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

4.14.2 String Literals as Arguments
Run Time Environment
In addition to being used as initializers, string literals may also be used as function
arguments. For example:
myputs("Enter the Dragon code:\n");
Here allocation of the string literal depends on the surrounding code. If the statement
appears in a boot or secure function, the literal will be allocated in a corresponding PSV
constant section. Otherwise it will be placed in general (non-secure) memory,
according to the constants memory model.
Recall that data stored in a secure segment can not be read by any other segment. For
example, it is not possible to call the standard C library function puts() with a string
that has been allocated in a secure segment. Therefore literals which appear as function
arguments can only be passed to functions in the same security segment. This is
also true for objects referenced by pointers and arrays. Simple scalar types such as
char, int, and float, which are passed by value, may be passed to functions in
different segments.
4.14.3 Const-qualified Variables in Secure Flash
const-qualified variables with initializers can be supported in secure Flash segments
using PSV constant sections managed by the compiler. For example:
const int __attribute__((boot)) time_delay = 55;
If the const qualifier was omitted from the definition of time_delay, this statement
would be rejected with an error message. (Initialized variables in secure RAM are not
supported).
Since the const qualifier has been specified, variable time_delay can be allocated
in a PSV constant section that is owned by the boot segment. It is also possible to specify
the PSV constant section explicitly with the space(auto_psv) attribute:
int __attribute__((boot,space(auto_psv))) bebop = 20;
Pointer variables initialized with string literals require special processing. For example:
char * const foo __attribute__((boot)) = "eek";
The compiler will recognize that string literal "eek" must be allocated in the same PSV
constant section as pointer variable foo. The logic for making that association is
already supported in the compiler for named PSV sections.
4.14.4 Object Compatibility Model
Since functions in secure segments set PSVPAG to their respective psv constant sections,
a convention must be established for managing multiple values of the PSVPAG
register. In previous versions of the compiler, a single value of PSVPAG was set during
program startup if the default constants-in-code memory model was selected. The
compiler relied upon that preset value for accessing const variables and string literals,
as well as any variables specifically nominated with space(auto_psv).
Compiler v3.0 will provide automatic support for multiple values of PSVPAG. Variables
declared with space(auto_psv) may be combined with secure segment constant
variables and/or managed psv pointer variables in the same source file. Precompiled
objects that assume a single, pre-set value of PSVPAG will be link-compatible with
objects that define secure segment psv constants or managed psv variables.
Even though PSVPAG is now considered to be a compiler-managed resource, there is
no change to the function calling conventions. Objects and libraries created with earlier
versions are compatible with 3.0 objects, with the exception of some Interrupt Service
Routines as noted in Section 8.10 “PSV Usage with Interrupt Service Routines”.
© 2008 Microchip Technology Inc. DS51284H-page 77