Datasheet

 2013-2014 Microchip Technology Inc. DS70000689D-page 41
dsPIC33EPXXXGM3XX/6XX/7XX
4.2 Data Address Space
The dsPIC33EPXXXGM3XX/6XX/7XX CPU has a
separate 16-bit wide data memory space. The Data
Space is accessed using separate Address Generation
Units (AGUs) for read and write operations. The data
memory maps, which are presented by device family
and memory size, are shown in Figure 4-5 through
Figure 4-7.
All Effective Addresses (EAs) in the data memory space
are 16 bits wide and point to bytes within the Data
Space. This arrangement gives a Base Data Space
address range of 64 Kbytes or 32K words.
The Base Data Space address is used in conjunction
with a Data Space Read or Write Page register
(DSRPAG or DSWPAG) to form an Extended Data
Space, which has a total address range of 16 Mbytes.
dsPIC33EPXXXGM3XX/6XX/7XX devices implement
up to 52 Kbytes of data memory (4 Kbytes of data
memory for Special Function Registers and up to
48 Kbytes of data memory for RAM). If an EA points to
a location outside of this area, an all zero word or byte
is returned.
4.2.1 DATA SPACE WIDTH
The data memory space is organized in byte-
addressable, 16-bit wide blocks. Data is aligned in
data memory and registers as 16-bit words, but all Data
Space EAs resolve to bytes. The Least Significant
Bytes (LSBs) of each word have even addresses, while
the Most Significant Bytes (MSBs) have odd
addresses.
4.2.2 DATA MEMORY ORGANIZATION
AND ALIGNMENT
To maintain backward compatibility with PIC
®
MCU
devices and improve Data Space memory usage
efficiency, the dsPIC33EPXXXGM3XX/6XX/7XX
instruction set supports both word and byte operations.
As a consequence of byte accessibility, 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++] results in a value of Ws + 1 for
byte operations and Ws + 2 for word operations.
A data byte read, reads the complete word that
contains 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
registers 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 that 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
operations, or translating from 8-bit MCU code. If a
misaligned read or write is attempted, an address error
trap is generated. If the error occurred on a read, the
instruction underway is completed. If the error occurred
on a write, the instruction is executed but the write does
not occur. In either case, a trap is then executed,
allowing the system and/or user application to examine
the machine state prior to execution of the address
Fault.
All byte loads into any W register are loaded into the
LSB; the MSB is not modified.
A Sign-Extend (SE) instruction is provided to allow user
applications to translate 8-bit signed data to 16-bit
signed values. Alternatively, for 16-bit unsigned data,
user applications can clear the MSB of any W register
by executing a Zero-Extend (ZE) instruction on the
appropriate address.
4.2.3 SFR SPACE
The first 4 Kbytes of the Near Data Space, from
0x0000 to 0x0FFF, is primarily occupied by Special
Function Registers (SFRs). These are used by the
dsPIC33EPXXXGM3XX/6XX/7XX core and peripheral
modules for controlling the operation of the device.
SFRs are distributed among the modules that they
control and are generally grouped together by module.
Much of the SFR space contains unused addresses;
these are read as ‘0’.
4.2.4 NEAR DATA SPACE
The 8-Kbyte area, between 0x0000 and 0x1FFF, is
referred to as the Near Data Space. Locations in this
space are directly addressable through a 13-bit abso-
lute address field within all memory direct instructions.
Additionally, the whole Data Space is addressable
using MOV instructions, which support Memory Direct
Addressing mode with a 16-bit address field, or by
using Indirect Addressing mode using a Working
register as an Address Pointer.
Note: The actual set of peripheral features and
interrupts varies by the device. Refer to the
corresponding device tables and pinout
diagrams for device-specific information.