70592C Datasheet, PDF (57/314 Pages) Microchip Technology – High-Performance, 16-bit Microcontrollers

PIC24HJXXXGPX06A/X08A/X10A
4.2.6 SOFTWARE STACK
In addition to its use as a working register, the W15
register in the PIC24HJXXXGPX06A/X08A/X10A
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 4-5. 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.
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. 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 0x2000 in RAM, initialize the
SPLIM with the value 0x1FFE.
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:
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++]
4.2.7 DATA RAM PROTECTION FEATURE
The PIC24H product family supports Data RAM protec-
tion 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 Seg-
ment 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 in Table 4-34 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 are
somewhat different 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 3-operand MCU instructions are of the form:
Operand 3 = Operand 1 <function> Operand 2
where:
Operand 1 is always a working register (i.e., 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 fol-
lowing 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 the
addressing modes given above.
Individual instructions may support
different subsets of these addressing
modes.
© 2011 Microchip Technology Inc.
DS70592C-page 57