User`s guide

C for DSP56800E

Optimizing Code

132

Targeting MC56F83xx/DSP5685x Controllers

External Library Compatibility

If you enable the large data model when the compiler builds your main application,

external libraries written in C also must be built with the large data model enabled.

The linker enforces this requirement, catching global objects located out of range for

particular instructions.

A more serious compatibility problem involves pointer parameters. Applications built

with the large data memory model may pass pointer parameter values in two words of

the stack. But libraries built using the small memory model may expect pointer

arguments to occupy a single word of memory. This incompatibility will cause

runtime stack corruption.

You may or may not build external libraries or modules written in assembly with

extended addressing modes. The linker does not enforce any compatibility rules on

assembly language modules or libraries.

The compiler encodes the memory model into the object file. The linker verifies that

all objects linked into an executable have compatible memory models. The ELF

header’s e_flags field includes the bit fields that contain the encoded data memory

model attributes of the object file:

#define EF_M56800E_LDMM 0x00000001 /* Large data memory model

flag */

Additionally, C language objects are identified by an ELF header flag.

#define EF_M56800E_C 0x00000002 /* Object file generated from C

source */

Optimizing Code

compiler assigns two (or more) register variables to the same register, if the code does

not use the variables at the same time. The code of Listing 6.6

does not use variables i

and j at the same time, so the compiler could store them in the same register:

Listing 6.6 Register Coloring Example

short i;

int j;