PIC24FJ128GA Datasheet, PDF (30/232 Pages) Microchip Technology – General Purpose, 16-Bit Flash Microcontrollers

PIC24FJ128GA FAMILY
3.2.2
DATA MEMORY ORGANIZATION
AND ALIGNMENT
To maintain backward compatibility with PICmicro^®
devices and improve data space memory usage effi-
ciency, the PIC24 instruction set supports both word
and byte operations. As a consequence of byte acces-
sibility, all effective address calculations are internally
scaled to step through word-aligned memory. For
example, the core recognizes that Post-Modified
Register Indirect Addressing mode [Ws++] will result in
a value of Ws + 1 for byte operations and Ws + 2 for
word operations.
Data byte reads will read the complete word which con-
tains the byte, using the LSb of any EA to determine
which byte to select. The selected byte is placed onto
the LSB of the data path. That is, data memory and reg-
isters are organized as two parallel byte-wide entities
with shared (word) address decode but separate write
lines. Data byte writes only write to the corresponding
side of the array or register which matches the byte
address.
All word accesses must be aligned to an even address.
Misaligned word data fetches are not supported, so
care must be taken when mixing byte and word opera-
tions, or translating from 8-bit MCU code. If a
misaligned read or write is attempted, an address error
trap will be generated. If the error occurred on a read,
the instruction underway is completed; if it occurred on
a write, the instruction will be executed but the write will
not occur. In either case, a trap is then executed, allow-
ing the system and/or user to examine the machine
state prior to execution of the address Fault.
All byte loads into any W register are loaded into the
Least Significant Byte. The Most Significant Byte is not
modified.
A sign-extend instruction (SE) is provided to allow
users to translate 8-bit signed data to 16-bit signed
values. Alternatively, for 16-bit unsigned data, users
can clear the MSB of any W register by executing a
zero-extend (ZE) instruction on the appropriate
address.
Although most instructions are capable of operating on
word or byte data sizes, it should be noted that some
instructions operate only on words.
3.2.3 NEAR DATA SPACE
The 8-Kbyte area between 0000h and 1FFFh is
referred to as the near data space. Locations in this
space are directly addressable via a 13-bit absolute
address field within all memory direct instructions. The
remainder of the data space is addressable indirectly.
Additionally, the whole data space is addressable using
MOV instructions, which support Memory Direct
Addressing with a 16-bit address field.
3.2.4 SFR SPACE
The first 2 Kbytes of the near data space, from 0000h
to 07FFh, are primarily occupied with Special Function
Registers (SFRs). These are used by the PIC24 core
and peripheral modules for controlling the operation of
the device.
SFRs are distributed among the modules that they con-
trol, and are generally grouped together by module.
Much of the SFR space contains unused addresses;
these are read as ‘0’. A diagram of the SFR space,
showing where SFRs are actually implemented, is
shown in Table 3-2. Each implemented area indicates
a 32-byte region where at least one address is imple-
mented as an SFR. A complete listing of implemented
SFRs, including their addresses, is shown in Tables 3-3
through 3-30.
TABLE 3-2: IMPLEMENTED REGIONS OF SFR DATA SPACE
SFR Space Address
xx00
xx20
xx40
xx60
xx80
000h
Core
ICN
100h
200h
Timers
I²C™
UART
Capture
—
SPI
300h
A/D
—
—
400h
—
—
—
—
500h
—
—
—
—
600h
PMP RTC/Comp CRC
—
700h
—
—
System NVM/PMD
Legend: — = No implemented SFRs in this block
Compare
—
—
—
—
—
—
xxA0
Interrupts
—
—
—
—
—
—
—
xxC0
xxE0
—
—
—
I/O
I/O
—
—
—
—
I/O
—
—
DS39747C-page 28
Preliminary
© 2006 Microchip Technology Inc.