The sound sheet

▸This sheet is part ofHow the sound works

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

What this sheet is for

The second Z80 sits at 11F, running at 3MHz like the main one, with its own program ROM at 9F and 2K of RAM at 8F. Its address space is completely separate: it cannot see the game's memory and the game cannot see its.

The only connection is the '374 at 8J. The main CPU writes a command number to C800, the latch holds it, and the sound CPU reads it at 6000 whenever it next looks. One byte, one direction, no acknowledgement. That is why a sound fault can never hang the game.

10E decodes what the sound CPU is reaching for and 6F joins two of its outputs so the program ROM answers across its full range - the same active-low AND trick used on the CPU sheet.

A separate address space with no window onto the game's memory at all. MAME's sound CPU map has the same four regions.

Two chips that are really four instruments

9C and 11C are Yamaha YM2203s, clocked at 1.5MHz, the crystal divided by eight. 7C picks which one is being addressed and whether the CPU is naming a register or handing over a value; 9E finishes that into a chip select for each.

Each YM2203 is two instruments in one package: three FM channels and three square-wave PSG channels. Two chips give six of each, which is why the Commando theme can carry a melody with real timbre over a hard percussive backing.

The sound CPU is interrupted periodically to keep the music in tempo - 11E holds that interrupt until the code answers it - and 8D and 10J stretch its clock briefly when it writes to the sound chips, because the YM2203 needs a moment to digest a write. 8E hands the whole section its clock, its interrupt and its reset from the rest of the board.

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