VDG ROM FONT

Tim Lindner posted an interesting VDG Study (2022) comparing and contrasting the VDG and GIME ROM font bitmaps shown on real hardware and the MAME emulator.

Allen Huffman referred to a similar 6847t1 Mystery thread on an Acorn Atom forum which led me to the following image by “robcfg”:

That got me wondering:

“How do you get from those bits to the glyphs shown on the screen?”

Combining a careful look at the patterns in the image with information from the original 6847 datasheet, it is possible to work out each character.

From the datasheet, the 6847 display is drawn as 256 x 192 pixels using 32 x 16 tiles which makes each tile 8 x 12 pixels.

Also from the data sheet are the notes that there are 64 bitmaps in the ROM, each 5 x 7 pixels and drawn in on the tile with a vertical offset of 3 pixels from the top, 2 pixels from the bottom, 2 pixels from the left, and 1 pixel from the right.

How the VDG draws each 5 x 7 bitmap pixel (◼) within the overall 8 x 12 tile space (◻):
◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻
◻◻◼◼◼◼◼◻
◻◻◼◼◼◼◼◻
◻◻◼◼◼◼◼◻
◻◻◼◼◼◼◼◻
◻◻◼◼◼◼◼◻
◻◻◼◼◼◼◼◻
◻◻◼◼◼◼◼◻
◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻

The data sheet seems straightforward enough, but did not contain the actual original bitmaps encoded in the ROM the way that the MC6847 T1 VDG data sheet does.

With the die shot from “robcfg”, and some thought, I was able to reconstruct the “missing” font bitmap diagrams that were not in the MC6847 data sheet.

Looking at the image, there are 5 horizontal “bands” of bits.

Each horizontal band is a group of 7 individual vertical bits.

There's our 5 x 7.

Vertically, there are 32 columns of 2 bits each in pairs.

That gives us our 64 possible bit maps.

So, each vertical column of 35 bits, grouped in 5 bands of 7 bits each must be a character.

Creating a 5 x 7 box on graph paper, I started with the top band of 7 in the leftmost column of bits.

I drew that pattern of bits vertically in the rightmost column of my graph paper box.

Then I moved down the image to the second band of 7 vertical bits and drew those in the next column to the left in my graph paper box.

The third band of 7 bits were copied into the middle column of my graph paper box.

The fourth band of 7 bits going down in the image was copied to the next-to-leftmost column of my graph paper box.

The bottom band of 7 bits in the image going down was copied into the leftmost column of my graph paper box.

And there it was: the commercial “@” sign, character zero of the VDG font.

So, was I lucky or right?

Have to do the rest to find out…

◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
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◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
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◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
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◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
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◻◻◻◻◼◻◻◻ ◻◻◼◻◻◻◼◻ ◻◻◻◼◻◼◻◻ ◻◻◼◻◼◻◼◻
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◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
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◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
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◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◼◻◻◻ ◻◻◻◼◻◼◻◻ ◻◻◻◼◻◼◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◼◻◻◻ ◻◻◻◼◻◼◻◻ ◻◻◻◼◻◼◻◻
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◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◼◻◼◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◼◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◼◻◼◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻

◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◼◻◻◻ ◻◻◼◼◻◻◼◻ ◻◻◻◼◻◻◻◻ ◻◻◻◼◼◻◻◻
◻◻◻◼◼◼◼◻ ◻◻◼◼◻◻◼◻ ◻◻◼◻◼◻◻◻ ◻◻◻◼◼◻◻◻
◻◻◼◻◻◻◻◻ ◻◻◻◻◻◼◻◻ ◻◻◼◻◼◻◻◻ ◻◻◻◼◼◻◻◻
◻◻◻◼◼◼◻◻ ◻◻◻◻◼◻◻◻ ◻◻◻◼◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◼◻ ◻◻◻◼◻◻◻◻ ◻◻◼◻◼◻◼◻ ◻◻◻◻◻◻◻◻
◻◻◼◼◼◼◻◻ ◻◻◼◻◻◼◼◻ ◻◻◼◻◻◼◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◼◻◻◻ ◻◻◼◻◻◼◼◻ ◻◻◻◼◼◻◼◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻

◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◼◻◻◻ ◻◻◻◻◼◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◼◻◻◻◻ ◻◻◻◻◻◼◻◻ ◻◻◻◻◼◻◻◻ ◻◻◻◻◼◻◻◻
◻◻◼◻◻◻◻◻ ◻◻◻◻◻◻◼◻ ◻◻◻◼◼◼◻◻ ◻◻◻◻◼◻◻◻
◻◻◼◻◻◻◻◻ ◻◻◻◻◻◻◼◻ ◻◻◼◼◼◼◼◻ ◻◻◼◼◼◼◼◻
◻◻◼◻◻◻◻◻ ◻◻◻◻◻◻◼◻ ◻◻◻◼◼◼◻◻ ◻◻◻◻◼◻◻◻
◻◻◻◼◻◻◻◻ ◻◻◻◻◻◼◻◻ ◻◻◻◻◼◻◻◻ ◻◻◻◻◼◻◻◻
◻◻◻◻◼◻◻◻ ◻◻◻◻◼◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻

◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◼◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◼◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◼◻◻
◻◻◼◼◻◻◻◻ ◻◻◼◼◼◼◼◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◼◻◻◻
◻◻◼◼◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◼◻◻◻◻
◻◻◻◼◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◼◼◻◻◻◻ ◻◻◼◻◻◻◻◻
◻◻◼◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◼◼◻◻◻◻ ◻◻◼◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻

◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◼◼◻◻◻ ◻◻◻◻◼◻◻◻ ◻◻◻◼◼◼◻◻ ◻◻◻◼◼◼◻◻
◻◻◼◻◻◼◻◻ ◻◻◻◼◼◻◻◻ ◻◻◼◻◻◻◼◻ ◻◻◼◻◻◻◼◻
◻◻◼◻◻◼◻◻ ◻◻◻◻◼◻◻◻ ◻◻◻◻◻◻◼◻ ◻◻◻◻◻◻◼◻
◻◻◼◻◻◼◻◻ ◻◻◻◻◼◻◻◻ ◻◻◻◼◼◼◻◻ ◻◻◻◻◼◼◻◻
◻◻◼◻◻◼◻◻ ◻◻◻◻◼◻◻◻ ◻◻◼◻◻◻◻◻ ◻◻◻◻◻◻◼◻
◻◻◼◻◻◼◻◻ ◻◻◻◻◼◻◻◻ ◻◻◼◻◻◻◻◻ ◻◻◼◻◻◻◼◻
◻◻◻◼◼◻◻◻ ◻◻◻◼◼◼◻◻ ◻◻◼◼◼◼◼◻ ◻◻◻◼◼◼◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻

◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◼◻◻ ◻◻◼◼◼◼◼◻ ◻◻◻◼◼◼◻◻ ◻◻◼◼◼◼◼◻
◻◻◻◻◼◼◻◻ ◻◻◼◻◻◻◻◻ ◻◻◼◻◻◻◻◻ ◻◻◻◻◻◻◼◻
◻◻◻◼◻◼◻◻ ◻◻◼◼◼◼◻◻ ◻◻◼◻◻◻◻◻ ◻◻◻◻◻◼◻◻
◻◻◼◼◼◼◼◻ ◻◻◻◻◻◻◼◻ ◻◻◼◼◼◼◻◻ ◻◻◻◻◼◻◻◻
◻◻◻◻◻◼◻◻ ◻◻◻◻◻◻◼◻ ◻◻◼◻◻◻◼◻ ◻◻◻◼◻◻◻◻
◻◻◻◻◻◼◻◻ ◻◻◼◻◻◻◼◻ ◻◻◼◻◻◻◼◻ ◻◻◼◻◻◻◻◻
◻◻◻◻◻◼◻◻ ◻◻◻◼◼◼◻◻ ◻◻◻◼◼◼◻◻ ◻◻◼◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻

◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◼◼◼◻◻ ◻◻◻◼◼◼◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◼◼◻◻◻
◻◻◼◻◻◻◼◻ ◻◻◼◻◻◻◼◻ ◻◻◻◼◼◻◻◻ ◻◻◻◼◼◻◻◻
◻◻◼◻◻◻◼◻ ◻◻◼◻◻◻◼◻ ◻◻◻◼◼◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◼◼◼◻◻ ◻◻◻◼◼◼◼◻ ◻◻◻◻◻◻◻◻ ◻◻◻◼◼◻◻◻
◻◻◼◻◻◻◼◻ ◻◻◻◻◻◻◼◻ ◻◻◻◼◼◻◻◻ ◻◻◻◼◼◻◻◻
◻◻◼◻◻◻◼◻ ◻◻◻◻◻◻◼◻ ◻◻◻◼◼◻◻◻ ◻◻◻◻◼◻◻◻
◻◻◻◼◼◼◻◻ ◻◻◻◼◼◼◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◼◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻

◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◼◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◼◻◻◻◻ ◻◻◻◼◼◻◻◻
◻◻◻◻◼◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◼◻◻◻ ◻◻◼◻◻◼◻◻
◻◻◻◼◻◻◻◻ ◻◻◼◼◼◼◼◻ ◻◻◻◻◻◼◻◻ ◻◻◻◻◻◼◻◻
◻◻◼◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◼◻ ◻◻◻◻◼◻◻◻
◻◻◻◼◻◻◻◻ ◻◻◼◼◼◼◼◻ ◻◻◻◻◻◼◻◻ ◻◻◻◻◼◻◻◻
◻◻◻◻◼◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◼◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◼◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◼◻◻◻◻ ◻◻◻◻◼◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻
◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻ ◻◻◻◻◻◻◻◻

Along the way of doing this, I found that both datasheets for the MC6847 and the MC6847T1 have a printing error in them.

There is a figure that illustrates inverse lit and unlit pixels using a bitmap of the tile like those show above.

But, the bitmap for the capital B they used for the figure does not match the capital B bitmap in the ROM.

Oops.

My personal feeling is this might have been the original ROM bitmap for the B, but then there was a change to add serifs for clarity on 1970's televisions and that change did not trigger drawing a new illustration.

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