PIC18F97J60_11 Datasheet, PDF (88/492 Pages) Microchip Technology – 64/80/100-Pin, High-Performance, 1-Mbit Flash Microcontrollers with Ethernet

PIC18F97J60 FAMILY
FIGURE 6-8:
USE OF THE BANK SELECT REGISTER (DIRECT ADDRESSING)
7
BSR⁽¹⁾
0
0 0 00001 0
Bank Select⁽²⁾
000h ^{Data Memory} _00h
Bank 0
100h
FFh
00h
Bank 1
200h
FFh
00h
Bank 2
300h
FFh
00h
7
From Opcode⁽²⁾
0
11 11 11 11 11 1 1 1
Bank 3
through
Bank 13
E00h
FFh
00h
Bank 14
F00h
FFh
00h
Bank 15
FFFh
FFh
Note 1:
2:
The Access RAM bit of the instruction can be used to force an override of the selected bank (BSR3:BSR0)
to the registers of the Access Bank.
The MOVFF instruction embeds the entire 12-bit address in the instruction.
6.3.2 ACCESS BANK
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.
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 96 bytes of
memory (00h-5Fh) in Bank 0 and the last 160 bytes of
memory (60h-FFh) in Bank 15. The lower block is
known as the “Access RAM” and is composed of
GPRs. The upper block is 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 6-7).
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.
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 60h and
above, this means that users can evaluate and operate
on SFRs more efficiently. The Access RAM below 60h
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.
The mapping of the Access Bank is slightly different
when the extended instruction set is enabled (XINST
Configuration bit = 1). This is discussed in more detail
in Section 6.6.3 “Mapping the Access Bank in
Indexed Literal Offset Mode”.
6.3.3
GENERAL PURPOSE
REGISTER 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.
DS39762F-page 88
 2011 Microchip Technology Inc.