CHIP-8

command module
Version: v0.0.0-...-547a445 Latest Latest
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Published: Jan 7, 2020 License: Zlib Imports: 18 Imported by: 0

README

CHIP-8 Emulator

CHIP-8 is an assembler, debugger, and emulator for the COSMAC ELF CHIP-8 interpreter and its derivative: the Super CHIP-8, which ran on HP-48 calculators. Everything is emulated as well as possible: the video display refreshes at 60 Hz and sound is emulated as well.

From the screen capture above you can see the disassembled program, register values, and a log which is used to show breakpoint information, memory dumps, and more.

CHIP-8 is written in Go and uses SDL for its rendering, input handling, and audio. It should easily run on Windows, OS X, and Linux. It - and the source code - are under the ZLIB license.

What's New?

Getting the latest version? I recommend looking at the CHANGELOG file.

Downloading

You can download the latest pre-built release for your platform here.

Building

If you'd like to build yourself, then you'll need the go-sdl2 package (which requires cgo to build). But, once you have that it should be as simple as running:

$ go build -ldflags "-H windowsgui"

If you'd like a nice application icon for Windows, then grab akavel/rsrc and build the syso file for your architecture and then build:

$ rsrc -arch [386|amd64] -ico chip-8.ico -o chip-8.syso
Icon  chip-8.ico  ID:  4

$ go build -ldflags "-H windowsgui"

That's it. You should have a chip-8 executable ready to run.

Usage

Simply launch the app and away you go!

If you get a panic on startup or the application fails to launch for an unknown reason, it's most likely due to one of the following:

  • SDL2.DLL is missing (or not installed).
  • FONT.BMP is missing.

Once launched simply drag a ROM or C8 source file into the app to load it. You can also press H at any time to see the list of key commands available to you. But here's a quick breakdown:

Emulation Description
Back Reset the emulator
Ctrl+Back Reset and break
[ Decrease emulation speed
] Increase emulation speed
F2 Reload ROM or C8 assembler file
F3 Open ROM or C8 assembler file
F4 Save ROM
Debugging Description
F5 Pause/break emulation
F6 Step over
F7 Step into
SHIFT+F7 Step out
F8 Dump memory at I register
F9 Toggle breakpoint

Note: You can launch the emulator with -eti. This will tell the emulator to assemble and load ROMs in a mode that supports the ETI-660. This flag should be rarely used. The ETI-660 loads CHIP-8 programs starting at address 0x600 instead of 0x200. Use this if you intend to assemble and run a ROM on actual ETI-660 hardware or if you have a ROM assembled for the ETI (good luck finding one!).

Virtual Key Mapping

The original CHIP-8 had 16 virtual keys had the layout on the left, which has been mapped (by default) to the keyboard layout on the right:

 1 2 3 C                                   1 2 3 4
 4 5 6 D    This is emulated with these    Q W E R
 7 8 9 E    keyboard keys -->              A S D F
 A 0 B F                                   Z X C V

The Assembler

While playing the games that exist for the CHIP-8 might be fun for a while, the real fun is in creating your own games and seeing just how creative you can be with such a limited machine!

Just about every assembler for the CHIP-8 is different, and this one is, too. It's designed with a few niceties in mind. So, bear this in mind and take a few minutes to peruse this documentation.

Heavily documented, example programs can be found in games/sources/.

Syntax

Each line of assembly uses the following syntax:

label instruction arg0, arg1, ... ; comment

A label must appear at the very beginning of the line, and there must be at least a single whitespace character before the instruction or directive of a line (i.e. an instruction cannot appear at the beginning of a line). There are plenty of examples in games/sources to learn from.

Registers

The CHIP-8 has 16, 8-bit virtual registers: V0, V1, V2, V3, V4, V5, V6, V7, V8, V9, VA, VB, VC, VD, VE, and VF. All of these are considered general purpose registers except for VF which is used for carry, borrow, shift, overflow, and collision detection.

There is a single, 16-bit address register: I, which is used for reading from - and writing to - memory.

Last, there are two, 8-bit, timer registers (DT for delays and ST for sounds) that continuously count down at 60 Hz. The delay timer is good for time limiting your game or waiting brief periods of time. While the sound timer is non-zero a tone will be emitted.

Finally, the Super CHIP-8, which was used on the HP-48 calculators, contained 8, 8-bit, user-flag registers: R0-R7. These cannot be directly used, but registers V0-V7 can be saved to - and loaded from - them. This can be quite handy at times. See the LD R, VX and LD VX, R instructions below.

Literals

Literal constants can be in decimal, hexadecimal, or binary. Only decimal values can be negative, and binary allows the use of . in place of 0 to make it easier to visualize sprites.

        LD      V0, #FF
        ADD     V0, -2

BALL    BYTE    %.1......
        BYTE    %111.....
        BYTE    %.1......

Text literals can be added with single-, double-, or back-quotes, but there is no escape character. Usually this is just to add text information to the final ROM.

        BYTE    "A little game made by ME!"
Instruction Set

While this information is readily available in a few other places, I'm adding it here so it isn't lost. There are also a few tid-bits here that I couldn't find anywhere else.

Each instruction is 16-bit and written MSB first. Each instruction is broken down into nibbles, where the nibbles (when combined) mean the following:

Operand Description
X Virtual register (V0-VF)
Y Virtual register (V0-VF)
N 4-bit nibble literal
NN 8-bit byte literal
NNN 12-bit literal address (typically a label)

Here is the CHIP-8 instructions. The Super CHIP-8 instructions follow after the basic instruction set.

Opcode Mnemonic Description
00E0 CLS Clear video memory
00EE RET Return from subroutine
0NNN SYS NNN Call CDP1802 subroutine at NNN
2NNN CALL NNN Call CHIP-8 subroutine at NNN
1NNN JP NNN Jump to address NNN
BNNN JP V0, NNN Jump to address NNN + V0
3XNN SE VX, NN Skip next instruction if VX == NN
4XNN SNE VX, NN Skip next instruction if VX != NN
5XY0 SE VX, VY Skip next instruction if VX == VY
9XY0 SNE VX, VY Skip next instruction if VX != VY
EX9E SKP VX Skip next instruction if key(VX) is pressed
EXA1 SKNP VX Skip next instruction if key(VX) is not pressed
FX0A LD VX, K Wait for key press, store key pressed in VX
6XNN LD VX, NN VX = NN
8XY0 LD VX, VY VX = VY
FX07 LD VX, DT VX = DT
FX15 LD DT, VX DT = VX
FX18 LD ST, VX ST = VX
ANNN LD I, NNN I = NNN
FX29 LD F, VX I = address of 4x5 font character in VX (0..F) (* see note)
FX55 LD [I], VX Store V0..VX (inclusive) to memory starting at I; I remains unchanged
FX65 LD VX, [I] Load V0..VX (inclusive) from memory starting at I; I remains unchanged
FX1E ADD I, VX I = I + VX; VF = 1 if I > 0xFFF else 0
7XNN ADD VX, NN VX = VX + NN
8XY4 ADD VX, VY VX = VX + VY; VF = 1 if overflow else 0
8XY5 SUB VX, VY VX = VX - VY; VF = 1 if not borrow else 0
8XY7 SUBN VX, VY VX = VY - VX; VF = 1 if not borrow else 0
8XY1 OR VX, VY VX = VX OR VY
8XY2 AND VX, VY VX = VX AND VY
8XY3 XOR VX, VY VX = VX XOR VY
8XY6 SHR VX VF = LSB(VX); VX = VX >> 1 (** see note)
8XYE SHL VX VF = MSB(VX); VX = VX << 1 (** see note)
FX33 BCD VX Store BCD representation of VX at I (100), I+1 (10), and I+2 (1); I remains unchanged
CXNN RND VX, NN VX = RND() AND NN
DXYN DRW VX, VY, N Draw 8xN sprite at I to VX, VY; VF = 1 if collision else 0

And here are the instructions added for the Super CHIP-8 (a.k.a. SCHIP-8/CHIP-48):

Opcode Mnemonic Description
00BN SCU N Scroll up N pixels (N/2 pixels in low res mode)
00CN SCD N Scroll down N pixels (N/2 pixels in low res mode)
00FB SCR Scroll right 4 pixels (2 pixels in low res mode)
00FC SCL Scroll left 4 pixels (2 pixels in low res mode)
00FD EXIT Exit the interpreter; this causes the VM to infinite loop
00FE LOW Enter low resolution (64x32) mode; this is the default mode
00FF HIGH Enter high resolution (128x64) mode
DXY0 DRW VX, VY, 0 Draw a 16x16 sprite at I to VX, VY (8x16 in low res mode) (*** see note)
FX30 LD HF, VX I = address of 8x10 font character in VX (0..F) (* see note)
FX75 LD R, VX Store V0..VX (inclusive) into HP-RPL user flags R0..RX (X < 8)
FX85 LD VX, R Load V0..VX (inclusive) from HP-RPL user flags R0..RX (X < 8)

To use the above instructions, they need to be enabled with the SUPER directive.

In addition to the regular CHIP-8 and CHIP-48 instructions, there was also an extended instruction set (the CHIP-8E) added in 1979 by Paul Moews. These are very nice to have, and not used in any ROMs that I've seen online. However, this assembler supports them if turned on using the EXTENDED directive.

Using the EXTENDED instruction set will likely ensure that your ROM will not work with any other CHIP-8 emulator on actual hardware. And, technically, the SUPER and EXTENDED directives should be mutually exclusive as there is no hardware that supports both of them (the CHIP-48 was exclusively for HP-48 calculators and CHIP-8E was a one-off change to the COSMAC ELF interpreter for a few games). But, this assembler allows you to use both and the emulator does't care either.

Opcode Mnemonic Description
5XY1 SGT VX, VY Skip next instruction if VX > VY
5XY2 SLT VX, VY Skip next instruction if VX < VY
9XY1 MUL VX, VY VX * VY; VF contains the most significant byte, VX contains the least significant
9XY2 DIV VX, VY VX / VY; VF contains the remainder and VX contains the quotient
9XY3 BCD VX, VY Store BCD representation of the 16-bit word VX, VY (where VX is the most significant byte) at I through I+4; I remains unchanged
FX94 LD A, VX Load I with the font sprite of the 6-bit ASCII value found in VX; V0 is set to the symbol length (**** see note)

It should be noted that the CHIP-8E also had a DISP instruction which output the value of VX to the hex display. That instruction is not supported, because the opcode is the same as a CHIP-48 instruction, and it is redundant as this app contains a debugger and all registers are visible at all times.

(*): This is implementation-dependent. Originally the CDP1802 CHIP-8 interpreter kept this memory somewhere else, but most emulators (including this one) put these sprites in the first 512 bytes of the program.

(**): So, in the original CHIP-8, the shift opcodes were actually intended to be VX = VY shift 1. But somewhere along the way this was dropped and shortened to just be VX = VX shift 1. No ROMS or emulators I could find implemented the original CHIP-8 shift instructions, and so neither does this one. However, the assembler will always write out a correct instruction so that any future emulators can implement the shift either way and it will work.

(***): When implementing 16x16 sprite drawing, note that the sprites are drawn row major. The first two bytes make up the first row, the next two bytes the second row, etc.

(****): The ASCII font is a "compressed", 6-bit font (64 characters). Use the ASCII directive to convert a text string and write it to the binary. If you'd like to see what the font looks like, load and run games/sources/ascii.c8.

Directives

The assembler understands - beyond instructions - the following directives:

Directive Description
SUPER The assembler will allow the use of CHIP-48 instructions.
EXTENDED The assembler will allow the use of CHIP-8E instructions
EQU Declare the label to equal a literal constant instead of the current address. Must be declared before being used.
VAR Declare the label to represent a general purpose, V-register instead of the current address. Must be declared before being used.
BREAK Create a breakpoint. No instruction is written, but the emulator will break before the next instruction is executed. All text after the directive will be output to the log.
ASSERT Create a conditional breakpoint. The emulator will only break if VF is non-zero when the assert is hit. All text after the directive will be output to the log.
ASCII Write a text string - converted to 6-bit ASCII characters - to the ROM.
BYTE Write bytes to the ROM. This can take bytes literals or text strings.
WORD Write 2-byte words to the ROM in MSB first byte order.
ALIGN Align the ROM to a power of 2 byte boundary.
PAD Write "zero" bytes to the ROM. Easier than using BYTE and typing out a bunch of 0's.

Debugging

While the program is running, pressing F5 or SPACE will pause emulation and break into the debugger. You should see the disassembled code with the current instruction highlighted red.

Once in the debugger, pressing F6 will single-step the current instruction and F7 will step "over" it (this is useful when on a CALL instruction and you'd rather just skip the call and break again once you've returned back). Press F8 to dump the memory near the I register. F9 will toggle a breakpoint on the current instruction.

When you've gotten whatever information you need, press F5 again to continue execution.

If you are debugging your own C8 assembler program, don't forget about the BREAK and ASSERT directives.

NOTE: the DT and ST registers still count down even while emulation is paused/broken. This is so the sound tone isn't constantly on while debugging and a delay would take a long time to reach zero if single stepping.

Saving ROMs

While a C8 file is loaded, pressing F4 will allow you to save the ROM file to disk. But be aware that if using the extended, CHIP-8E instructions, it's quite possible that any saved ROMs will not work with other CHIP-8 emulators. And, if using SCHIP or CHIP-8E instructions, these ROMs will not work with the original CHIP-8 interpreter if loaded onto actual hardware.

CHIP-8 Tips & Tricks

Assembly language - if you're not used to it - can be a bit daunting at first. Here's some tips to keep in mind (for CHIP-8 and assembly programming in general) that can help you along the way...

  • If you want to subtract a constant value from a register, remember it's easier to just add a negative value instead.

  • Want to compare greater than or equal to? Use SUB and SUBN. Remember VF is 1 if there is not a borrow (the result is >= 0). Use SUBN when you want to compare, but not store the result in what you're comparing. Examples:

    • 10-2 .. VF=1
    • 8-8 .. VF=1
    • 7-20 .. VF=0
  • Remember that for purposes of borrowing in subtraction, all operands are unsigned!

    • (-2)-3 = #FE-3 .. VF=1
    • (-3)-(-4) = #FD-#FC .. VF=1
    • (-2)-(-1) = #FE-#FF .. VF=0
  • Need a switch statement? Use SE and SNE followed by JP instructions to build a jump table. Use a JP V0, address instead when possible.

  • Perform tail calls whenever possible. If you see a CALL followed by a RET, just change the CALL to a JP and get rid of the RET.

  • Need a random point on the screen? RND VX, #3F for X and RND VY, #1F for Y. Use #7F and #3F if in high res mode.

  • When setting up global use of registers, leave V0-V2 always free as scratch. Most memory reads/writes use them, especially after performing a LD to BCD.

  • Only draw when you absolutely have to. Video RAM isn't cleared every "frame" like modern games, so once you draw something it's there until you clear it.

  • Since all drawing operations are an XOR, you can draw something simply to test VF, and then draw it again to completely undo the operation.

  • While there's no floating point math, it's pretty easy to you 2 bytes (or registers) to perform fixed point operations.

Example CHIP-8 Programs

There are a few example programs in games/sources/ for you you play around with, modify, and learn from.

Thanks!

Special thanks to Andy Kelts for creating the nice icons and my good friend Mark Allender for his continual harassment. And if you think this is cool, check out Mark's Apple II emulator!

Roadmap

Here are a few features I'm still planning on adding...

  • Saving/restoring of VM images.
  • Including C8 source files.
  • Importing 1-bit images into C8 files.
  • Saving screen shots and maybe videos to GIF.
  • Use OpenGL and add a CRT shader.

That's all folks!

If you have any feedback, please email me. If you find a bug or would like a feature, feel free to open an issue.

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