DSPIC30F5015 datasheet(27/234 Pages) MICROCHIP

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DS70149E-page 27

dsPIC30F5015/5016

3.1.1

DATA ACCESS FROM PROGRAM

MEMORY USING TABLE

INSTRUCTIONS

This architecture fetches 24-bit wide program memory.

Consequently, instructions are always aligned. How-

ever, as the architecture is modified Harvard, data can

also be present in program space.

There are two methods by which program space can

be accessed; via special table instructions, or through

the remapping of a 16K word program space page into

the upper half of data space (see Section 3.1.2 “Data

Access from Program Memory Using Program

Space Visibility”). The TBLRDL and TBLWTL instruc-

tions offer a direct method of reading or writing the least

significant word of any address within program space,

without going through data space. The TBLRDH and

TBLWTH

instructions are the only method whereby the

upper 8 bits of a program space word can be accessed

as data.

The PC is incremented by two for each successive

24-bit program word. This allows program memory

addresses to directly map to data space addresses.

Program memory can thus be regarded as two 16-bit

word wide address spaces, residing side by side, each

with the same address range. TBLRDL and TBLWTL

access the space that contains the least significant

word, and TBLRDH and TBLWTH access the space that

contains the MSB.

Figure 3-2 shows how the EA is created for table oper-

ations and data space accesses (PSV = 1). Here,

P<23:0> refers to a program space word, whereas

D<15:0> refers to a data space word.

A set of table instructions are provided to move byte or

word-sized data to and from program space.

TBLRDL:

Table Read Low

Word: Read the least significant word of the

program address;

P<15:0> maps to D<15:0>.

Byte: Read one of the LSBs of the program

address;

P<7:0> maps to the destination byte when byte

select = 0;

P<15:8> maps to the destination byte when byte

select = 1.

TBLWTL:

Table Write Low (refer to Section 6.0

“Flash Program Memory” for details on Flash

Programming).

TBLRDH:

Table Read High

Word: Read the most significant word of the

program address;

P<23:16> maps to D<7:0>; D<15:8> always

is = 0.

Byte: Read one of the MSBs of the program

address;

P<23:16> maps to the destination byte when

byte select = 0;

The destination byte will always be = 0 when

byte select = 1.

TBLWTH:

Table Write High (refer to Section 6.0

“Flash Program Memory” for details on Flash

Programming).

FIGURE 3-3:

PROGRAM DATA TABLE ACCESS (LEAST SIGNIFICANT WORD)

PC Address

0x000000

0x000002

0x000004

0x000006

00000000

Program Memory

‘Phantom’ Byte

(Read as ‘0’).

TBLRDL.W

TBLRDL.B (Wn<0> = 1)

TBLRDL.B (Wn<0> = 0)

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