[Java] from scratch - Very Simple GameBoy Assembler (CPU: Sharp LR35902) + Hello World program

Very simple GameBoy assembler (CPU: Sharp LR35902) implemented in java + Hello World assembly example program.

Some details about this assembler:
- can generate only 32Kb 'ROM ONLY' (00h) Cartridge Type
- can't evaluate expressions
- labels must have at least 3 bytes
- support non local + local labels (similar to nasm)
- supported pseudo instructions: org, equ & db
- decimal literal numbers: 127, 10
- hex literal numbers: $ab, 0abh
- binary literal numbers: %10010001, 10010001b

Runnable jar (java 8): https://github.com/leonardo-ono/JavaGBAsm/raw/master/src/dist/jgbasm.jar

Source code: https://github.com/leonardo-ono/JavaGBAsm

Hello World example program: https://github.com/leonardo-ono/JavaGBAsm/tree/master/example

Note 1: For simplicity, the Hello World example program does not contain the correct header. You can use some external program like rgbfix to fix it. Emulator like BGB lets you run it anyway with warnings, but on a real device, it will not run without a correct header.

Note 2: 19/jan/2020 IMPORTANT: added wait_for_vblank function in hello.asm. According to http://bgb.bircd.org/pandocs.htm#videodisplay: "CAUTION: stopping LCD operation (Bit 7 from 1 to 0) may be performed during V-Blank ONLY, disabeling the display outside of the V-Blank period may damage the hardware."

Tools:
Gameboy Tile Designer: http://www.devrs.com/gb/hmgd/gbtd.html

References:
https://media.ccc.de/v/33c3-8029-the_ultimate_game_boy_tal #t=34
https://gbdev.gg8.se/files/docs/mirrors/pandocs.htm #thecartridgeheader
https://www.pastraiser.com/cpu/gameboy/gameboy_opcodes.html
http://marc.rawer.de/Gameboy/Docs/GBCPUman.pdf
https://eldred.fr/gb-asm-tutorial/hello-world.html