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. Rp2040 has similar support, and the Esp32 has enhanced VMM (Virtual Memory Management) hardware.

Sming version 4.3 introduced partition tables to support multiple architectures, different hardware variants and custom flash layouts without restriction. It is binary compatible with the Esp32 IDF partition tables but with a consistent API across all architectures.

See Hardware configuration for details.

A typical layout for a 4MByte Esp8266 device might look like this:

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
102000	ROM_1_ADDR		rom1.bin	Second ROM image
200000	RBOOT_SPIFFS_0
300000	RBOOT_SPIFFS_1
3FA000		4		Partition table
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

Note

This was the previous layout for 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

The actual layout in use can be seen by running make map.

Speed and caching

Flash memory 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.