PIC24FJ16MC101_12 Datasheet, PDF (54/350 Pages) Microchip Technology – 16-bit Microcontrollers (up to 32 KB Flash and 2 KB SRAM)

PIC24FJ16MC101/102 AND PIC24FJ32MC101/102/104
4.2.5 SOFTWARE STACK
In addition to its use as a working register, the W15
register in the PIC24FJ16MC101/102 and
PIC24FJ32MC101/102/104 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 pre-decrements 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.
Note:
A PC push during exception processing
concatenates the SRL register to the MSb
of the PC prior to the push.
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. However, 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 0x0C00 in RAM, initialize the SPLIM with the
value 0x0BFE.
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 SFR space.
A write to the SPLIM register should not be immediately
followed by an indirect read operation using W15.
FIGURE 4-6:
0x0000 ₁₅
CALL STACK FRAME
0
PC<15:0>
000000000 PC<22:16>
<Free Word>
W15 (before CALL)
W15 (after CALL)
POP : [--W15]
PUSH : [W15++]
4.2.6 DATA RAM PROTECTION FEATURE
The PIC24FXXXX product family supports Data RAM
protection features that enable segments of RAM to be
protected when used in conjunction with Boot and
Secure Code Segment Security. BSRAM (Secure RAM
segment for BS) is accessible only from the Boot
Segment Flash code, when enabled. SSRAM (Secure
RAM segment for RAM) is accessible only from the
Secure Segment Flash code, when enabled. See
Table 4-1 for an overview of the BSRAM and SSRAM
SFRs.
4.3 Instruction Addressing Modes
The addressing modes shown in Table 4-34 form the
basis of the addressing modes that are optimized to
support the specific features of individual instructions.
The addressing modes provided in the MAC class of
instructions differ from those provided in 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:
Not all instructions support all of the
addressing modes given above.
Individual instructions can support
different subsets of these addressing
modes.
DS39997C-page 54
Preliminary
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