https://github.com/digitsensitive/chip8

https://github.com/digitsensitive/chip8

Last synced: 8 months ago
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• Host: GitHub
• URL: https://github.com/digitsensitive/chip8
• Owner: digitsensitive
• Created: 2020-08-16T06:53:59.000Z (over 5 years ago)
• Default Branch: master
• Last Pushed: 2020-08-23T12:34:47.000Z (over 5 years ago)
• Last Synced: 2025-02-14T08:47:10.495Z (10 months ago)
• Language: C++
• Size: 19.5 KB
• Stars: 1
• Watchers: 2
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md

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README

          
# CHIP-8

## Systems memory map

- 0x000 - 0x1FF: CHIP-8 interpreter (contains font set in emu)

- 0x200 - 0xFFF: Program ROM and work RAM

Details:

- 0x050 - 0x0A0: Used for the built in 4x5 pixel font set (0-F)

- 0xEA0 - 0xEFF: Reserved for the call stack, internal use, and other variables

- 0xF00 - 0xFFF: Reserved for display refresh

## Memory

- Most commonly implemented on 4K systems (f.e. Cosmac VIP, Telmac 1800)

- These machines had 4096 (0x1000) memory locations

- Each location with 8 bits (a byte)

  --> 4096 * 8 = 32768 bits = 4096 byte = 

- The CHIP-8 interpreter itself occupies the first 512 bytes of the memory space

- Most programs written for the original system begin at memory location 512 (0x200)

- The uppermost 256 bytes (0xF00-0xFFF) are reserved for display refresh

- The 96 bytes below that (0xEA0-0xEFF) were reserved for the call stack, internal use, and other variables.

- In modern CHIP-8 implementations, where the interpreter is running natively 

outside the 4K memory space, there is no need to avoid the lower 512 bytes 

of memory (0x000-0x200), and it is common to store font data there.

## Registers

- 16 * 8-bit data registers named V0 to VF.

- The VF register doubles as a flag for some instructions; thus, it should be avoided.

- In an addition operation, VF is the carry flag, while in subtraction, it is 

  the "no borrow" flag. In the draw instruction VF is set upon pixel collision.

- The address register, which is named I, is 16 bits wide and is used with 

  several opcodes that involve memory operations.

## Stack

- The stack is only used to store return addresses when subroutines are called

- The original RCA 1802 version allocated 48 bytes for up to 12 levels of nesting

- Modern implementations usually have more

## Timers

- Two timers

- They both count down at 60 hertz, until they reach 0.

- Delay timer: This timer is intended to be used for timing the events of games. 

  Its value can be set and read.

- Sound timer: This timer is used for sound effects. When its value is nonzero, 

  a beeping sound is made.

## Input

- Input is done with a hex keyboard that has 16 keys ranging 0 to F

- The '8', '4', '6', and '2' keys are typically used for directional input

- Three opcodes are used to detect input

  - One skips an instruction if a specific key is pressed, 

  - while another does the same if a specific key is not pressed

  - The third waits for a key press, and then stores it in one of the data registers.

## Graphics and sound

- Original CHIP-8 Display resolution is 64×32 pixels (= 2048) 

  and color is monochrome (= black and white)

- Graphics are drawn to the screen solely by drawing sprites,

  which are 8 pixels wide and may be from 1 to 15 pixels in height

- Sprite pixels are XOR'd with corresponding screen pixels. In other words, 

  sprite pixels that are set flip the color of the corresponding screen pixel,

  while unset sprite pixels do nothing

- The carry flag (VF) is set to 1 if any screen pixels are flipped from set to 

  unset when a sprite is drawn and set to 0 otherwise.

  This is used for collision detection.

As previously described, a beeping sound is played when the value of the sound timer is nonzero.

## Opcode table

- 35 / each two bytes long and stored big-endian

- Big-endian is an order in which the "big end" 

  (most significant value in the sequence) is stored first 

  (at the lowest storage address).

- Use on of these types:

    unsigned short

    unsigned short int

    uint16_t

    u_int16_t

  -->	0 to 65,535

- NNN: address

- NN: 8-bit constant

- N: 4-bit constant

- X and Y: 4-bit register identifier

- PC : Program Counter

- I : 16bit register (For memory address) (Similar to void pointer)

- VN: One of the 16 available variables. N may be 0 to F (hexadecimal)