B board · sheet 8/9 · manual page 18

The scroll video RAM sheet

The tilemap itself: which tile sits where, what colour it uses, and whether it is allowed in front of the sprites.

Draft. This sheet was read from the original 1985 drawing and cross-checked against MAME, but it has not yet been walked pin by pin against a physical board. The logic described here is what the drawing shows.
▸This sheet is part ofHow the background scrolls

The subsystem followed end to end, across every sheet that builds it.

What this sheet is for

7A, an M58725, holds the map. The multiplexers at 9A, 9B and 9C share it between the CPU and the video hardware, exactly as the character RAM sheet does on the other board.

The same problem produced the same solution twice, on two different boards, by the same engineers. Worth noticing.

What a map entry holds

Each entry is a tile number plus attributes: which colour set to use, whether to flip the tile, and the priority bit.

That priority bit is SCRWIN, and it leaves this sheet on the band cable to reach the colour mixer on the CPU board. It is the single most consequential bit in the background subsystem, and it is why this machine's scenery can be in front of the player.

Every chip on this sheet

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

1J74LS325A74LS2736A74LS2736B74LS1747AM587259A74LS1579B74LS1579C74LS15710A74LS24510B74LS28310D74LS27311A74LS27311B74LS28311C74LS8614A74LS74
← open the schematic for this sheet