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FAQ update for io.write clarification (#2463)

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Tim Godfrey 2018-08-18 12:26:18 -05:00 committed by Marcel Stör
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docs/en/lua-developer-faq.md
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@ -43,7 +43,7 @@ Whilst the Lua standard distribution includes a stand-alone Lua interpreter, Lua
The ESP8266 was designed and is fabricated in China by [Espressif Systems](http://espressif.com/new-sdk-release/). Espressif have also developed and released a companion software development kit (SDK) to enable developers to build practical IoT applications for the ESP8266. The SDK is made freely available to developers in the form of binary libraries and SDK documentation. However this is in a *closed format*, with no developer access to the source files, so anyone developing ESP8266 applications must rely solely on the SDK API (and the somewhat Spartan SDK API documentation). (Note that for the ESP32, Espressif have moved to an open-source approach for its ESP-IDF.)
The NodeMCU Lua firmware is an ESP8266 application and must therefore be layered over the ESP8266 SDK. However, the hooks and features of Lua enable it to be seamlessly integrated without losing any of the standard Lua language features. The firmware has replaced some standard Lua modules that don't align well with the SDK structure with ESP8266-specific versions. For example, the standard `io` and `os` libraries don't work, but have been largely replaced by the NodeMCU `node` and `file` libraries. The `debug` and `math` libraries have also been omitted to reduce the runtime footprint (`modulo` can be done via `%`, `power` via `^`).
The NodeMCU Lua firmware is an ESP8266 application and must therefore be layered over the ESP8266 SDK. However, the hooks and features of Lua enable it to be seamlessly integrated without losing any of the standard Lua language features. The firmware has replaced some standard Lua modules that don't align well with the SDK structure with ESP8266-specific versions. For example, the standard `io` and `os` libraries don't work, but have been largely replaced by the NodeMCU `node` and `file` libraries. The `debug` and `math` libraries have also been omitted to reduce the runtime footprint (`modulo` can be done via `%`, `power` via `^`). Note that the `io.write()` function described in Lua's [Simple I/O Model](https://www.lua.org/pil/21.1.html) is not replaced by the `file` library. To write to the same serial port that the `print(string)` function uses by default, use `uart.write(0,string)`.
NodeMCU Lua is based on [eLua](http://www.eluaproject.net/overview), a fully featured implementation of Lua 5.1 that has been optimized for embedded system development and execution to provide a scripting framework that can be used to deliver useful applications within the limited RAM and Flash memory resources of embedded processors such as the ESP8266. One of the main changes introduced in the eLua fork is to use read-only tables and constants wherever practical for library modules. On a typical build this approach reduces the RAM footprint by some 20-25KB and this makes a Lua implementation for the ESP8266 feasible. This technique is called LTR and this is documented in detail in an eLua technical paper: [Lua Tiny RAM](http://www.eluaproject.net/doc/master/en_arch_ltr.html).