DSPIC30F3012-30IP Datasheet, PDF (39/210 Pages) Microchip Technology – High-Performance, 16-bit Digital Signal Controllers

dsPIC30F2011/2012/3012/3013
All byte loads into any W register are loaded into the
LSB. The MSB is not modified.
A Sign-Extend (SE) instruction is provided to allow
users to translate 8-bit signed data to 16-bit signed
values. Alternatively, for 16-bit unsigned data, users
can clear the MSB of any W register by executing a
Zero-Extend (ZE) instruction on the appropriate
address.
Although most instructions are capable of operating on
word or byte data sizes, it should be noted that some
instructions, including the DSP instructions, operate
only on words.
3.2.5 NEAR DATA SPACE
An 8 Kbyte near data space is reserved in X address
memory space between 0x0000 and 0x1FFF, which is
directly addressable via a 13-bit absolute address field
within all memory direct instructions. The remaining X
address space and all of the Y address space is
addressable indirectly. Additionally, the whole of X data
space is addressable using MOV instructions, which
support memory direct addressing with a 16-bit
address field.
3.2.6 SOFTWARE STACK
The dsPIC DSC devices contain a software stack. W15
is used as the Stack Pointer.
The Stack Pointer always points to the first available
free word and grows from lower addresses towards
higher addresses. It pre-decrements for stack pops
and post-increments for stack pushes, as shown in
Figure 3-10. 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.
Note:
A PC push during exception processing
concatenates the SRL register to the MSB
of the PC prior to the push.
FIGURE 3-10:
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++]
There is a Stack Pointer Limit register (SPLIM)
associated with the Stack Pointer. 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
Effective Address (EA) is generated using W15 as a
source or destination pointer, the address thus
generated is compared with the value in SPLIM. If the
contents of the Stack Pointer (W15) and the SPLIM reg-
ister are equal, and a push operation is performed, a
stack error trap does not occur. The stack error trap
occurs 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, thus preventing 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.
© 2010 Microchip Technology Inc.
DS70139G-page 39