Tero's Arduino Blog

Using Arduino with Ada

Storing large data amount to flash memory

If you paid attention in my Olimex MOD-LCD3310 article, you noticed that I stored a large array for font/characters to RAM memory. This is highly inefficient since the array takes space both on the flash and on the memory at the same time.

To make the space usage more efficient, you can specify the array to be only on the flash. This way, you can use the precious RAM to other things. The downside is that you cannot access the array on the flash as easily as from RAM.

However, AVR-Ada provides you relatively simple means to place data on flash and to retrieve it from there. You need to do only two things, add a linker pragma like:

pragma Linker_Section (Fonts, ".progmem");

and use AVR.Programspace package to fetch the data:

Place := Fonts (0, I)'Address;
Offset := AVR.Programspace.Get_Byte (Place);

Notice how the (flash) address of Fonts array is specified by Address attribute. You don't need to know any details how the address is calculated, the compiler handles everything for you.

The updated code is available at my arduino-mod-lcd3310 repository as usual. The revision number for this change is ab6d9f7edc6f.

And here the .bss usage before and after Linker_Section pragma:

$ avr-size main-ram.elf
   text    data     bss     dec     hex filename
   2570     582     512    3664     e50 main-ram.elf
$ avr-size main-progmem.elf
   text    data     bss     dec     hex filename
   3038     112     511    3661     e4d main-progmem.elf

The data (RAM) usage is higher without .progmem Linker_Section pragma, and with the pragma the text (flash section) is higher.

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