Background
At present, there are two types of LCD 3D printers on the market: FDM (Fused Deposition Manufacturing) and Light Curing 3D Printing. Among them, the FDM technology is more mature, the price is lower, and the market share is higher. You can even buy a desktop FDM printer for around 200 USD right now on eBay.

Most FDM 3D printers will use an open-source firmware, Marlin. Marlin firmware runs on the main board of the LCD 3D printer, manages all real-time activities of the machine, and coordinates all operations of heating devices, stepper motors, sensors, LCD displays, and other devices. Marlin’s control language is G-code. After the 3D model is created, it will generate a file in .gcode format through the slicing software. The .gcode file will be transparently transmitted to Marlin through a memory card or U disk. Marlin will parse the .gcode file to obtain G-code, and then use G-code to control heating. It also uses G-code language to communicate with LCD screens. The G-code language is published on the open-source website:
http://marlinfw.org/meta/gcode/ and will be updated as the 3D printer functions increase.