The scroll ROM sheet

What this sheet is for

Six tile ROM positions — 1B, 1C, 1E over 3B, 3C, 3E — and the six 74LS194 shift registers at 4A through 4F that serialise what they return.

Background tiles are 16 by 16, four times the area of a character, which is why the background needs as much ROM as the sprites do.

Six sockets, three chips

The drawing shows six 27128s. The photographed board fits three 27256s instead, labelled 6, 7 and 8, using the same jumper option as the sprite ROMs. Each large part covers a pair of the drawing's positions.

Both arrangements hold 96K of tiles. If you are comparing the board against the drawing and counting sockets, this is where the numbers stop matching and nothing is actually wrong.

Three planes, in parallel

The ROMs are read simultaneously, not in sequence. Each supplies one bit-plane of the pixel's colour index, so three chips read at once give three bits per pixel with no multiplexing and no extra time.

It costs more sockets and saves all the logic that reading them in turn would need. On a board where every gate was counted, that was the right trade.

Every chip on this sheet

Click any of them to open the schematic with that chip selected in both panels.