MPLAB C Compiler for PIC24 MCUs and dsPIC DSCs ... - Microchip

6.1 INTRODUCTION
MPLAB ® C COMPILER FOR
PIC24 MCUs AND dsPIC ® DSCs
USER’S GUIDE
Chapter 6. Additional C Pointer Types
MPLAB C Compiler for PIC24 MCUs and dsPIC ® DSCs (formerly MPLAB C30) offers
some extended pointer modes to help access more of the unique features of
Microchip’s 16-bit product architecture. Extended pointers and their use will be covered
in this chapter.
• Managed PSV Pointers - for reading more data through the PSV
• PMP Pointers - for accessing data via the PMP peripheral (where available)
• External Pointers - for accessing external memory in a user-defined fashion
Although the concentration will be on pointer access, defining variables and ensuring
that the data is allocated in the correct region of the 16-bit architectures (bi-polar) memory
is also covered.
This chapter will make use of concepts introduced in Chapter 2. “Differences
Between 16-Bit Device C and ANSI C”.
6.2 MANAGED PSV POINTERS
The dsPIC30F/33F and PIC24F/H families of processors contain hardware support for
accessing data from within program Flash using a hardware feature that is commonly
called Program Space Visibility (PSV). More detail about how PSV works can be found
in device data sheets or family reference manuals. Also, see Section 4.14 “Program
Space Visibility (PSV) Usage” and Section 8.10 “PSV Usage with Interrupt Service
Routines”.
Briefly, the architecture allows the mapping of one 32K page of Flash into the upper
32K of the data address space via the Special Function Register (SFR) PSVPAG. By
default the compiler only supports direct access to one single PSV page, referred to as
the auto_psv space. In this model, 16-bit data pointers can be used. However, on
larger devices, this can make it difficult to manage large amounts of constant data
stored in Flash.
The extensions presented here allow the definition of a variable as being a ‘managed’
PSV variable. This means that the compiler will manipulate both the offset (within a
PSV page) and the page itself. As a consequence, data pointers must be 32 bits. The
compiler will probably generate more instructions than the single PSV page model, but
that is the price being paid to buy more flexibility and shorter coding time to access
larger amounts of data in Flash.
6.2.1 Defining Data for Managed PSV Access
Chapter 2. “Differences Between 16-Bit Device C and ANSI C” introduces C extensions
which allows the identification of extra information for a variable or function. The
compiler provides the space attribute to help place variables into different areas
(spaces) of memory.
© 2008 Microchip Technology Inc. DS51284H-page 81