Bitfields for feature byte 4:
Bit(s)	Description	)
 7	IBM "private" (set on N51SX, CL57SX)
 6	system has EEPROM
 5-3	ABIOS presence
	001 not supported
	010 supported in ROM
	011 supported in RAM (must be loaded)
 2	"private"
 1	system supports memory split at/above 16M
 0	POSTEXT directly supported by POST
SeeAlso: #00509,#00512,#00514