Arduino eeprom_put sketch writes data in two locations?

Question

Using precisely this code - the eeprom_put example in the Arduino IDE, and am seeing some strange results...

The sketch does write correctly to eeAddress = 0, and the corresponding eeprom_get sketch does return the data.

Please note that I've initialized the entire EEPROM with 'FF's before any test...

The weird part is this sketch writes exactly the same data starting at Address = 2048. The loop is not running; I see no jump to offset 2048... I gotta be missing something!

I'm dumping EEPROM using avrdude, and viewing results in an external hex editor.

How is this stuff stored? Apart from not being able to directly view a float - OK, I get it - should the other data be simply viewable in hex?

My own experiments in writing hex bytes directly also work, but also write in two locations.

Small update to this: Precisely the same behavior seen with a RobotDyn Mega. Both the 'clone' Mega and the RobotDyn report: Device signature = 0x1e9801 (probably m2560)

#include <EEPROM.h>

struct MyObject {
  float field1;
  byte field2;
  char name[10];
};

void setup() {

  Serial.begin(9600);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only
  }

  float f = 123.456f;  //Variable to store in EEPROM.
  int eeAddress = 0;   //Location we want the data to be put.


  //One simple call, with the address first and the object second.
  EEPROM.put(eeAddress, f);

  Serial.println("Written float data type!");

  /** Put is designed for use with custom structures also. **/

  //Data to store.
  MyObject customVar = {
    3.14f,
    65,
    "Working!"
  };

  eeAddress += sizeof(float); //Move address to the next byte after float 'f'.

  EEPROM.put(eeAddress, customVar);
  Serial.print("Written custom data type! \n\nView the example sketch eeprom_get to see how you can retrieve the values!");
}

void loop() {
  /* Empty loop */
}

Answer 1

What is probably happening is aliasing. Let's say your processor has a 16 bit address bus. 16 bits can count up to 2^16 = 65536 = 64k memory addresses. Each address is a byte (B) so your processor has 64k-Bytes of address space. It can physically address 64kB of memory.

What happens if you only have one memory chip with 4KBytes of memory, with only 12 address lines? Well, if the designer was lazy (or cost-conscious), the first 12 address lines on the processor will be connected to the 12 lines on the memory chip. The other 4 lines on the processor will be ignored.

So it doesn't matter what the processor does with address lines 13-16 -- they're not connected to the memory chip. Therefore the contents of the memory chip appears where you expect, starting at address 0, but also again at address 4096, and again at address 8192 and so on.