MEM A000h:8504h - S3 ViRGE - MEMORY-MAPPED SUBSYSTEM REGISTERS                  
Size:	12 BYTEs
Note:	the S3 graphics processor registers can be mapped at either
	  linear 000A0000h or at offset 16M from the start of the linear
	  frame buffer


Format of S3 memory-mapped subsystem registers:
Offset	Size	Description	(Table M0071)
8504h	DWORDDoubleword; four bytes.	 Commonly used to hold a 32-bit segment:offset or selector:offset address.	subsystem Control/Status Register (see PORTIBM PC Portable (uses same BIOS as XT) 42E8h,PORTIBM PC Portable (uses same BIOS as XT) 9AE8h)
		on read:
		    bit 13 indicates whether graphics processor is busy
		    bits 12-8 indicate number of free FIFO slots
8508h	DWORDDoubleword; four bytes.	 Commonly used to hold a 32-bit segment:offset or selector:offset address.	???
850Ch	DWORDDoubleword; four bytes.	 Commonly used to hold a 32-bit segment:offset or selector:offset address.	advanced function control (see PORTIBM PC Portable (uses same BIOS as XT) 4AE8h)
SeeAlso: #M0073,#M0057,#M0072