INT 21 - Phar Lap 386/DOS-Extender - GET SEGMENT LINEAR BASE ADDRESS
AX = 2508h
BX = segment selector
Return: CF clear if successful
ECX = linear base address of segment
CF set if invalid segment selector
Note:   this function is also supported by FlashTek X-32VM

See also: 212509

212508 - Phar Lap 386/DOS-Extender - GET SEGMENT LINEAR BASE ADDRESS