Accessing byte data stored on flash

As ram is short and rom is plentiful on the ESP8266 you might want to store some of your data in rom. The performance won’t be as good as keeping it in ram, but if you need to store a lot of data you may not have any choice. The problem you’ll find is that memory mapped flash needs to be accessed in 4 byte aligned chunks. So as Pete discovered if you try to store a uint8 array in rom you won’t be able to read it in the normal way.

Here is a simple example of how to store and access a uint8 (and uint16) array in rom:

uint8 ICACHE_RODATA_ATTR data[] = {

uint16 ICACHE_RODATA_ATTR data16[] = {

uint8 ICACHE_FLASH_ATTR read_rom_uint8(const uint8* addr){
	uint32 bytes;
	bytes = *(uint32*)((uint32)addr & ~3);
	return ((uint8*)&bytes)[(uint32)addr & 3];

uint16 read_rom_uint16(const uint16* addr){
	uint32 bytes;
	bytes = *(uint32*)((uint32)addr & ~3);
	return ((uint16*)&bytes)[((uint32)addr >> 1) & 1];

void ICACHE_FLASH_ATTR user_init(void) {
	os_printf("%d\r\n", read_rom_uint8(&data[0]));
	os_printf("%d\r\n", read_rom_uint8(data + 13));

	os_printf("%d\r\n", read_rom_uint16(&data16[1]));


  1. Hi,

    I am looking at replacing the SPI flash chip by an SD card, which can be switched to SPI mode in certain conditions. Espressif specs talks about booting from SD card, but Mr Lee on Twitter says it is not supported.

    Do you know if rBoot could support booting from an SD card?

    1. rBoot itself is booted by the esp8266 in the normal way and it then goes on to load the user rom. This means that rBoot needs to be stored somewhere that the esp8266 can already boot from (i.e. spi flash).

Leave a comment