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

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16-Bit C Compiler User’s Guide 2.3.1 Specifying Attributes of Variables The compiler keyword __attribute__ allows you to specify special attributes of variables or structure fields. This keyword is followed by an attribute specification inside double parentheses. The following attributes are currently supported for variables: • address (addr) • aligned (alignment) • boot • deprecated • fillupper • far • mode (mode) • near • noload • packed • persistent • reverse (alignment) • section ("section-name") • secure • sfr (address) • space (space) • transparent_union • unordered • unused • weak You may also specify attributes with __ (double underscore) preceding and following each keyword (e.g., __aligned__ instead of aligned). This allows you to use them in header files without being concerned about a possible macro of the same name. To specify multiple attributes, separate them by commas within the double parentheses, for example: __attribute__ ((aligned (16), packed)). Note: It is important to use variable attributes consistently throughout a project. For example, if a variable is defined in file A with the far attribute, and declared extern in file B without far, then a link error may result. address (addr) The address attribute specifies an absolute address for the variable. This attribute can be used in conjunction with a section attribute. This can be used to start a group of variables at a specific address: int foo __attribute__((section("mysection"),address(0x900))); int bar __attribute__((section("mysection"))); int baz __attribute__((section("mysection"))); A variable with the address attribute cannot be placed into the auto_psv space (see the space() attribute or the -mconst-in-code option); attempts to do so will cause a warning and the compiler will place the variable into the PSV space. If the variable is to be placed into a PSV section, the address should be a program memory address. int var __attribute__ ((address(0x800))); DS51284H-page 12 © 2008 Microchip Technology Inc.
Differences Between 16-Bit Device C and ANSI C aligned (alignment) This attribute specifies a minimum alignment for the variable, measured in bytes. The alignment must be a power of two. For example, the declaration: int x __attribute__ ((aligned (16))) = 0; causes the compiler to allocate the global variable x on a 16-byte boundary. On the dsPIC DSC device, this could be used in conjunction with an asm expression to access DSP instructions and addressing modes that require aligned operands. As in the preceding example, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable to the maximum useful alignment for the dsPIC DSC device. For example, you could write: short array[3] __attribute__ ((aligned)); Whenever you leave out the alignment factor in an aligned attribute specification, the compiler automatically sets the alignment for the declared variable to the largest alignment for any data type on the target machine – which in the case of the dsPIC DSC device is two bytes (one word). The aligned attribute can only increase the alignment; but you can decrease it by specifying packed (see below). The aligned attribute conflicts with the reverse attribute. It is an error condition to specify both. The aligned attribute can be combined with the section attribute. This will allow the alignment to take place in a named section. By default, when no section is specified, the compiler will generate a unique section for the variable. This will provide the linker with the best opportunity for satisfying the alignment restriction without using internal padding that may happen if other definitions appear within the same aligned section. boot This attribute can be used to define protected variables in Boot Segment (BS) RAM: int __attribute__((boot)) boot_dat[16]; Variables defined in BS RAM will not be initialized on startup. Therefore all variables in BS RAM must be initialized using inline code. A diagnostic will be reported if initial values are specified on a boot variable. An example of initialization is as follows: int __attribute__((boot)) time = 0; /* not supported */ int __attribute__((boot)) time2; void __attribute__((boot)) foo() { time2 = 55; /* initial value must be assigned explicitly */ } deprecated The deprecated attribute causes the declaration to which it is attached to be specially recognized by the compiler. When a deprecated function or variable is used, the compiler will emit a warning. A deprecated definition is still defined and, therefore, present in any object file. For example, compiling the following file: int __attribute__((__deprecated__)) i; int main() { return i; } will produce the warning: © 2008 Microchip Technology Inc. DS51284H-page 13
Page 1 and 2: MPLAB ® C COMPILER FOR PIC24 MCUs
Page 5 and 6: Table of Contents A.5 Identifiers .
Page 7 and 8: INTRODUCTION MPLAB ® C COMPILER FO
Page 9 and 10: CONVENTIONS USED IN THIS GUIDE The
Page 11 and 12: THE MICROCHIP WEB SITE Preface C Re
Page 13 and 14: 1.1 INTRODUCTION 1.2 HIGHLIGHTS MPL
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4.1 INTRODUCTION 4.2 HIGHLIGHTS 4.3
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4.5 MEMORY SPACES Run Time Environm
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Run Time Environment 1. It is possi
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4.8 SOFTWARE STACK Run Time Environ
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FIGURE 4-2: CALL OR RCALL Stack gro
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4.11 FUNCTION CALL CONVENTIONS Run
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Run Time Environment The compiler a
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4.14.2 String Literals as Arguments
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5.1 INTRODUCTION 5.2 HIGHLIGHTS MPL
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6.1 INTRODUCTION MPLAB ® C COMPILE
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6.3 PMP POINTERS Additional C Point
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6.4 EXTERNAL POINTERS 6.3.3 Define
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ead_external16 Additional C Pointer
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Additional C Pointer Types } else {
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7.5 USING SFRS Device Support Files
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7.6 USING MACROS Device Support Fil
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EXAMPLE 7-2: EEDATA ACCESS VIA PSV
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Interrupts When using the interrupt
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8.4.1 dsPIC30F DSCs (Non-SMPS) Inte
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8.4.3 PIC24F MCUs Interrupt Vectors
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Interrupts TABLE 8-3: INTERRUPT VEC
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Interrupts TABLE 8-4: INTERRUPT VEC
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85 _Interrupt85 _AltInterrupt85 Res
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8.8 ENABLING/DISABLING INTERRUPTS I
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Interrupts 2. Unsafe: read bar inte
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LATGbits.LATG2 = 0; /* No problem l
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MPLAB ® C COMPILER FOR PIC24 MCUs
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Mixing Assembly Language and C Modu
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A.5 IDENTIFIERS A.6 CHARACTERS Impl
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Implementation-Defined Behavior The
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A.15 PREPROCESSING DIRECTIVES Imple
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A.17 SIGNALS A.18 STREAMS AND FILES
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A.24 GETENV A.25 SYSTEM A.26 STRERR
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B.1 INTRODUCTION MPLAB ® C COMPILE
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__builtin_btg Built-in Functions De
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__builtin_clr_prefetch Argument: xp
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__builtin_dmaoffset Built-in Functi
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__builtin_fbcl Built-in Functions A
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__builtin_modsd Argument: dividend
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__builtin_mpy Error Messages An err
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__builtin_mulss Built-in Functions
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__builtin_psvoffset Built-in Functi
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__builtin_subab Built-in Functions
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__builtin_tblwth Built-in Functions
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C.1 INTRODUCTION C.2 ERRORS MPLAB
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Diagnostics ambiguous abbreviation
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Diagnostics cast specifies function
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F Diagnostics ‘identifier’ fail
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Diagnostics initializer for static
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Diagnostics invalid use of undefine
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N Diagnostics negative width in bit
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R Diagnostics redeclaration of ‘i
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Diagnostics symbol ‘symbol’ not
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Diagnostics void expression between
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Diagnostics anonymous union declare
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Diagnostics comparison between sign
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duplicate ‘const’ The ‘const
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Diagnostics function might be possi
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Diagnostics ‘identifier’ is nar
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Diagnostics library function ‘ide
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P Diagnostics parameter has incompl
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Diagnostics shift count >= width of
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Diagnostics too many arguments for
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V Diagnostics __VA_ARGS__ can only
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MPLAB C Compiler for PIC18 MCUs vs.
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E.1 INTRODUCTION E.2 HIGHLIGHTS MPL
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G.1 PREAMBLE MPLAB ® C COMPILER FO
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G.3 VERBATIM COPYING G.4 COPYING IN
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GNU Free Documentation License incl
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Glossary File Registers On-chip dat
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Glossary Machine Language A set of
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Glossary Stack, Software Memory use
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Symbols __builtin_add..............
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-fpack-struct .....................
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-idirafter.........................
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Peephole Optimization..............
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-Wsequence-point...................
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MPLAB C Compiler for PIC24 MCUs and dsPIC DSCs ... - Microchip

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