Why do MCU compilers for chips like AVR or ESP (used widely by Arduino) keep all strings in SRAM heap by default?

userP8m6 0
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There is a common technique used in Arduino world, where you can use PROGMEM macros in order to keep strings and other similar data in flash memory instead of SRAM to keep lower RAM usage, while sacrificing some performance - https://www.arduino.cc/reference/en/language/variables/utilities/progmem/

Basically, instead of storing these in SRAM, there is just some reference to a FLASH address where the string is stored and loaded from on the fly, in order to save RAM.

But I can't understand why do MCU compilers put all strings including local strings from functions into heap memory and keep them there all the time in the first place. Also I don't understand how compiler can "store anything in RAM instead of flash" - RAM is volatile, so compiler can hardly "store" anything in there as it's cleared on every reset. These strings still must be present in program image stored on FLASH, so why does it copy them from FLASH to RAM on each launch of the MCU? I was thinking that maybe whole program image must be loaded into RAM for execution, but then that doesn't make sense as these chips use harvard architecture and program is executed from FLASH already (and most of these chips have much bigger FLASH than RAM anyway, so whole image would never fit into RAM).

While I understand how to use workarounds that prevent this behaviour, I can't understand why this behaviour exists in the first place. Can someone shed some light on it? Why are all strings loaded into HEAP on start of the program by default? Is that for performance reasons?

arduino-esp32 compiler-construction arduino c
1 Answers

The AVR architecture is different from many other common architectures in that the the code and data exist in completely different memory spaces (though the program memory can be accessed as data, as shown in PROGMEM documentation page to which you linked). This is one type of modified Harvard architecture.

Most other architectures that you're likely to use present themselves to the user as having code and data exist in the same memory space. While this is often also done with a modified Harvard architecture, they present themselves to the user as a von Neumann architecture, having a unified code and data memory space.

On AVR, to make initialized global or static data available to use as any other in-memory data, part of the program startup code copies the initialization data from program memory into RAM. This is generally done to program segments with names like .data or .rodata, depending on whether or not the variables in question are const.

Note that, contrary to what you say in your question, this data is not copied to the heap, it's stored in some portion of RAM chosen during program linking.

Using PROGMEM and the associated functions, you can directly access the data stored in the flash memory of the AVR device. This constant data is placed in a segment that won't be copied to RAM on startup, like .progmem.data, and so doesn't have space in RAM reserved for it.

The case with the Xtensa architecture, used by the ESP8266 and some members of the ESP32 family, is completely different. Contrary to what you state in your question, I don't believe that static or global objects which are const are copied into RAM by default, only those which can be modified (the .data segment would be copied as initialization to RAM, while the .rodata segment would not be).

Thomas Jager0
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