Devlog 3.

The emulator is almost completed and and can run .ch8 binaries

not fully complete it still requires some polishing like right now the graphics is flickering a bit, probably due to a bug in graphics engine logic or I might need to change how fast cycles are being executed.

Checkout my previous devlogs to know what is chip8?

What this time?

This time I felt like nobody in the world can explain the logic to translate those opCodes. Coming from high level low level is absolute new, i thought I might find some resemblance (guess what?? I was wrong)

I still can’t say in understand it 100%, but the struggle is on

Here’s what I learnt and built.

we load the ch8 binary file -> Save instruction in the RAM -> a continuous while loop which executes each instruction code

How does the program even runs?

• Basically there is a while loop, which keeps on running unless user closes the program or program ends
• This is where we write logic to
• find which instruction code to execute next
• Control the execution speed (clock speed of cpu)
• Map keyboard events (more on this below)

How do we know what code to run next?

• There is a function executeCycle, inside this funciotn we have -
• A variable programCounter which tracks the current instruction code position and calculates next one too.
• Then we have a huge switch case flow, which basically maps all the 35 standard instruction codes form chip8 to modern computer.
• Then based upon the code, we perform tasks like painting a pixel or change some data in RAM.

To keep this short, I will not get into the details about translating the chip8 instruction.

Biggest man in the room - Graphics Engine (XOR)

• we basically have black and white screen, 64px length and 32px height (we use multiplier so the size is not exactly 64x32, as that will be two small)
• we then have 1s, 0s to represent black and white pixels
• the engine basically deleted every pixel and re draws it again, this causes a flashing effect as moder screens are faster and also show the transition.

Collision detection

• the value of a pixel is set to 1 (white) and 0 (black)
• We take XOR of the Current value of the pixel and the to be updated value
• if two pixels are white (1) the xor will be 0, and when this happens we set the Vf to 1, indicating a collision has occured.
• Vf is the 16th register which is left empty and use only to show collision.

Keypad

keypad

The original chip8 keypad was like this

Chip8                                             Modern keyboard
1   2   3   C					1    2   3   4
4  5   6   D	        ---> 	        Q  W  E   R
7  8   9   E					A   S   D   F
A  0   B   F					Z   X   C   V

What’s left

1. implement UI to control chip 8 settings like speed, screen size etc.
2. Add a list of software that user can run, as well as option to run his own