art@ACC.ARPA (01/23/86)
> Does anyone out there know 1) how many mapping registers there are and > 2) how wide they are? I have a suspicion that they are 15 bits wide > (24-9), but don't know for sure, nor do I have any idea how many are > available. The uVAX-II has 8192 map regs to map the entire 4 meg (22 bit) QBUS address space. Each map reg is a longword (32 bits) and has the generic map reg format (i.e. high bit = valid, low bits = PFN). Current memory limits may preclude more than about 15 bits being useful. The map regs begin at %X20088000 in uVAX-II physical address space. Because the VMS data structures and routines used to deal with UNIBUS only accomodated 496 map regs, VMS currently treats the QBUS as an 18 bit bus and only uses the first 496 QBUS map regs. The uVAX-I has no bus adapter between the QBUS and memory, and thus no map regs. Therefore uVAX-I QBUS devices have to DMA directly to/from physical pages (which will not be contiguous unless special precautions are taken by the kernel). <Art@ACC.ARPA> ------