The ins and outs of writing a ZX Spectrum emulator using Lua 5.3 using bitwise operators, dynamic code generation and string.unpack.

1. Ignacio Burgueño
Lua Workshop 2015
Stockholm, Sweden
At the end of the rainbow

2. About Me
Ignacio Burgueño
Developer at inConcert
@iburgueno
https://github.com/ignacio

3. What will be talking about?
● Writing a ZX Spectrum emulator in Lua 5.3
● Fun with Bitwise operators

4. A bit of history

11. Fairlight
Bo Jangeborg, 1985

13. What is needed?
● Replace hardware parts with software
● Which parts?

14. Architecture
https://commons.wikimedia.org/wiki/File:ZXspectrum_mb.jpg

15. Architecture
"ZX Spectrum block diagram" taken from the book “The ZX Spectrum ULA” by Chris Smith

16. Memory

17. CPU
"Z80 arch" by Appaloosa - Own work. Licensed under CC BY-SA 3.0 via Commons -
https://commons.wikimedia.org/wiki/File:Z80_arch.svg#/media/File:Z80_arch.svg
Instruction
Register
Instruction
Decoder
Control
Logic
I R
1+_ 1+_
MUXMUX
W ZW' Z'
BB' CC'
DD' EE'
H LH' L'
IX
IY
PC
SP
TEMP
ACU
A
A'
F
F'
+
BUFFERSBUFFERSBUFFERS
ALU
Internal Data Bus 8 Bit
Address Bus 16 Bit
Control Bus
Control Section
Z80 Architecture

18. CPU

19. CPU – Instruction Set
● 8-bit arithmetic and logic operations
● 16-bit arithmetic
● 8-bit load
● 16-bit load
● Bit set, reset, and test
● Call, return, and restart
● Exchange, block transfer, and search
● General purpose arithmetic and CPU control
● Input and output
● Jump
● Rotate and shift LD A,42 0x3e 0x2a

20. CPU – Fetch/Execute Cycle

21. CPU – Fetch/Execute Cycle

22. CPU – Fetch/Execute Cycle

23. CPU – Fetch/Execute Cycle

24. CPU - Opcodes

25. CPU – Instruction families
● Same operation is applied to different registers
● One instruction for each combination
● Avoid writing the same function over and over
● Generate them using a template

26. CPU – Instruction families

27. CPU – Instruction families

28. CPU – Instruction families

29. See what's going on

30. Drawing to the Screen

31. Drawing to the Screen

32. Drawing to the Screen

33. Drawing to the Screen

34. Drawing to the Screen
luasdl2
https://github.com/Tangent128/luasdl2
More information on the subject:
How "oldschool" graphics worked video series

35. Getting software in it
(aka, how do I load games on this?)

36. Parsing Binary data
Offset (bytes) Size Description
0 1 byte I
1 8 words HL', DE', BC', AF'
9 10 words HL, DE, BC, IY, IX
19 1 byte Interrupt
20 1 byte R
21 4 words AF, SP
25 1 byte Interruption Mode
26 1 byte Border Color
27 49152 bytes RAM dump 16384 .. 65535
Total: 49179 bytes
File Formats - SNA

37. File Formats - SNA
● Parsing binary formats
● Don't want to split strings
● Enter string.unpack

38. File Formats - SNA
● Cumbersome to read
● What value ends up where?
● Extend the unpack format string

39. File Formats - SNA

40. File Formats - SNA

41. Testing
● Run little code snippets
● Make sure registers have the correct values
● Correct memory locations are read and written at the
right times

42. Testing
● Run little code snippets
● Make sure registers have the correct values
● Correct memory locations are read and written at the
right times

43. Demo

44. Where to get it?
https://github.com/ignacio/luagleck
Where do I get software to try?
http://www.worldofspectrum.org/archive.html

45. Thank you!
Any questions?
Ignacio Burgueño - @iburgueno