You can find a map for the ESP8266 memory layout in the Wiki.
The ESP8266 has several types of memory, and it is important to have a basic apprecation of what they are and how they’re used.
Data RAM where variables, etc. are stored. Contains the stack (which starts at the top of RAM) and the heap (which starts near the bottom, after any static data).
Instruction RAM. All code executes from here, but there’s not much of it so so only parts of your application are loaded at any one time. This caching happens in the background by mapping certain memory address ranges into physical flash locations.
If a location is accessed which is already in the cache (a ‘hit’) then the access runs at full RAM speed. If it’s not in the cache (a ‘miss’) then an interrupt (exception) is generated internally which performs the flash read operation.
This is why interrupt service routines must not access PROGMEM directly, and must
be marked using
IRAM_ATTR to ensure it’s always available.
You may get a performance improvement using
IRAM_ATTR, but as frequently-used
code will be cached anyway it won’t necessarily run faster.
If the code is timing critical it may benefit from pre-caching. See Esp8266 SPI Flash Support.
Main storage for your application, libraries, the Espressif SDK code, etc. Flash memory is accessed via external serial bus and is relatively slow. For the ESP8266, it’s approximately 12x slower, though this only applies on cache misses.
See Flash memory for further details.
Fixed code stored in the Esp8266 itself containing very low-level support code which is factory-programmed and cannot be changed.
At startup, only the non-volatile Flash and ROM memories contain anything useful, whilst DRAM and IRAM will probably just contain garbage. The Arduino platform was initially designed to work with much simpler hardware, where the program was executed directly from Flash memory on hardware reset.
The ESP8266 and ESP32 are far more complex, and most of the low-level initialisation happens in the ROM code. The ROM essentially contains the systems BIOS, with various low-level routines which may be used instead of accessing hardware directly. It is also responsible for setting up memory caching.
Control is passed to runtime libraries provided by Espressif, stored in Flash memory. Both ROM and runtime code are closed-source and not generally available for inspection, though disassemblies do exist.
The first point we really see what’s going on is in the bootloader (rBoot). The bootloader identifies the correct program image (as there can be more than one), loads the starting portion into IRAM and jumps there. It also configures the caching mechanism so that the correct program image is loaded. You can find more details about this in the rBoot documentation.
Code is copied from flash into IRAM, and const data copied into DRAM. Also static and global variable values are initialised from tables stored in flash. Static and global variables without an initialised value are initialised to 0.