Bitfields for Intel 82371FB/82371SB top of memory register:
Bit(s)	Description	)
 7-4	top of ISA memory (in megabytes, less 1; i.e. 0001 = 2M)
 3	ISA/DMA lower BIOS forwarding enable
 2	(82371SB) enable A000/B000 segment forwarding to PCI bus
 1	enable forwarding ISA/DMA 512K-640K region to PCI bus
 0	reserved
SeeAlso: #01167,#01217