The Chip-8 language is a very simple virtual machine specification for which we can easily build a runtime in Java. Due to its simplicity, the implementation for a Chip-8 interpreter can easily be grasped and explained in detail during this single session. Despite this simplicity, the presentation covers many of the issues faced when we use Java to emulate/simulate other architectures. It also looks at some common issues faced by emulation authors when using the Java platform to implement runtimes for more-complex architectures such as 8-bit/16-bit computers or even early home video game systems.

3. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Let’s Write Our Own CHIP-8 Interpreter!
CON3584
David Buck
Principal Member of Technical Staff
Java Platform Group
October 3rd, 2017
Confidential – Oracle Internal/Restricted/Highly Restricted 3

4. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Safe Harbor Statement
The following is intended to outline our general product direction. It is intended for
information purposes only, and may not be incorporated into any contract. It is not a
commitment to deliver any material, code, or functionality, and should not be relied upon
in making purchasing decisions. The development, release, and timing of any features or
functionality described for Oracle’s products remains at the sole discretion of Oracle.
Confidential – Oracle Internal/Restricted/Highly Restricted 4

5. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
• JVM Sustaining Engineer
• OpenJDK 8 Update Project
Maintainer
• JavaOne Rock Star
• Co-author of Oracle WebLogic
Server 11g 構築・運用ガイド
• @DavidBuckJP
• https://blogs.oracle.com/buck/
Hi There!

6. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Agenda
CHIP-8 History
CHIP-8 Architecture
Emulating CHIP-8
Advanced Emulation Topics
1
2
3
Confidential – Oracle Internal/Restricted/Highly Restricted 6
4

7. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Introducing CHIP-8
Confidential – Oracle Internal/Restricted/Highly Restricted 7

8. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Before CHIP-8
Confidential – Oracle Internal/Restricted/Highly Restricted 8
https://en.wikipedia.org/w/index.php?curid=4327124

9. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Intel 8080
Confidential – Oracle Internal/Restricted/Highly Restricted 9
By Konstantin Lanzet - CPU collectionCamera: Canon EOS 400D, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=7028099

10. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Not a Poor Nerd’s Hobby…
• 1975 kit price: 439 USD
• In 2017 currency: 1,997 USD
Confidential – Oracle Internal/Restricted/Highly Restricted 10

11. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
COSMAC ELF
Confidential – Oracle Internal/Restricted/Highly Restricted 11
By Swtpc6800 en:User:Swtpc6800 Michael Holley - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3471056

12. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
COSMAC ELF
• DIY project documented in Popular Electronics in 1976-1977
• Could be built for under 100 USD (1976 prices)
• Kits and pre-assembled boards offered from many vendors
• Very popular both then and now
Confidential – Oracle Internal/Restricted/Highly Restricted 12

13. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Pixie
• RCA 1861 video generator IC
• 64×32 square pixels
• Needed 256 bytes of external RAM
Confidential – Oracle Internal/Restricted/Highly Restricted 13

14. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
RCA COSMAC 1802
Confidential – Oracle Internal/Restricted/Highly Restricted 14
By Konstantin Lanzet (with permission) - CPU collection Konstantin Lanzet, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4774289

15. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
RCA COSMAC 1802
• Complementary Symmetry Monolithic Array Computer
• 1 chip revision of an earlier 2 chip design (1801)
• Very low cost compared to 8080
• Still in use in many applications today
Confidential – Oracle Internal/Restricted/Highly Restricted 15

16. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Radiation Hardened Versions
Confidential – Oracle Internal/Restricted/Highly Restricted 16
By NASA - http://solarsystem.nasa.gov/multimedia/display.cfm?IM_ID=2071 (image link)http://photojournal.jpl.nasa.gov/catalog/PIA18176 (image link)
Public Domain, https://commons.wikimedia.org/w/index.php?curid=408298

17. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
COSMAC VIP
• Developed by Joseph Weisbecker
• Version of the ELF targeting the video game market
• Came pre-assembled
• Hex-keyboard was standard
Confidential – Oracle Internal/Restricted/Highly Restricted 17

18. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Hex Keyboard
Confidential – Oracle Internal/Restricted/Highly Restricted 18
By John crane 59 - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15795865

19. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Hex Keyboard
Confidential – Oracle Internal/Restricted/Highly Restricted 19
1 3
4 5 6
7 8 9
0A B
C
D
E
F
2

20. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
COSMAC VIP
Confidential – Oracle Internal/Restricted/Highly Restricted 20
• CPU: 1802
• ROM: 512-byte
• RAM: 2k~4k on-board (up to 32k external)
• Cassette interface: 100 bps
• 40 years old this year!

21. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Similar to “CPU Trainers” of the day…
Confidential – Oracle Internal/Restricted/Highly Restricted 21
Public Domain, https://commons.wikimedia.org/w/index.php?curid=422438

22. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
RCA-Studio II
Confidential – Oracle Internal/Restricted/Highly Restricted 22
By Evan-Amos - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=38826714

23. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
CHIP-8
• A “language” for programming video games
• Developed by Joseph Weisbecker for the VIP
• Interpreter built into the VIP ROM
• Made writing games for the VIP much easier
Confidential – Oracle Internal/Restricted/Highly Restricted 23

24. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
“Language”?
• Looks like machine code
• It was really a very early virtual machine specification
Confidential – Oracle Internal/Restricted/Highly Restricted 24

25. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Why use a virtual machine?
• Abstract away much of the real hardware
– Linier display buffer -> X,Y mapped grid
– Sprite support
– Sound support (q-line on 1802)
– Key debouncing
• Very easy to program
• Very high code density
Confidential – Oracle Internal/Restricted/Highly Restricted 25

26. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Other Platforms Followed (Telmac 1800)
Confidential – Oracle Internal/Restricted/Highly Restricted 26
By jpl - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=40541200

27. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Fast Forward About 10 Years
HP 28c released
• HP’s first graphing calculator
• Light-years beyond anything else
– RPL (lambda expressions!)
– Symbolic math (CAS)
Confidential – Oracle Internal/Restricted/Highly Restricted 27
By Kurt Moerman (Kpmkpm) - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=4020998

28. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
The Legendary 48G
• Released in 1990
• HP’s most powerful calculator for almost a decade
Confidential – Oracle Internal/Restricted/Highly Restricted 28
By Feureau.com - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=9953201

29. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Meanwhile…
• TI-81
• Z80 based
• BASIC-like programming language
• Very easy to program
• Very easy to make games
Confidential – Oracle Internal/Restricted/Highly Restricted 29
By Calcvids - I took this photo of my TI-81, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=24252718

30. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
HP48G owners were jealous!
• RPL was horrible for gaming
– Too abstract / high-level
– Slow
– Steep learning curve
Confidential – Oracle Internal/Restricted/Highly Restricted 30

31. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
CHIP-8 to the rescue!
• Andreas Gustafsson wrote a CHIP-8 interpreter for the HP48
• HP48 users got access to library of preexisting software from the 70s
• New CHIP-8 software started to be written
• Super CHIP (SCHIP) expanded to take advantage of HP48 hardware
Confidential – Oracle Internal/Restricted/Highly Restricted 31

32. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
CHIP-8 Today
• A great way for future emulator authors to cut their teeth
• Probably the easiest platform to emulate that plays games
• Possibly the only platform with more emulators than native software
Confidential – Oracle Internal/Restricted/Highly Restricted 32

33. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
CHIP-8 Architecture
Confidential – Oracle Internal/Restricted/Highly Restricted 33

34. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Memory Map
Confidential – Oracle Internal/Restricted/Highly Restricted 34
CHIP-8 Interpreter (ROM)
Font Data
Free Memory (RAM)
Display Memory (RAM)
0x200
0x1FF
0xFFF
0xF00
0xEFF
0x000

35. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
An Embarrassment of Registers
• 15 8-bit general registers (V0-VE)
• 1 8-bit “Flag” register (VF)
– Only used for carry (ALU) and sprite hit detection
• 1 16-bit address register
– Only 12-bit LSBs normally used
• 1 16-bit PC
Confidential – Oracle Internal/Restricted/Highly Restricted 35

36. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Sound
• Write-only sound timer that decrements 60 ticks a second
• Buzzer / beeping sound plays until timer reaches zero
• No music here!
Confidential – Oracle Internal/Restricted/Highly Restricted 36

37. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Display
• 64x32 monochrome pixels
• Sprites
– 8 pixels wide
– 1~15 pixels tall
– Drawn to video memory by XOR:ing
– Collision flag (FV) set (or cleared)
Confidential – Oracle Internal/Restricted/Highly Restricted 37

38. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Timer
• Like sound timer, but no output.
• Decremented 60 ticks a second
• Stops at zero
• Can be both read or written to (unlike RO sound timer)
Confidential – Oracle Internal/Restricted/Highly Restricted 38

39. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Opcodes
• AAA: address
• KK: 8-bit constant (byte)
• K: 4-bit constant (nibble)
• X, Y: 4-bit register identifier
• I, PC: 16bit register
Confidential – Oracle Internal/Restricted/Highly Restricted 39

40. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Call instruction
• 0AAA – jump to 1802 code at AAA (not used in modern CHIP-8)
Confidential – Oracle Internal/Restricted/Highly Restricted 40

41. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Display
• 00E0 CLS
• FX29 set I register to font for hex digit stored in X
• DXYK draw sprite K pixels high at X, Y coordinates
Confidential – Oracle Internal/Restricted/Highly Restricted 41

42. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Flow Control Instructions
• 00EE return from sub ★
• 1AAA jump to AAA
• 2AAA call sub at AAA ★
• 3XKK if VX == KK skip next instruction
• 4XKK if VX != KK skip next instruction
• 5XY0 if VX == VY skip next instruction
★ push and pop return address use implementation-defined stack
Confidential – Oracle Internal/Restricted/Highly Restricted 42

43. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Loads / Stores / Moves
• 6XKK Vx = NN
• 8XY0 VX = VY
• AAAA I = AAA
Confidential – Oracle Internal/Restricted/Highly Restricted 43

44. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
ALU (Math) Instructions
• 7XKK Vx += KK
• 8XY1 Vx=Vx|Vy
• 8XY2 Vx=Vx&Vy
• 8XY3 Vx=Vx^Vy
• 8XY4 Vx += Vy
• 8XY5 Vx -= Vy
• 8XY6 Vx=Vy=Vy>>1
• 8XY7 Vx=Vy-Vx
• 8XYE Vx=Vy=Vy<<1
Confidential – Oracle Internal/Restricted/Highly Restricted 44

45. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Input Instructions
• EX9E if (key()==Vx) skip next instruction
• EXA1 if (key()!=Vx) skip next instruction
• FX0A Vx = next key press (blocking wait)
Confidential – Oracle Internal/Restricted/Highly Restricted 45

46. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Timers / Sound Instructions
• FX07 Vx = delay_timer
• FX15 delay_timer = Vx
• FX18 sound_timer = Vx
Confidential – Oracle Internal/Restricted/Highly Restricted 46

47. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Other Instructions
• CXKK Vx = random_number & KK
• FX33 stores 3 digits of BCD for value of Vx into I, I+1, I+2
• FX55 writes all V registers between V0 and Vx to I
• FX65 loads all V registers between V0 and Vx from I
Confidential – Oracle Internal/Restricted/Highly Restricted 47

48. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Emulating CHIP-8
Confidential – Oracle Internal/Restricted/Highly Restricted 48

49. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Overall Design
Confidential – Oracle Internal/Restricted/Highly Restricted 49
Chip8
(Interpreter) DisplayKeyPad
Memory

50. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Overall Design
• Allows Display and KeyPad to be swapped out with different
implementations
– AWT
– Swing
– JavaFX
– LWJGL
• Memory is isolated to be closer aligned to more advanced emulators
Confidential – Oracle Internal/Restricted/Highly Restricted 50

51. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Memory
• One single 4K byte array
• Handles font data (must be initialized)
• Handles reading in of program (“rom”) during startup
Confidential – Oracle Internal/Restricted/Highly Restricted 51

52. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Display
• ASCII ART inspired
• Appends characters to a StringBuilder
– White Pixel ‘#’
– Black Pixel ‘ ‘
• Note that screen buffer memory is independent from main memory
• Screen buffer is shared between Display and Chip8 (the interpreter)
• Screen refresh timing / method can have a huge performance impact
Confidential – Oracle Internal/Restricted/Highly Restricted 52

53. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
KeyPad
• Depends on enabling raw input from terminal
• Semantics of key press detection depend on automatic key repeat
– By extension also depends on a very low repeat delay
• Shutdown hook is registered to restore terminal to useable state
Confidential – Oracle Internal/Restricted/Highly Restricted 53

54. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Interpreter Loop
• Is a naive implementation
• Uses a simple series of nested switch statements
• Register values are stored in next larger primitive type
• Each iteration of the main loop updates timers as needed
• Each iteration of the main loop refreshes display as needed
Confidential – Oracle Internal/Restricted/Highly Restricted 54

55. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Debugging
• You often want some way to debug what is running on the VM
– Trace
– Breakpoints
– Memory dumps
Confidential – Oracle Internal/Restricted/Highly Restricted 55

56. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Other General Emulation Tips
• Start with small test cases, not full games
• Write your own tests if you cannot find any
• Have some way to throttle execution speed
• Compare your emulator’s behavior to other emulators
Confidential – Oracle Internal/Restricted/Highly Restricted 56

57. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Advanced Emulation Topics
Confidential – Oracle Internal/Restricted/Highly Restricted 57

58. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Your code should model the hardware
• Figure out how instructions are encoded / decoded (e.g. z80 decode)
• If there are multiple pieces of similar hardware, instantiating objects is
probably best
• Multiple threads for interpreter loop / IO refresh (including display) may be
the easiest design
Confidential – Oracle Internal/Restricted/Highly Restricted 58

59. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
My CHIP-8 Design
Confidential – Oracle Internal/Restricted/Highly Restricted 59
Chip8
(Interpreter) DisplayKeyPad
Memory

60. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Typical Emulator Design
Confidential – Oracle Internal/Restricted/Highly Restricted 60
Memory
(and/or IO bus) DisplayKeyPad
CPU

61. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Dynamic Recompilation
• Use ASM to generate bytecode on the fly!
• Way easier than you might imagine
• Run your interpreter through ASMifier
(temporarily refactoring each opcode implementation into its own method may help
make things clearer)
• Write a new version of interpreter loop where implementation code is
replaced by the corresponding ASMifier output
• The interpreter loop will now just linearly scan the “rom”, and generate
corresponding bytecode.
Confidential – Oracle Internal/Restricted/Highly Restricted 61

62. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Conclusions
• Emulation / Interpreters are fun to write in Java
• They are not as hard as many people imagine
• You can improve your general programming skillset by writing one
• Start out small. You can’t get much simpler than CHIP-8
Confidential – Oracle Internal/Restricted/Highly Restricted 62

63. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
THANK YOU!
Confidential – Oracle Internal/Restricted/Highly Restricted 63

64. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
• Octo - Chip-8 Assembly Language & On-line IDE
https://johnearnest.github.io/Octo/
• Cowgod's Chip-8 Technical Reference
http://devernay.free.fr/hacks/chip8/C8TECH10.HTM
• Chip-8 Software for HP48 Archive
http://www.hpcalc.org/hp48/games/chip/
• Cosmac ELF
(A great fan site with a lots of ELF content)
http://www.cosmacelf.com/
• Matthew Mikolay's Retro Computing Site
(Has scans of VIPER magazine among a ton of other great
information)
http://retro.mattmik.com/
Confidential – Oracle Internal/Restricted/Highly Restricted 64
References

65. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Stay connected
• Join us: DevOps Corner (Developer Lounge – Moscone West)
• Learn more: openjdk.java.net | wercker.com/java
• Follow: @OpenJDK, @wercker #JavaOne #DevOps
65

66. Copyright © 2017, Oracle and/or its affiliates. All rights reserved. |
Safe Harbor Statement
The preceding is intended to outline our general product direction. It is intended for
information purposes only, and may not be incorporated into any contract. It is not a
commitment to deliver any material, code, or functionality, and should not be relied upon
in making purchasing decisions. The development, release, and timing of any features or
functionality described for Oracle’s products remains at the sole discretion of Oracle.
Confidential – Oracle Internal/Restricted/Highly Restricted 66