Frequently Asked Questions

What can be done about an OUT OF RAM error?

The compiler makes every effort to optimize usage of RAM. Understanding the RAM allocation can be a help in designing the program structure. The best re-use of RAM is accomplished when local variables are used with lots of functions. RAM is re-used between functions not active at the same time. See the NOT ENOUGH RAM error message in this manual for a more detailed example.

RAM is also used for expression evaluation when the expression is complex. The more complex the expression, the more scratch RAM locations the compiler will need to allocate to that expression. The RAM allocated is reserved during the execution of the entire function but may be re-used between expressions within the function. The total RAM required for a function is the sum of the parameters, the local variables and the largest number of scratch locations required for any expression within the function. The RAM required for a function is shown in the call tree after the RAM=. The RAM stays used when the function calls another function and new RAM is allocated for the new function. However when a function RETURNS the RAM may be re-used by another function called by the parent. Sequential calls to functions each with their own local variables is very efficient use of RAM as opposed to a large function with local variables declared for the entire process at once.

Be sure to use SHORT INT (1 bit) variables whenever possible for flags and other boolean variables. The compiler can pack eight such variables into one byte location. This is done automatically by the compiler whenever you use SHORT INT. The code size and ROM size will be smaller.

Finally, consider an external memory device to hold data not required frequently. An external 8 pin EEPROM or SRAM can be connected to the PIC® MCU with just 2 wires and provide a great deal of additional storage capability. The compiler package includes example drivers for these devices. The primary drawback is a slower access time to read and write the data. The SRAM will have fast read and write with memory being lost when power fails. The EEPROM will have a very long write cycle, but can retain the data when power is lost.