Datasheet

dsPIC33FJ06GS001/101A/102A/202A and dsPIC33FJ09GS302
DS75018C-page 66  2011-2012 Microchip Technology Inc.
4.3.1 SOFTWARE STACK
In addition to its use as a working register, the W15
register in the 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 4-5.
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 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
0x1000 in RAM, initialize the SPLIM with the value
0x0FFE.
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 4-5: CALL STACK FRAME
4.4 Instruction Addressing Modes
The addressing modes shown in Table 4-39 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.
4.4.1 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.
4.4.2 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.
<Free Word>
PC<15:0>
000000000
015
W15 (before CALL)
W15 (after CALL)
Stack Grows Toward
Higher Address
0x0000
PC<22:16>
POP : [--W15]
PUSH : [W15++]
Note: Not all instructions support all of the
addressing modes shown above.
Individual instructions can support
different subsets of these addressing
modes.