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Retro6K ROM Cells by vidthekid Retro6K ROM Cells by vidthekid
I'm designing a retro-style video display system that has 6144 bytes of video RAM and employs a few tricks common in early-80s consumer electronics. It builds a 256×144 pixel screen (16:9 with square pixels or 4:3 with oblong pixels) using a 32×18 array of cells which are 8×8 pixels each. Each cell on the screen is one of 256 available cell prototypes, which may contain up to four colors; these colors can be selected independently for each screen cell, from a software-selected palette of 32 colors. Those 32 colors are selected from 256 fixed, hardware-defined colors.

Although software is free to provide its own prototype cells by writing to video memory, software size can be reduced by having a ROM in the system containing default prototype cells useful for general purposes. What we see here is the contents of that ROM chip.

The first 100 cells are normal text characters. For plain text, assign the cell colors like this: desired background color, background color, background color, foreground color. For bold text: background, background, foreground, foreground. For text with shadow: background, shadow, shadow, foreground. For bold text with shadow: background, shadow, foreground, foreground. Software can use these cells in more creative ways, potentially.

The next 28 cells are for drawing large letters two columns wide and three rows tall, with smoothed edges. Color assignment would be background, intermediate, intermediate, foreground, where the intermediate colors are visually between the foreground and background colors. Several characters from the next block of cell prototypes are required for this purpose as well, though their color assignments will consist of only the desired foreground and background colors in arrangements which vary depending on the cell being used.

The next 48 cells are for drawing various partial blocks and patterns. Then there's a cell for squares of various sizes or outlined/nested squares, and a cell for circles of various sizes or outlined/nested circles.

The last 78 cells are line-drawing and box-drawing elements, with some support for boxes filled with color.
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Submitted on
August 14, 2018
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