PIC24FV32KA304 datasheet(53/320 Pages) MICROCHIP

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PIC24FV32KA304 FAMILY

DS39995B-page 53

 2011 Microchip Technology Inc.

4.2.5

SOFTWARE STACK

In addition to its use as a working register, the W15

Stack Pointer. The pointer always points to the first

available free word and grows from lower to higher

addresses. It predecrements for stack pops and

post-increments for stack pushes, as shown in

Figure 4-4.

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 (SPLIM) register,

associated 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

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,

0DF6, in RAM, initialize the SPLIM with the value,

0DF4.

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 Special Function Register (SFR)

space.

FIGURE 4-4:

CALL

STACK FRAME

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.

Apart from the normal execution, the PIC24F

architecture provides two methods by which the

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, PSV

Table instructions allow an application to read or write

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 (lsw) 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 Most Significant bit (MSb) of

TBLPAG is 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 8-bit Program Space

Visibility Page Address register (PSVPAG) is used to

define a 16K word page in the program space. When

the MSb of the EA is ‘1’, PSVPAG is concatenated with

the lower 15 bits of the EA to form a 23-bit program

space address. Unlike the table operations, this limits

remapping operations strictly to the user memory area.

Table 4-27 and Figure 4-5 show how the program EA is

created for table operations and remapping accesses

from the data EA. Here, P<23:0> bits refer to a program

space word, whereas the D<15:0> bits refer to a data

space word.

Note:

A PC push during exception processing

will concatenate the SRL register to the

MSB of the PC prior to the push.

Note:

A write to the SPLIM register should not

be immediately followed by an indirect

read operation using W15.

PC<15:0>

000000000

W15 (before CALL)

W15 (after CALL)

0000h

PC<22:16>

POP

: [--W15]

PUSH : [W15++]

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