INT 2F - SRSoft MODAL PCIBM PC v2+ - ALLOCATE COMMON MEMORY
	AX = AB03h
	BX = required size of common memory for variables/semaphores in bytes
Return: AX = 5253h ('SR' byte-swapped) if installed
SeeAlso: AX=AB00h"MODAL",AX=AB02h"MODAL",AX=AB04h"MODAL"