DSPIC33FJ64GS406 datasheet(99/456 Pages) MICROCHIP

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DS70591E-page 99

dsPIC33FJ32GS406/606/608/610 and dsPIC33FJ64GS406/606/608/610

4.2.7

SOFTWARE STACK

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

dsPIC33FJ64GS406/606/608/610 devices is also used

as a Software Stack Pointer. The Stack 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 4-6. For a PC push during any CALL instruc-

tion, the MSb of the PC is zero-extended before the

push, ensuring that the MSb is always clear.

The Stack Pointer Limit register (SPLIM) 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’

because 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. For example, to cause a

stack error trap when the stack grows beyond address

0x1800 in RAM, initialize the SPLIM with the value,

0x17FE.

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

generated when the Stack Pointer address is found to

be less than 0x0800. This prevents the stack from

interfering with the Special Function Register (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

4.3

Instruction Addressing Modes

The addressing modes shown in Table 4-66 form the

basis of the addressing modes optimized to support the

specific features of individual instructions. The

addressing modes provided in the MAC class of

instructions differ from those in the other instruction

types.

4.3.1

FILE REGISTER INSTRUCTIONS

Most file register instructions use a 13-bit address field

(f) to directly address data present in the first

8192 bytes of data memory (Near Data Space). Most

file register instructions employ a working register, W0,

which is denoted as WREG in these instructions. The

destination is typically either the same file register or

WREG (with the exception of the MUL instruction),

which writes the result to a register or register pair. The

MOV instruction allows additional flexibility and can

access the entire data space.

4.3.2

MCU INSTRUCTIONS

The three-operand MCU instructions are of the form:

Operand 3 = Operand 1 <function> Operand 2

where Operand 1 is always a working register (that is,

the addressing mode can only be register direct), which

is referred to as Wb. Operand 2 can be a W register,

fetched from data memory, or a 5-bit literal. The result

location can be either a W register or a data memory

location. The following addressing modes are

supported by MCU instructions:

• Register Direct

• Register Indirect

• Register Indirect Post-Modified

• Register Indirect Pre-Modified

• 5-Bit or 10-Bit Literal

Note:

A PC push during exception processing

concatenates the SRL register to the MSb

of the PC prior to the push.

PC<15:0>

000000000

W15 (before CALL)

W15 (after CALL)

0x0000

PC<22:16>

POP : [--W15]

PUSH : [W15++]

Note:

Not all instructions support all the

addressing modes given above. Individual

instructions can support different subsets

of these addressing modes.

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