Flash memory

Introduction

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.