192GA110 datasheet(49/292 Pages) MICROCHIP

49 / 292 page

Preliminary

DS39905B-page 47

PIC24FJ256GA110 FAMILY

3.3.2

DATA ACCESS FROM PROGRAM

MEMORY USING TABLE

INSTRUCTIONS

The TBLRDL and TBLWTL instructions offer a direct

method of reading or writing the lower word of any

address within the program space without going through

data space. The TBLRDH and TBLWTH instructions are

the only method to read or write the upper 8 bits of a

program space word 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 which contains the least significant

data word, and TBLRDH and TBLWTH access the space

which contains the upper data byte.

Two table instructions are provided to move byte or

word-sized (16-bit) data to and from program space.

Both function as either byte or word operations.

TBLRDL (Table Read Low): In Word mode, it

maps the lower word of the program space

location (P<15:0>) to a data address (D<15:0>).

In Byte mode, either the upper or lower byte of

the lower program word is mapped to the lower

byte of a data address. The upper byte is

selected when byte select is ‘1’; the lower byte

is selected when it is ‘0’.

TBLRDH (Table Read High): In Word mode, it

maps the entire upper word of a program address

(P<23:16>) to a data address. Note that

D<15:8>, the ‘phantom’ byte, will always be ‘0’.

In Byte mode, it maps the upper or lower byte of

the program word to D<7:0> of the data

address, as above. Note that the data will

always be ‘0’ when the upper ‘phantom’ byte is

selected (byte select = 1).

In a similar fashion, two table instructions, TBLWTH

and TBLWTL, are used to write individual bytes or

words to a program space address. The details of

their operation are explained in Section 4.0 “Flash

Program Memory”.

For all table operations, the area of program memory

space to be accessed is determined by the Table

Memory Page Address register (TBLPAG). TBLPAG

covers the entire program memory space of the

device, including user and configuration spaces. When

TBLPAG<7> = 0, the table page is located in the user

memory space. When TBLPAG<7> = 1, the page is

located in configuration space.

FIGURE 3-6:

ACCESSING PROGRAM MEMORY WITH TABLE INSTRUCTIONS

Note:

Only table read operations will execute in

the configuration memory space, and only

then, in implemented areas such as the

Device ID. Table write operations are not

allowed.

00000000

‘Phantom’ Byte

TBLRDH.B (Wn<0> = 0)

TBLRDL.W

TBLRDL.B (Wn<0> = 1)

TBLRDL.B (Wn<0> = 0)

TBLPAG

000000h

800000h

020000h

030000h

Program Space

Data EA<15:0>

The address for the table operation is determined by the data EA

within the page defined by the TBLPAG register.

Only read operations are shown; write operations are also valid in

the user memory area.

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