3 Emulated systems

This CHIP-8 emulator is compatible with other kinds of systems. CHIP-8 is based in the origina lspecification written for the first computers using CHIP-8 virtual machines inside, such as the COSMAC VIP. However, through the years, hobbyists interested in CHIP-8 emulation have developed emulators for other kinds of machines such as calculators or PCs, and they have extended the original specification for adding support to new features if the host hardware allowed so.

In particular, this emulator is at the moment compatible with CHIP-8 and SUPERCHIP ROMs.

3.1 CHIP-8 platform

This is the original platform for the CHIP-8 virtual machine. It is a simple architecture that has the following hardware resources.

• 4 kB of RAM memory.
• A register bank made of 16 registers, each one being 8 bits. These registers are refered to as V0, V1, ..., V9, VA, VB, VC, VD, VE and VF.
• A stack that can hold 16 16-bit values.
• A stack pointer for pointing to the next free location in the stack. As an example, if the stack pointer holds the value 5, that means that the next value pushed to the stack will be in the position 5. Pushing a value to the stack will increase the value of the stack pointer by 1 to make it point to the next free location. Popping something will decrease the value by 1, and then whatever is pointed by it, will be transfered to the target destination.
• A special register named I that is used by some operations such as painting to the screen.

The CHIP-8 screen is a rasterized 64 x 32 pixels display. It is monochrome and up to 2048 pixels can be represented. Usually these pixels will be black and white, but there is no official statement on this, which means that is possible for some systems to use different colors, such as green over black.

CHIP-8 also has a 16 keys keyboard, as indicated in Using the emulator. It also has a buzzer that can play sounds on a particular and single frequency.

Talking about the interpreter, the CHIP-8 specification sets 34 instructions that can manipulate the data in memory, registers and stack, and interact with other IO devices such as sending images to the screen, reading keys from the keyboard or making the speaker buzz.

3.2 SUPERCHIP platform

SUPERCHIP is an extension over the original CHIP-8 specification that appeared in the early 90s in order to run CHIP-8 games in some graphical calculators. Thanks to the new hardware capabilities of the machine, this specification adds the following features to CHIP-8:

• A new graphical mode. By default the emulator runs in compatibility mode in order to play old games that do not support the new mode, but a new mode can be enabled that has a 128x64 monochrome display, with up to 4 times more pixels to display.
• Instructions for scrolling the screen into different directions, making faster some operations that rely on this, such as letting the user see more parts of a big screen.
• Interaction with the underlying operating system that runs the SUPERCHIP emulator. For the first time, it is possible to stop the emulation without having to close the emulator. Plus, it is possible to receive input from the operating system and send output to the operating system using a new register bank called R, whose values can be provided before running the game, and requested after finishing the game.

There are 10 new instructions that cna be used in the programs. SUPERCHIP is still compatible with the original instruction set because these new opcodes are mapped to opcodes that would trigger on a crash on regular CHIP-8 emulators. A classic ROM can still work because the original instruction set is, of course, provided.

3.3 MEGACHIP platform

This mode is not implemented yet in this emulator, although their features makes this platform very appealing.

MEGACHIP is a new format born after the improvement of personal computers. Old limitations have no place on regular devices we have today, and therefore we can have better multimedia capabilities for running games.

MEGACHIP adds support for the following features, that should be designed to be backwards compatible with CHIP-8 and SUPERCHIP formats:

• 256x192 raster resolution.
• For the first time, multiple colors using an indexed 256 color palette.
• Up to 32 MB of RAM.
• 8 bit digital sound.

There are 11 new instructions that are of course mapped to opcodes that neither CHIP-8 nor SUPERCHIP uses. Therefore, it is possible to run old CHIP-8 and SUPERCHIP games without side effects.