Flash, SRAM, EEPROM, PROGMEM and F()

Where data lives

The ATmega328P on the Uno R3 has 32 KB flash (program code and constants; part is used by the bootloader), 2 KB SRAM (variables, the stack and the heap) and 1 KB EEPROM (non-volatile data that survives power-off, such as settings). SRAM is the scarce resource: string literals are copied from flash into SRAM at start-up on AVR, so many Serial.print("long text") calls quietly eat RAM. The F() macro keeps a literal in flash for print functions, and PROGMEM stores larger constant tables in flash, read back with functions like pgm_read_byte. EEPROM cells have a limited number of write cycles (the datasheet specifies around 100,000), so write only when a value changes. Avoid heavy String and malloc use on tiny-RAM boards, because heap fragmentation can cause hard-to-find crashes. These details are AVR-specific; other chips handle constants differently.

Saving RAM and storing settings

Arduino C++ on AVR (Uno R3).

#include <EEPROM.h>
#include <avr/pgmspace.h>

const uint8_t gammaTable[8] PROGMEM = {0, 2, 8, 20, 40, 70, 120, 255};
const int ADDR_BRIGHTNESS = 0;

void setup() {
  Serial.begin(115200);
  Serial.println(F("This text stays in flash, not SRAM"));

  uint8_t level = pgm_read_byte(&gammaTable[3]);  // read from flash
  Serial.println(level);

  uint8_t saved = EEPROM.read(ADDR_BRIGHTNESS);
  EEPROM.update(ADDR_BRIGHTNESS, 128);  // writes only if the value differs
  Serial.println(saved);
}

void loop() {}

Bookshelf, desk and notebook

Flash is a bookshelf of reference books you rarely change, SRAM is a small desk you work on, and EEPROM is a notebook that keeps notes after you leave but wears out if you rewrite the same page too often.

Quick check: What does Serial.println(F("Hello")) achieve on an AVR board?

• Stores the text in EEPROM permanently
• Prints the text faster over USB
• Keeps the string literal in flash instead of copying it into SRAM
• Formats the text as floating point