Flash memory


ESP8266 flash memory sizes vary from 512Kbytes on the ESP-01 up to 4Mbytes on the ESP12F. Up to 16MBytes are supported for custom designs.

You can find general details for the memory layout in the ESP8266 Wiki.

This is the layout for Sming with a 4MByte flash device:

Address Config variable Size Source filename Description
(hex) (if any) (KB) (if applicable)  
000000   1 rboot.bin Boot loader
001000   4   rBoot configuration
002000 ROM_0_ADDR   rom0.bin First ROM image
100000 RBOOT_SPIFFS_0      
202000 ROM_1_ADDR   rom1.bin Second ROM image
300000 RBOOT_SPIFFS_1      
3FB000   4 blank.bin RF Calibration data (Initialised to FFh)
3FC000   4 esp_init_data_default.bin PHY configuration data
3FD000   12 blank.bin System parameter area

Partition Tables

{ todo }

Whilst SDK version 3 requires a partition table, previous versions do not but this can be added so that we can use it as a common reference for all the above locations.

Speed and caching

Flash memory on the ESP8266 is accessed via an external SPI bus, so reading it takes about 12x longer than reading from internal RAM. To mitigate this, some of the internal RAM is used to cache data. Part of this is managed in hardware, which means if the data required is already in the cache then there is no difference in speed. In general, then, frequently accessed data is read as if it were already in RAM.

Bear in mind that every time new data is read via the cache, something else will get thrown away and have to be re-read. Therefore, if you have large blocks of infrequently accessed data then it’s a good idea to read it directly using flashmem_read(). You can get the address for a memory location using flashmem_get_address().

See Program Space for details of how to store data in flash, and access it.