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DS30010118D-page 61

PIC24FJ256GA705 FAMILY

4.3.3

READING DATA FROM PROGRAM

MEMORY USING EDS

The upper 32 Kbytes of Data Space may optionally be

mapped into any 16K word page of the program space.

This provides transparent access of stored constant

data from the Data Space without the need to use

special instructions (i.e., TBLRDL/H).

Program space access through the Data Space occurs

when the MSb of EA is ‘1’ and the DSRPAG<9> bit is

also ‘1’. The lower eight bits of DSRPAG are concate-

nated to the Wn<14:0> bits to form a 23-bit EA to

access program memory. The DSRPAG<8> decides

which word should be addressed; when the bit is ‘0’,

the lower word, and when ‘1’, the upper word of the

program memory is accessed.

The entire program memory is divided into 512 EDS

pages, from 200h to 3FFh, each consisting of 16K words

of data. Pages, 200h to 2FFh, correspond to the lower

words of the program memory, while 300h to 3FFh

correspond to the upper words of the program memory.

Using this EDS technique, the entire program memory

can be accessed. Previously, the access to the upper

word of the program memory was not supported.

Table 4-15 provides the corresponding 23-bit EDS

address for program memory with EDS page and

source addresses.

For operations that use PSV and are executed outside a

REPEAT loop, the MOV and MOV.D instructions will require

one instruction cycle in addition to the specified execution

time. All other instructions will require two instruction

cycles in addition to the specified execution time.

For operations that use PSV, which are executed inside

a REPEAT loop, there will be some instances that

require two instruction cycles in addition to the

specified execution time of the instruction:

• Execution in the first iteration

• Execution in the last iteration

• Execution prior to exiting the loop due to an

interrupt

• Execution upon re-entering the loop after an

interrupt is serviced

Any other iteration of the REPEAT loop will allow the

instruction accessing data, using PSV, to execute in a

single cycle.

TABLE 4-15:

EDS PROGRAM ADDRESS WITH DIFFERENT PAGES AND ADDRESSES

EXAMPLE 4-3:

EDS READ CODE FROM PROGRAM MEMORY IN ASSEMBLY

DSRPAG

(Data Space Read Register)

Source Address while

Indirect Addressing

23-Bit EA Pointing

to EDS

Comment

200h

•

2FFh

8000h to FFFFh

000000h to 007FFEh

•

7F8000h to 7FFFFEh

Lower words of 4M program

instructions (8 Mbytes) for

read operations only.

300h

•

3FFh

000001h to 007FFFh

•

7F8001h to 7FFFFFh

Upper words of 4M program

instructions (4 Mbytes remaining;

4 Mbytes are phantom bytes) for

read operations only.

000h

Invalid Address

Address error trap.(1)

Note 1:

When the source/destination address is above 8000h and DSRPAG/DSWPAG is ‘0’, an address error trap

will occur.

; Set the EDS page from where the data to be read

mov

#0x0202, w0

mov

w0, DSRPAG

;page 0x202, consisting lower words, is selected for read

mov

#0x000A, w1

;select the location (0x0A) to be read

bset

w1, #15

;set the MSB of the base address, enable EDS mode

;Read a byte from the selected location

mov.b

[w1++], w2

;read Low byte

mov.b

[w1++], w3

;read High byte

;Read a word from the selected location

mov

[w1], w2

;

;Read Double - word from the selected location

mov.d

[w1], w2

;two word read, stored in w2 and w3

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