Variables in Program Memory

This feature allows the programmer full flexibility in deciding the placement of variables in memory. Variables can be now declared as part of the program memory, using a very simple and intuitive syntax. For example:

__pmem int c; // 'c' is an integer that will be stored in program memory.

This feature is very useful when data memory is tight, because some or all of the data can be moved to program memory. It can be handled exactly the same way as normal data. This is almost completely transparent to the programmer, with a few exceptions that will be presented in the next paragraphs.

The CPU architecture only allows post increment addressing of words (16-bit data) in program memory. While the compiler circumvents this restriction and allows full access to all data types in program memory, the performance is decreased. If placement of some variables in program memory is needed, and at the same time the execution speed is important, here are some pointers that can be used to organize the code:

Try to keep all variables that are used in a loop (the loop counter included) in data memory. This condition becomes more important as the loop nesting level increases.
If possible, place only int (16-bit) data in program memory. Data types with different dimensions are accessed via sequences of code rather than single instructions. 16-bit data is fastest, followed by 32-bit data and 8-bit data.
Data in program memory can be loaded and stored in a limited number of DALU registers. Because of this, a number of register save/restore sequences can appear if there are not enough available DALU registers. This could be a problem with computational intensive code because the operations do not take place only in registers anymore, and the execution of the code will be slower. This can be avoided by using as many variables in data memory as possible.

This topic contains the following sub-topics: