The character generator sheet

▸This sheet is part ofHow a frame is drawn

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

What this sheet is for

The previous sheet decided which character sits at this position on screen. This sheet turns that number into actual dots.

5D holds the shapes: every digit of the score, every letter, the tiles that spell out COMMANDO. It is addressed by the character number plus the row within the character, which 4E remembers and 4D adjusts for vertical flip.

The pixels come out eight at a time and have to leave one at a time, so they are loaded into the '194 shift registers at 4F and 5F and clocked out at the 6MHz dot rate by 5H. Two chips because the text layer is two bits deep: four colours, one of which is transparent.

Fetch eight pixels at once, clock them out one per dot, and pick which end to read from if the character is mirrored. The sprite and background boards do exactly the same thing with more chips and more bits.

Mirroring for free, again

A '194 can shift in either direction, and 3H chooses which end of the pair to listen to. That is the entire implementation of horizontal mirroring for the text layer, and it is the same trick the sprite board uses at 10K and the background board at 7C.

The alternative would have been storing a second, mirrored copy of every character in ROM. Capcom did that nowhere in this machine. 4C combines a character's own flip flags with the whole-cabinet flip so a mirrored letter on an upside-down screen still comes out right, which is the kind of detail that is easy to get wrong and very obvious when it is.

Every chip on this sheet

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

← open the schematic for this sheet