#+TITLE: Terminology
#+AUTHOR: Riyyi
#+LANGUAGE: en
#+OPTIONS: toc:nil

GameBoy terminology.

** Terminology

*** Hardware [[#references][(1)]][[#references][(2)]]

| Name            | Modern equivalent | Description                               |
|-----------------+-------------------+-------------------------------------------|
| APU             | Sound card        | Audio Processing Unit                     |
| PPU             | GPU               | Pixel Processing Unit                     |
| Bootrom         | BIOS              |                                           |
| HRAM (High RAM) | CPU cache         | Embedded memory inside the CPU, 127 bytes |
| WRAM (Work RAM) | RAM               |                                           |

*** Flags

The half-carry flag ~H~ is set when an arithmetic operation on two numbers
causes a carry from bit #3 to bit #4 for 8-bit operations or a carry from bit
#11 to bit #12 for 16-bit operations. For 16-bit operation this indicates the
lower 4 bits of the *second* byte[[#references][(3)]].

We start counting from the right as bit *#0*!

*** Opcode

| Name      | Bits | Sign     | Mnemonics |    [[https://gbdev.io/gb-opcodes/optables/][1]] |   [[https://rgbds.gbdev.io/docs/v0.5.2/gbz80.7/][2]] |  [[https://gekkio.fi/files/gb-docs/gbctr.pdf][3]]/[[http://marc.rawer.de/Gameboy/Docs/GBCPUman.pdf][3]] |   [[https://gbdev.io/pandocs/CPU_Instruction_Set.html][4]] | Description                  | Note                   |
|-----------+------+----------+-----------+------+-----+------+-----+------------------------------+------------------------|
|           |      |          |           |  <r> | <r> |  <r> | <r> |                              |                        |
| immediate |    8 | unsigned | i8        |   d8 |  n8 |    n |     | Next byte in memory          |                        |
| immediate |   16 | unsigned | i16       |  d16 | n16 |   nn |     | Next byte in memory          | little-endian          |
| register  |    8 | unsigned | r8        |      |  r8 |    r |     | Any of the registers         |                        |
| register  |   16 | unsigned | r16       |      | r16 |   rr |     | Any of the registers         | little-endian          |
| address   |    8 | unsigned | a8        |      |     |  (n) |     | Address at value             |                        |
| address   |   16 | unsigned | a16 or () |  a16 | n16 | (nn) |  nn | Address at value             | little-endian          |
| immediate |    8 | signed   | s8        |   r8 |  e8 |    e |  dd | Next byte in memory          | used as offset         |
| immediate |    8 | unsigned | io8       | (a8) | n16 |  (n) |   n | 0xff00 + next byte in memory | write to I/O-port      |
| condition |      |          | <ZNHC>    |      |  cc |      |   f | Execute if condition met     |                        |
| constant  |    8 | unsigned | b         |      |  u3 |    b |   n | Single bit at position 'x'   |                        |
| constant  |    8 | unsigned | vec       |      | vec |      |   n | One of RST vectors           | absolute fixed address |

d = data \\
n = constant

**** Flag conditions

Flag conditions, sometimes referred to as ~cc~, are the state the flags need to
be in to execute the opcode. There are four possible checks[[#references][(4)]][[#references][(5)]]:

| Code | Meaning                        | State |
|------+--------------------------------+-------|
| Z    | Execute if Zero flag is set    |     1 |
| NZ   | Execute if Zero flag is unset  |     0 |
| C    | Execute if Carry flag is set   |     1 |
| NC   | Execute if Carry flag is unset |     0 |

**** RST vectors

Absolute fixed address, possible values are[[#references][(4)]]: ~0x00~, ~0x08~, ~0x10~, ~0x18~,
~0x20~, ~0x28~, ~0x30~, and ~0x38~.

*** Variables

| Name   | Meaning            |
|--------+--------------------|
| length | amount of elements |
| size   | amount of bytes    |
|--------+--------------------|
| lhs    | left hand side     |
| rhs    | right hand side    |

** References

1. [[https://gbdev.io/pandocs/Memory_Map.html]["Memory Map"]]. rgbdev.io. Retrieved 22 Aug 2022.
2. [[https://gbdev.io/pandocs/Specifications.html]["Specifications"]]. rgbdev.io. Retrieved 22 Aug 2022.
3. [[https://robdor.com/2016/08/10/gameboy-emulator-half-carry-flag/]["A Visual Guide to the Gameboy's Half-Carry Flag"]]. robdor.com. Retrieved 24 Aug 2022.
4. [[https://rgbds.gbdev.io/docs/v0.5.2/gbz80.7#LEGEND]["gbz80(7) — CPU opcode reference"]]. rgbds.gbdev.io. Retrieved 26 Aug 2022.
5. [[http://www.devrs.com/gb/files/opcodes.html]["GameBoy Opcode Summary"]]. devrs.com. Retrieved 25 Aug 2022.