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PIC24FJ256GA705 FAMILY

DS30010118D-page 58

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4.3

Interfacing Program and Data

Memory Spaces

The PIC24F architecture uses a 24-bit wide program

space and 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 PIC24F architecture

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 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 remapping 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.

4.3.1

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.

For table operations, the 8-bit Table Memory Page

Address 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 MSBs of TBLPAG are

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 10-bit Extended Data

Space Read register (DSRPAG) is used to define a

16K word page in the program space. When the Most

Significant bit (MSb) of the EA is ‘1’, and the MSb (bit 9)

of DSRPAG is ‘1’, the lower eight bits of DSRPAG are

concatenated with the lower 15 bits of the EA to form a

23-bit program space address. The DSRPAG<8> bit

decides whether the lower word (when the bit is ‘0’) or

the higher word (when the bit is ‘1’) of program memory

is mapped. Unlike table operations, this strictly limits

remapping operations to the user memory area.

Table 4-14 and Figure 4-7 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.

TABLE 4-14:

PROGRAM SPACE ADDRESS CONSTRUCTION

Access Type

Access

Space

Program Space Address

<23>

<22:16>

<15>

<14:1>

<0>

Instruction Access

(Code Execution)

User

PC<22:1>

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)

User

DSRPAG<7:0>(2)

Data EA<14:0>(1)

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 DSRPAG<0>.

DSRPAG<9> is always ‘1’ in this case. DSRPAG<8> decides whether the lower word or higher word of

program memory is read. When DSRPAG<8> is ‘0’, the lower word is read, and when it is ‘1’, the higher

word is read.

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