YM2151 Programming

The YM2151 is a Yamaha FM-based soundchip. The focus of this article is programming this chip with regard to the Atari 7800 XM module, though there will get information that applies to other systems that support the YM2151.

If you wish to just use the existing XMYM tracker (instead of programming the YM2151 at a low level) you can skip this article, and get a hold of the DefleMask composing software, and the dmf2asm converter, so you can make your music and convert it to XMYM compatible format.

Mapping the XM YM2151 into address space

The XM Ctrl register is used to map devices in and out of address space. Writing the value #$84 will will make the YM2151 and cartridge ROM available. If you want other hardware present, like the pokey sound chip, you'll need to modify the value appropriately. (see the official XM developer docs)

The YM2151 is now ready and active at addresses $460 and $461.

Updating Registers on the YM2151

The YM2151 can be busy with updates you've previously sent to it, so previous to any communications you need to check if it's ready with something like...

 WaitForYMReady
   bit $461
   bmi WaitForYMReady

To update the YM registers, you write the YM register location to the $460 location, and then the value you wish to write to the YM register to $461. For convenience, the following subroutine will write the A register contents to the X register location.

YM2151BASE = $460
WriteToYM_X
  ; utility function: writes A to the YM, at register location X
  ; on entry: X=YM Register, A=Value to write
WaitForYMXReady1
  bit YM2151BASE+1
  bmi WaitForYMXReady1
  stx YM2151BASE
WaitForYMXReady2
  bit YM2151BASE+1
  bmi WaitForYMXReady2
  sta YM2151BASE+1
  rts

The YM2151 Register Map

Here's the summary of the YM2151 register locations. It's important to note that these aren't locations in 6502 address space, but locations internal to the YM2151, and can only be reached through YM2151 register update mechanism.

YM2151 Registers

Location	Bits Used	Register	Register Description	Alt Reg Name	Purpose
01	%TTTTTTTT	TEST	Test	-	not normally used
08	%-SSSSCCC	SN,CN	Slot Number, Channel Number	-	Send Note-on or Note-off