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remove guidance to fall back to 1.5.4.1 for 512Kb modules (#2222)

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Arnim Läuger 2018-01-10 07:17:12 +01:00 committed by Marcel Stör
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docs/en/lua-developer-faq.md
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@ -64,7 +64,7 @@ The NodeMCU libraries act as C wrappers around registered Lua callback functions
* The ESP8266 uses a combination of on-chip RAM and off-chip Flash memory connected using a dedicated SPI interface. Code can be executed directly from Flash-mapped address space. In fact the ESP hardware actually executes code in RAM, and in the case of Flash-mapped addresses it executes this code from a RAM-based L1 cache which maps onto the Flash addresses. If the addressed line is in the cache then the code runs at full clock speed, but if not then the hardware transparently handles the adress fault by first copying the code from Flash to RAM. This is largely transparent in terms of programming ESP8266 applications, though the faulting access runs at SRAM speeds and this code runs perhaps 13× slower than already cached code. The Lua firmware largely runs out of Flash, but even so, both the RAM and the Flash memory are *very* limited when compared to systems that most application programmers use.
* Over the last two years, both the Espressif non-OS SDK developers and the NodeMCU team have made a range of improvements and optimisations to increase the amount of RAM available to developers, from a typical 15Kb or so with Version 0.9 builds to some 45Kb with the current firmware Version 2.x builds. See the [ESP8266 Non-OS SDK API Reference](https://espressif.com/sites/default/files/documentation/2c-esp8266_non_os_sdk_api_reference_en.pdf) for more detals on the SDK.
* The early ESP8266 modules were typically configured with 512Kb Flash. Fitting a fully featured Lua build with a number of optional libraries and still enough usable Flash to hold a Lua application was a struggle. However the code-size of the SDK has grown significantly between the early versions and the current 2.0 version. Applications based on the current SDK can no longer fit in 512Kb Flash memory, and so all currently produced ESP modules now contain a minimum of 1Mb with 4 and 16Mb becoming more common. The current NodeMCU firmware will fit comfortably in a 1Mb Flash and still have ample remaining Flash memory to support Lua IoT applications. Note that the [`1.5.4.1-final` branch](https://github.com/nodemcu/nodemcu-firmware/tree/1.5.4.1-final) is the last available release if you still wish to develop applications for 512Kb modules
* The early ESP8266 modules were typically configured with 512Kb Flash. Fitting a fully featured Lua build with a number of optional libraries and still enough usable Flash to hold a Lua application needs a careful selection of libraries and features. The current NodeMCU firmware will fit comfortably in a 1Mb Flash and still have ample remaining Flash memory to support Lua IoT applications.
* The NodeMCU firmware makes any unused Flash memory available as a [SPI Flash File System (SPIFFS)](https://github.com/pellepl/spiffs) through the `file` library. The SPIFFS file system is designed for SPI NOR flash devices on embedded targets, and is optimised for static wear levelling and low RAM footprint. For further details, see the link. How much Flash is available as SPIFFS file space depends on the number of modules included in the specific firmware build.
* The firmware has a wide range of libraries available to support common hardware options. Including any library will increase both the code and RAM size of the build, so our recommended practice is for application developers to choose a custom build that only includes the library that are needed for your application and hardware variants. The developers that don't want to bother with setting up their own build environment can use Marcel Stör's excellent [Cloud build service](http://nodemcu-build.com) instead.
* There are also further tailoring options available, for example you can choose to have a firmware build which uses 32-bit integer arithmetic instead of floating point. Our integer builds have a smaller Flash footprint and execute faster, but working in integer also has a number of pitfalls, so our general recommendation is to use floating point builds.