C51: Mixing Memory Models

Information in this article applies to:

• C51 Version 5.50 and Later

QUESTION

I have written the following code:

void foo(void)
{
  for (i=8;i>0;i--) {
    P1.2 = ~P1.2;
  }
}

In the SMALL memory model, the compiler generates assembly code that uses the DJNZ instruction. In the COMPACT and LARGE memory models, it generates codes that doesn't use the DJNZ instruction and appears less efficient.

It is important that this function executes as fast as possible, yet I want to use the LARGE memory model. Can I mix the SMALL and LARGE memory models somehow?

ANSWER

Yes. You can use the LARGE memory model for your project as a whole, and specify the SMALL memory model for individual functions. For example you could write the following code:

void foo(void) small
{
  for (i=8;i>0;i--) {
    P1.2 = ~P1.2;
  }
}

Be aware that if you do this for reentrant functions, then a separate reentrant stack needs to be initialized for each memory model that has reentrant functions defined. This takes up valuable data memory.

MORE INFORMATION

• Refer to Memory Models in the Cx51 User's Guide.

Last Reviewed: Thursday, February 25, 2021