Microchip Technology MA330011 Data Sheet
dsPIC33F
DS70165E-page 72
Preliminary
©
2007 Microchip Technology Inc.
3.6
Interfacing Program and Data
Memory Spaces
Memory Spaces
The dsPIC33F architecture uses a 24-bit wide program
space and a 16-bit wide data space. The architecture is
also a modified Harvard scheme, meaning that data
can also be present in the program space. To use this
data successfully, it must be accessed in a way that
preserves the alignment of information in both spaces.
space and a 16-bit wide data space. The architecture is
also a modified Harvard scheme, meaning that data
can also be present in the program space. To use this
data successfully, it must be accessed in a way that
preserves the alignment of information in both spaces.
Aside from normal execution, the dsPIC33F architec-
ture provides two methods by which program space
can be accessed during operation:
ture provides two methods by which program space
can be accessed during operation:
• Using table instructions to access individual bytes
or words anywhere in the program space
• Remapping a portion of the program space into
the data space (Program Space Visibility)
Table instructions allow an application to read or write
to small areas of the program memory. This capability
makes the method ideal for accessing data tables that
need to be updated from time to time. It also allows
access to all bytes of the program word. The remap-
ping method allows an application to access a large
block of data on a read-only basis, which is ideal for
look ups from a large table of static data. It can only
access the least significant word of the program word.
to small areas of the program memory. This capability
makes the method ideal for accessing data tables that
need to be updated from time to time. It also allows
access to all bytes of the program word. The remap-
ping method allows an application to access a large
block of data on a read-only basis, which is ideal for
look ups from a large table of static data. It can only
access the least significant word of the program word.
3.6.1
ADDRESSING PROGRAM SPACE
Since the address ranges for the data and program
spaces are 16 and 24 bits, respectively, a method is
needed to create a 23-bit or 24-bit program address
from 16-bit data registers. The solution depends on the
interface method to be used.
spaces are 16 and 24 bits, respectively, a method is
needed to create a 23-bit or 24-bit program address
from 16-bit data registers. The solution depends on the
interface method to be used.
For table operations, the 8-bit Table Page register
(TBLPAG) is used to define a 32K word region within
the program space. This is concatenated with a 16-bit
EA to arrive at a full 24-bit program space address. In
this format, the Most Significant bit of TBLPAG is used
to determine if the operation occurs in the user memory
(TBLPAG<7> =
(TBLPAG) is used to define a 32K word region within
the program space. This is concatenated with a 16-bit
EA to arrive at a full 24-bit program space address. In
this format, the Most Significant bit of TBLPAG is used
to determine if the operation occurs in the user memory
(TBLPAG<7> =
0
) or the configuration memory
(TBLPAG<7> =
1
).
For remapping operations, the 8-bit Program Space
Visibility register (PSVPAG) is used to define a
16K word page in the program space. When the Most
Significant bit of the EA is ‘
Visibility register (PSVPAG) is used to define a
16K word page in the program space. When the Most
Significant bit of the EA is ‘
1
’, PSVPAG is concatenated
with the lower 15 bits of the EA to form a 23-bit program
space address. Unlike table operations, this limits
remapping operations strictly to the user memory area.
space address. Unlike table operations, this limits
remapping operations strictly to the user memory area.
Table 3-37 and Figure 3-9 show how the program EA is
created for table operations and remapping accesses
from the data EA. Here, P<23:0> refers to a program
space word, whereas D<15:0> refers to a data space
word.
created for table operations and remapping accesses
from the data EA. Here, P<23:0> refers to a program
space word, whereas D<15:0> refers to a data space
word.
TABLE 3-37:
PROGRAM SPACE ADDRESS CONSTRUCTION
Access Type
Access
Space
Program Space Address
<23>
<22:16>
<15>
<14:1>
<0>
Instruction Access
(Code Execution)
(Code Execution)
User
0
PC<22:1>
0
0xx xxxx xxxx xxxx xxxx xxx0
TBLRD/TBLWT
(Byte/Word Read/Write)
User
TBLPAG<7:0>
Data EA<15:0>
0xxx xxxx xxxx xxxx xxxx xxxx
Configuration TBLPAG<7:0>
Data
EA<15:0>
1xxx xxxx xxxx xxxx xxxx xxxx
Program Space Visibility
(Block Remap/Read)
(Block Remap/Read)
User
0
PSVPAG<7:0>
Data EA<14:0>
(1)
0 xxxx xxxx xxx xxxx xxxx xxxx
Note 1:
Data EA<15> is always ‘
1
’ in this case, but is not used in calculating the program space address. Bit 15 of
the address is PSVPAG<0>.