PIC18FXX20
4.9.1
GENERAL PURPOSE REGISTER
FILE
4.9
Data Memory Organization
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. The data
memory map is in turn divided into 16 banks of 256
bytes each. The lower 4 bits of the Bank Select Regis-
ter (BSR<3:0>) select which bank will be accessed.
The upper 4 bits for the BSR are not implemented.
The register file can be accessed either directly or indi-
rectly. Indirect addressing operates using a File Select
Register and corresponding Indirect File Operand. 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 general purpose reg-
isters by all instructions. The top section of Bank 15
(F60h to FFFh) contains SFRs. All other banks of data
memory contain GPR registers, starting with Bank 0.
The data memory space contains both Special Func-
tion 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 application. The SFRs start at the last location of
Bank 15 (0FFFh) and extend downwards. Any remain-
ing space beyond the SFRs in the Bank may be imple-
mented as GPRs. GPRs start at the first location of
4.9.2
SPECIAL FUNCTION REGISTERS
The Special Function Registers (SFRs) are registers
used by the CPU and Peripheral Modules for control-
ling the desired operation of the device. These regis-
ters are implemented as static RAM. A list of these
registers is given in Table 4-2 and Table 4-3.
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. The
SFRs are typically distributed among the peripherals
whose functions they control.
Bank
0 and grow upwards. Any read of an
unimplemented location will read as ‘0’s.
PIC18FX520 devices have 2048 bytes of data RAM,
extending from Bank 0 to Bank 7 (000h through 7FFh).
PIC18FX620 and PIC18FX720 devices have
3840 bytes of data RAM, extending from Bank 0 to
Bank 14 (000h through EFFh). The organization of the
data memory space for these devices is shown in
Figure 4-6 and Figure 4-7.
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 a
File Select Register (FSRn) and a corresponding Indi-
rect File Operand (INDFn). Each FSR holds a 12-bit
address value that can be used to access any location
in the Data Memory map without banking.
The unused SFR locations are unimplemented and
read as '0's. The addresses for the SFRs are listed in
Table 4-2.
The instruction set and architecture allow operations
across all banks. This may be accomplished by indirect
addressing, or by the use of the MOVFFinstruction. The
MOVFF instruction is a two-word/two-cycle instruction
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,
regardless 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
provides a detailed description of the Access RAM.
2003 Microchip Technology Inc.
Advance Information
DS39609A-page 47
MICROCHIP [ MICROCHIP ]
