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Overlay Code with Parallax Propeller and GCC

Written April 7th, 2016 by

Programs that use overlay code were very popular in the era of early home computer systems when the amount of memory was not much, and the ability to load portions of code only when necessary allowed to make programs much larger than the available memory.

With microcontrollers we are in a similar situation with sometimes very complex programs and a rather limited memory, just think for example to the management of the SD card file system or the internet access libraries, that can severely limit the memory available for the program itself, especially if they are to be used simultaneously.

The Parallax Propeller is a microcontroller with 32K of internal RAM memory and uses an external EEPROM to load the code to run at power up. Since only the first 32K of the EEPROM are used for the program it is possible to dedicate the exceeding space of larger memories for data storage. Fortunately with the GCC compiler and the standard tools it is also possible to store portions of the program code to be loaded when necessary.

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