Ostrich emulator

A simple Game Boy emulator written in C# using SDL2 and ImGUI.

FAQs

• ROM selection: If you're on Linux using some minimal VM you'll have to enter the path manually.

Dev requirements

• Dotnet SDK 10
• libsdl2 & libglfw3

on UbuntuDebian using Wayland install the following packages:
sudo apt install dotnet-sdk-10.0 libglfw3 libsdl2-2.0-0 libgl1 libegl1 libx11-6 libwayland-client0

Display

The display uses only integer scaling in relation to the emulators display buffer. This is so that the rendered image better reflects the hardware being emulated.

Window Pipeline:

    Window
    ├─ Padding
    │   └─ Content Area
    │       └─ Integer-scaled emulator image
    └─ ImGui overlay

Padding has been added around the content area. This was present on original hardware and games would draw to the edge of the screen with the knowledge that the padding was there.

Project Structure

App (FrontEnd), it holds knowledge of Silk.NET, OpenGL, ImGui and is generally responsible for HCI tasks.

Emulation (Contracts), is an interface layer defining what should be exposed.

GameBoy (Core), implements the contracts and holds knowledge of the emulated hardware.

Reference Graph

Ostrich.App ──▶ Ostrich.Emulation Ostrich.GameBoy ──▶ Ostrich.Emulation

LR 35902 Opcodes

Base Opcodes (Grouped)

Opcode	Type	Pseudo C	Description
00	misc	;	NOP
01,11,21,31	ld16	rr = d16	LD BC/DE/HL/SP, immediate 16-bit
02,12	mem	[rr] = A	LD (BC/DE),A
0A,1A	mem	A = [rr]	LD A,(BC/DE)
03,13,23,33	alu16	rr++	INC BC/DE/HL/SP
0B,1B,2B,3B	alu16	rr--	DEC BC/DE/HL/SP
04..3D*	alu8	r++ / r--	INC/DEC 8-bit register or (HL)
06,0E,16,1E,26,2E,36,3E	ld8	r = d8	LD r,d8 (includes LD (HL),d8)
07	rotate	A = RLC(A)	RLCA
0F	rotate	A = RRC(A)	RRCA
17	rotate	A = RL(A,Carry)	RLA
1F	rotate	A = RR(A,Carry)	RRA
08	mem	[a16] = SP	LD (a16),SP
09,19,29,39	alu16	HL += rr	ADD HL,BC/DE/HL/SP
10	ctrl	stop	STOP
18	flow	PC += e8	JR e8
20,28,30,38	flow	if(cond) PC += e8	JR NZ/Z/NC/C,e8
22,32	mem	[HL++] = A / [HL--] = A	LD (HL+),A / LD (HL-),A
2A,3A	mem	A=[HL++] / A=[HL--]	LD A,(HL+) / LD A,(HL-)
27	alu	A = DAA(A)	DAA
2F	alu	A = ~A	CPL
37	flags	Carry = 1	SCF
3F	flags	Carry = !Carry	CCF
40..7F	ld8	r1 = r2	LD r,r' (76 is HALT)
76	ctrl	halt	HALT
80..87	alu	A += r	ADD A,r
88..8F	alu	A += r + Carry	ADC A,r
90..97	alu	A -= r	SUB A,r
98..9F	alu	A -= r + Carry	SBC A,r
A0..A7	alu	A &= r	AND A,r
A8..AF	alu	A ^= r	XOR A,r
B0..B7	alu	A |= r	OR A,r
B8..BF	cmp	flags = A - r	CP A,r
C0,C8,D0,D8	flow	if(cond) RET	RET NZ/Z/NC/C
C9	flow	RET	RET
D9	flow	RET; IME=1	RETI
C1,D1,E1,F1	stack	rr = POP()	POP BC/DE/HL/AF
C5,D5,E5,F5	stack	PUSH(rr)	PUSH BC/DE/HL/AF
C2,CA,D2,DA	flow	if(cond) PC=a16	JP NZ/Z/NC/C,a16
C3	flow	PC = a16	JP a16
E9	flow	PC = HL	JP HL
C4,CC,D4,DC	flow	if(cond) CALL a16	CALL NZ/Z/NC/C,a16
CD	flow	CALL a16	CALL a16
C6,CE	alu	A += d8 (+Carry)	ADD A,d8 / ADC A,d8
D6,DE	alu	A -= d8 (+Carry)	SUB d8 / SBC A,d8
E6,EE,F6,FE	alu	A op d8	AND/XOR/OR/CP d8
C7,CF,D7,DF,E7,EF,F7,FF	flow	CALL vec	RST 00/08/10/18/20/28/30/38
E0,F0	mem	[FF00+a8]=A / A=[FF00+a8]	LDH (a8),A / LDH A,(a8)
E2,F2	mem	[FF00+C]=A / A=[FF00+C]	LD (C),A / LD A,(C)
EA,FA	mem	[a16]=A / A=[a16]	LD (a16),A / LD A,(a16)
E8	alu16	SP += e8	ADD SP,e8
F8	alu16	HL = SP + e8	LD HL,SP+e8
F9	ld16	SP = HL	LD SP,HL
F3,FB	ime	IME = 0/1	DI / EI
D3,DB,DD,E3,E4,EB,EC,ED,F4,FC,FD	invalid	—	Unused/invalid on LR35902

* includes (HL) forms (34,35)

CB-Prefixed Opcodes (Grouped)

Opcode Range	Type	Pseudo C	Description
CB 00..07	rot	RLC r	Rotate left circular
CB 08..0F	rot	RRC r	Rotate right circular
CB 10..17	rot	RL r	Rotate left through carry
CB 18..1F	rot	RR r	Rotate right through carry
CB 20..27	shift	SLA r	Shift left arithmetic
CB 28..2F	shift	SRA r	Shift right arithmetic (MSB preserved)
CB 30..37	bitop	SWAP r	Swap high/low nibbles
CB 38..3F	shift	SRL r	Shift right logical
CB 40..7F	bit	BIT b,r	Test bit b
CB 80..BF	bit	RES b,r	Reset bit b
CB C0..FF	bit	SET b,r	Set bit b