Hello. I'm trying to figure out how it is possible to display a single pixel anywhere on the screen. Do you write a value to a specific address? If you could do that in Easy6502, why can't you do it when writing code for an nes game?

In brief, there are many different ways of doing video. Mostly, RAM was rather expensive compared to the desired resolution, so you have more than one pixel controlled by a byte. But also, RAM is rather slow compared to video, so if one byte controlled one pixel, you'd need a great number of RAM accesses to fetch the video data.

So, it's more normal to see video modes where one byte controls 2, 4 or even 8 pixels. Or, in the case of a character-mode display, in a sense one byte controls 64 pixels.

Easy6502 is a simple model, and is written as software, so it can present a more user-friendly interface.

Also, keep in mind the NES only has 2K of RAM by default. Some carts added more RAM but as a general rule, you're working with a 2K system. In order to have single pixel access (especially in multiple colors), 2K of RAM would only support very small resolutions. Which is why the NES (and many 8-bit computers) worked in tiles (chars) and even meta-tiles (collections of tiles).

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Also, the 1.79MHz 6502 speed is way too slow to draw NES style games on a framebuffer. If you were drawing a pixel at a time, you'd be getting seconds per frame instead of frames per second, which is not conducive to arcadey gameplay.

Consider the PPU to be a hardware decompressor from a compressed dictionary of tile/sprite/palette definitions into a streaming full-color video signal. This dictionary is the language that those sorts of games speak naturally, setting a byte (for instance) to change the position of a sprite, instead of all the effort of redrawing it in software pixel by pixel. Anything you wish to display needs to speak that language.

The easiest way to have a single byte show something on such a display is to change a tile, not to change a pixel.

If you want literally 1 pixel to show anywhere on the screen, define a sprite with 1 visible pixel, and position it by setting its X and Y coordinates.

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AcheronVM: A Reconfigurable 16-bit Virtual CPU for the 6502 Microprocessor