DSPIC30F2023 Microchip Technology Inc., DSPIC30F2023 Datasheet - Page 38

• Manufacturer Part Number: DSPIC30F2023
• Description: 28/44-pin Dspic30f1010/202x Enhanced Flash Smps 16-bit Digital Signal Controller
• Manufacturer: Microchip Technology Inc.

dsPIC30F1010/202X
3.2.2
The X data space is used by all instructions and sup-
ports all Addressing modes. There are separate read
and write data buses. The X read data bus is the return
data path for all instructions that view data space as
combined X and Y address space. It is also the X
address space data path for the dual operand read
instructions (MAC class). The X write data bus is the
only write path to data space for all instructions.
The X data space also supports modulo addressing for
all instructions, subject to Addressing mode restric-
tions. Bit-Reversed Addressing is only supported for
writes to X data space.
The Y data space is used in concert with the X data
space by the MAC class of instructions (CLR, ED,
EDAC, MAC, MOVSAC, MPY, MPY.N and MSC) to pro-
vide two concurrent data read paths. No writes occur
across the Y bus. This class of instructions dedicates
two W register pointers, W10 and W11, to always
address Y data space, independent of X data space,
whereas W8 and W9 always address X data space.
Note that during accumulator write back, the data
address space is considered a combination of X and Y
data spaces, so the write occurs across the X bus.
Consequently, the write can be to any address in the
entire data space.
The Y data space can only be used for the data
prefetch operation associated with the MAC class of
instructions. It also supports modulo addressing for
automated circular buffers. Of course, all other instruc-
tions can access the Y data address space through the
X data path, as part of the composite linear space.
The boundary between the X and Y data spaces is
defined as shown in Figure 3-6 and is not user pro-
grammable. Should an EA point to data outside its own
assigned address space, or to a location outside phys-
ical memory, an all-zero word/byte will be returned. For
example, although Y address space is visible by all
non-MAC instructions using any Addressing mode, an
attempt by a MAC instruction to fetch data from that
space, using W8 or W9 (X space pointers), will return
0x0000.
TABLE 3-2:
All effective addresses are 16 bits wide and point to
bytes within the data space. Therefore, the data space
address range is 64 Kbytes or 32K words.
DS70178C-page 36
Attempted Operation
EA = an unimplemented address
W8 or W9 used to access Y data
space in a MAC instruction
W10 or W11 used to access X
data space in a MAC instruction
DATA SPACES
EFFECT OF INVALID
MEMORY ACCESSES
Data Returned
0x0000
0x0000
0x0000
Preliminary
3.2.3
The core data width is 16 bits. All internal registers are
organized as 16-bit wide words. Data space memory is
organized in byte addressable, 16-bit wide blocks.
3.2.4
To help maintain backward compatibility with PIC
MCU devices and improve data space memory usage
efficiency, the dsPIC30F instruction set supports both
word and byte operations. Data is aligned in data mem-
ory and registers as words, but all data space EAs
resolve to bytes. Data byte reads will read the complete
word, which 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 X data path (no byte
accesses are possible from the Y data path as the MAC
class of instruction can only fetch words). 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 which
matches the byte address.
As a consequence of this byte accessibility, all effective
address calculations (including those generated by the
DSP operations, which are restricted to word sized
data) are internally scaled to step through word-aligned
memory. For example, the core would recognize 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.
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. Should a mis-
aligned read or write be attempted, an address error
trap will be generated. If the error occurred on a read,
the instruction underway is completed, whereas if it
occurred on a write, the instruction will be executed but
the write will not occur. In either case, a trap will then
be executed, allowing the system and/or user to exam-
ine the machine state prior to execution of the address
fault.
FIGURE 3-8:
0001
0003
0005
15
DATA SPACE WIDTH
DATA ALIGNMENT
Byte 1
Byte 3
Byte 5
MSB
DATA ALIGNMENT
© 2006 Microchip Technology Inc.
8 7
Byte 0
Byte 2
Byte 4
LSB
0
0000
0002
0004
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