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

16-Bit C Compiler User’s Guide
4.6 MEMORY MODELS
The compiler supports several memory models. Command-line options are available
for selecting the optimum memory model for your application, based on the specific
dsPIC DSC device part that you are using and the type of memory usage.
TABLE 4-1: MEMORY MODEL COMMAND LINE OPTIONS
Option Memory Definition Description
-msmall-data Up to 8 KB of data memory. Permits use of PIC18 like instructions
This is the default.
for accessing data memory.
-msmall-scalar Up to 8 KB of data memory. Permits use of PIC18 like instructions
This is the default.
for accessing scalars in data memory.
-mlarge-data Greater than 8 KB of data
memory.
Uses indirection for data references.
-msmall-code Up to 32 Kwords of program Function pointers will not go through a
memory. This is the default. jump table. Function calls use RCALL
instruction.
-mlarge-code Greater than 32 Kwords of Function pointers might go through a
program memory.
jump table. Function calls use CALL
instruction.
-mconst-in-data Constants located in data Values copied from program memory
memory.
by startup code.
-mconst-in-code Constants located in program Values are accessed via Program
memory. This is the default. Space Visibility (PSV) data window.
The command-line options apply globally to the modules being compiled. Individual
variables and functions can be declared as near or far to better control the code
generation. For information on setting individual variable or function attributes, see
Section 2.3.1 “Specifying Attributes of Variables” and Section 2.3.2 “Specifying
Attributes of Functions”.
4.6.1 Near and Far Data
If variables are allocated in the near data section, the compiler is often able to generate
better (more compact) code than if the variables are not allocated in the near data
section. If all variables for an application can fit within the 8 KB of near data, then the
compiler can be requested to place them there by using the default -msmall-data
command line option when compiling each module. If the amount of data consumed
by scalar types (no arrays or structures) totals less than 8 KB, the default
-msmall-scalar may be used. This requests that the compiler arrange to have just
the scalars for an application allocated in the near data section.
If neither of these global options is suitable, then the following alternatives are
available.
DS51284H-page 66 © 2008 Microchip Technology Inc.