Many PIC^® processors come with on-chip Electronically Erasable Programmable Read Only Memory (EEPROM) which is typically used to store configuration data or program state. Reading, writing or pre-initialize EEPROM requires following certain steps and the compiler has addressed EEPROM access in the following ways:

• Reading from eeprom is done via the EEPROM_READ macro or eeprom_read function. You supply an address in eeprom and it returns one byte.
  
• Writing to eeprom is done via the EEPROM_WRITE macro or eeprom_write function. Once again, you supply an address and one byte will be written to eeprom memory.
• Pre-initialization of eeprom is done via the __EEPROM_DATA macro in which you can supply 8 bytes at a time.

Although that covers the basics for eeprom access and save much reading of datasheets, it is a little limiting in how it can be accessed. For example, what if you want to read and/or write more than one byte at a time. Typically, a helper function like this is used:

This type of function will work fine, however it is quite slow because a function call and (re)loading of EEPROM special function registers are need for each byte. Another issue is with pre-initializing EEPROM via the EEPROM_DATA macro. For multi-byte objects such as structures or floating-point numbers, pre-initialization is difficult as it is necessary to calculate what bytes make up the object and make sure they are inserted in the right order.

These issues have been addressed by the introduction of the eeprom qualifier. The eeprom qualifier allows you to have a variable that live in eeprom and all of the hard work is handled by the compiler.

In the above example, "eenumber" and "eestruct" are EEPROM variables which are automatically allocated read and written by the compiler. As shown with eenumber above, pre-initialization becomes a simple assignment. Similarly, storing of multi-byte objects to EEPROM becomes a "one-liner". This is demonstrated where the ram structure "ramstruct" is assigned to "eestruct". Additionally, the reading/writing is done in bulk and so should be much faster.

Look out for the eeprom qualifier in the latest PIC compilers (HI-TECH PICC^™, HI-TECH PICC-18^™ and HI-TECH C for dsPIC/PIC24) from HI-TECH Software.