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In the Channel F, VRAM is a 128x64 pixel ( 8192 x 2 bits / 2kByte ) portion of RAM in the console, that can only be accessed through the ports. A portion of this VRAM is used as the display on the television screen. Column 125 and 126 controls the palette for the row of VRAM it is written on. The rest of the VRAM is unused by the console, it can be written to but there's no way of reading it back, there's however Schach RAM available in some cases depending on your cartridge platform.

Regarding the hardware the DRAM is usually Fairchild's own with their own markings, it's a MK4027 compatible DRAM in the first generation of machines (built-in speaker) and a MK4015N compatible DRAM in the second generation (VLSI circuits). Good to know if you're looking for spare parts in case you have odd colored bars in your display.

The portion of VRAM which is displayed starts at about column 4, row 4 and expands 102 pixels horizontally and 58 pixels vertically, giving a resolution of 102x58 pixels. This resolution is used in the MESS emulation system. However, most CRT televisions display a smaller resolution horizontally because of overscan, so the safe area to use is about 95x58 pixels. Many TV sets are adjusted wrong so that picture is offset in one or two directions. Zoomed and off center is what to normally expect, more or less.

screenshot from MESS displaying the visible and invisible portions of VRAM

screenshot of normal size, videocart 5

A screenshot from MESS displaying both the visible and invisible parts of VRAM, the other one shows a screen of normal size in approx. the same scale.

See Also