PIC18CXX2
4.9.2
SPECIAL FUNCTION REGISTERS
4.9
Data Memory Organization
The Special Function Registers (SFRs) are registers
used by the CPU and Peripheral Modules for controlling
the desired operation of the device. These registers are
implemented as static RAM. A list of these registers is
given in Table 4-1 and Table 4-2.
The data memory is implemented as static RAM. Each
register in the data memory has a 12-bit address,
allowing up to 4096 bytes of data memory. Figure 4-6
and Figure 4-7 show the data memory organization for
the PIC18CXX2 devices.
The SFRs can be classified into two sets; those asso-
ciated with the “core” function and those related to the
peripheral functions. Those registers related to the
“core” are described in this section, while those related
to the operation of the peripheral features are
described in the section of that peripheral feature.
Banking is required to allow more than 256 bytes to be
accessed. The data memory map is divided into as
many as 16 banks that contain 256 bytes each. The
lower 4 bits of the Bank Select Register (BSR<3:0>)
select which bank will be accessed. The upper 4 bits
for the BSR are not implemented.
The SFRs are typically distributed among the peripher-
als whose functions they control.
The data memory contains Special Function Registers
(SFR) and General Purpose Registers (GPR). The
SFRs are used for control and status of the controller
and peripheral functions, while GPRs are used for data
storage and scratch pad operations in the user’s appli-
cation. The SFRs start at the last location of Bank 15
(OxFFF) and grow downwards. Any remaining space
beyond the SFRs in the Bank may be implemented as
GPRs. GPRs start at the first location of Bank 0 and
grow upwards. Any read of an unimplemented location
will read as ’0’s.
The unused SFR locations will be unimplemented and
read as '0's. See Table 4-1 for addresses for the SFRs.
The entire data memory may be accessed directly or
indirectly. Direct addressing may require the use of the
BSR register. Indirect addressing requires the use of
the File Select Register (FSR). Each FSR holds a 12-
bit address value that can be used to access any loca-
tion in the Data Memory map without banking.
The instruction set and architecture allow operations
across all banks. This may be accomplished by indirect
addressing or by the use of the MOVFF instruction.
The MOVFF instruction is a two word/two cycle instruc-
tion that moves a value from one register to another.
To ensure that commonly used registers (SFRs and
select GPRs) can be accessed in a single cycle regard-
less of the current BSR values, an Access Bank is
implemented. A segment of Bank 0 and a segment of
Bank 15 comprise the Access RAM. Section 4.10 pro-
vides a detailed description of the Access RAM.
4.9.1
GENERAL PURPOSE REGISTER FILE
The register file can be accessed either directly or indi-
rectly. Indirect addressing operates through the File
Select Registers (FSR). The operation of indirect
addressing is shown in Section 4.12.
Enhanced MCU devices may have banked memory in
the GPR area. GPRs are not initialized by a Power-on
Reset and are unchanged on all other resets.
Data RAM is available for use as GPR registers by all
instructions. The top half of bank 15 (0xF80 to 0xFFF)
contains SFRs. All other banks of data memory contain
GPR registers starting with bank 0.
DS39026B-page 40
Preliminary
7/99 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
