
12-bit address, allowing up to 4096 bytes of addressable
memory. The memory space is divided into 16 banks
that contain 256 bytes each. All of the PIC18F97J60
family devices implement all available banks and pro-
vide 3808 bytes of data memory available to the user.
Figure 6-7 shows the data memory organization for the
devices.
The data memory contains Special Function Registers
(SFRs) and General Purpose Registers (GPRs). The
SFRs are used for control and status of the controller
and peripheral functions, while GPRs are used for data
storage and scratchpad operations in the user’s
application. Any read of an unimplemented location will
read as ‘0’s.
The instruction set and architecture allow operations
across all banks. The entire data memory may be
accessed by Direct, Indirect or Indexed Addressing
modes. Addressing modes are discussed later in this
section.
To ensure that commonly used registers (most SFRs
and select GPRs) can be accessed in a single cycle,
PIC18 devices implement an Access Bank. This is a
256-byte memory space that provides fast access to
the majority of SFRs and the lower portion of GPR
Bank 0 without using the BSR. Section 6.3.2 “Access
Bank” provides a detailed description of the Access
RAM.
6.3.1 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 Most Significant bits of a
location’s address; the instruction itself includes the
8 Least Significant bits (LSbs). Only the four lower bits
of the BSR are implemented (BSR3:BSR0). The upper
four bits are unused; they will always read ‘0’ and can-
not 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 shown in
Figure 6-8.
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
program data to an 8-bit address of F9h, while the BSR
is 0Fh, will end up resetting the program counter.
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 6-7 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.
DS39762F-page 86
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