garbage/doc/terminology.org

Terminology

GameBoy terminology.

Terminology

Hardware (1)(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(3).

We start counting from the right as bit #0!

Opcode

Name	Bits	Sign	Mnemonics	1	2	3/3	4	Description	Note
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(4)(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(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
lsb	least significant byte
msb	most significant byte

References

1. "Memory Map". rgbdev.io. Retrieved 22 Aug 2022.
2. "Specifications". rgbdev.io. Retrieved 22 Aug 2022.
3. "A Visual Guide to the Gameboy's Half-Carry Flag". robdor.com. Retrieved 24 Aug 2022.
4. "gbz80(7) — CPU opcode reference". rgbds.gbdev.io. Retrieved 26 Aug 2022.
5. "GameBoy Opcode Summary". devrs.com. Retrieved 25 Aug 2022.