D301 datasheet(57/416 Pages) MICROCHIP

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DS30010118D-page 57

PIC24FJ256GA705 FAMILY

TABLE 4-13:

EDS MEMORY ADDRESS WITH DIFFERENT PAGES AND ADDRESSES

4.2.6

SOFTWARE STACK

Apart from its use as a Working register, the W15

Stack Pointer (SSP). The pointer always points to the

first available free word and grows from lower to higher

addresses. It pre-decrements for stack pops and post-

increments for stack pushes, as shown in Figure 4-6.

Note that for a PC push during any CALL instruction,

the MSB of the PC is zero-extended before the push,

ensuring that the MSB is always clear.

The Stack Pointer Limit Value register (SPLIM), associ-

ated with the Stack Pointer, sets an upper address

boundary for the stack. SPLIM is uninitialized at Reset.

As is the case for the Stack Pointer, SPLIM<0> is

forced to ‘0’ as all stack operations must be word-

aligned. Whenever an EA is generated using W15 as a

source or destination pointer, the resulting address is

compared with the value in SPLIM. If the contents of

the Stack Pointer (W15) and the SPLIM register are

equal, and a push operation is performed, a stack error

trap will not occur. The stack error trap will occur on a

subsequent push operation. Thus, for example, if it is

desirable to cause a stack error trap when the stack

grows beyond address 2000h in RAM, initialize the

SPLIM with the value, 1FFEh.

Similarly, a Stack Pointer underflow (stack error) trap is

generated when the Stack Pointer address is found to

be less than 0800h. This prevents the stack from

interfering with the SFR space.

A write to the SPLIM register should not be immediately

followed by an indirect read operation using W15.

FIGURE 4-6:

CALL STACK FRAME

DSRPAG

(Data Space Read

DSWPAG

(Data Space Write

Source/Destination

Address while

Indirect

Addressing

24-Bit EA

Pointing to EDS

Comment

x(1)

0000h to 1FFFh

000000h to

001FFFh

Near Data Space(2)

2000h to 7FFFh

002000h to

007FFFh

001h

8000h to FFFFh

008000h to

00FFFEh

EPMP Memory Space

002h

010000h to

017FFEh

003h

•

1FFh

003h

•

1FFh

018000h to

0187FEh

•

FF8000h to

FFFFFEh

000h

Invalid Address

Address Error Trap(3)

Note 1:

If the source/destination address is below 8000h, the DSRPAG and DSWPAG registers are not considered.

This Data Space can also be accessed by Direct Addressing.

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

trap will occur.

Note:

A PC push during exception processing

will concatenate the SRL register to the

MSB of the PC prior to the push.

PC<15:0>

000000000

W15 (before CALL)

W15 (after CALL)

0000h

PC<22:16>

POP : [--W15]

PUSH : [W15++]

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