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I'm trying to pinpoint some strange behaviour involving the use of PROGMEM and the size of a Sketch.

I've written a library that contains a number of functions, in addition to several large global byte arrays and 32-bit uint arrays. These arrays are stored in PROGMEM using the pgmspace library. The library compiles fine, and all code works properly when uploaded to an Arduino.

I'm performing testing on the functions using a testing sketch that in turn uses my library. In order to test the performance, I've been switching between using PROGMEM and not using PROGMEM in the library (all arrays are under 2KB). When using PROGMEM, it drops the Global Variable dynamic memory count by the expected amount... but the the sketch program size remains either constant, or seems to arbitrarily increase or decrease by a very small amount, depending on the function. For example, a uint8_t byte[256] array, when moved to the PROGMEM, causes the Global Variable size to go down by about 256 bytes... but the sketch size only increases by about 12 bytes.

Each entry in the array is unique and is accessed, so the compiler shouldn't be optimizing it away. I tried just using avr-size directly on the ELF file, but the sketch size was the same. I can't tell if the data is being stored in PROGMEM all the time (even when not declared as PROGMEM, due to some sort of issue with PROGMEM in libraries?), or if PROGMEM data simply isn't being counted as part of the sketch size, or ??? I have tried moving the functions directly out of the library into the sketch, changing data types, adding/removing static and const declarations... but the lack of change in sketch size continues to confuse me. Any help would be greatly appreciated.

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    That is a whole point to not having constant data in the RAM. However they must be stored somewhere, they're just not copied into the RAM. – KIIV Jan 15 '18 at 15:19
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all constant data are part of the sketch. where else should they be? in runtime they are loaded into RAM. PROGMEM directive prevents the loading into RAM.

  • Ah, so either way, the storage size required remains constant? Now it makes sense. – fritterdonut Jan 15 '18 at 16:07

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