art@ACC.ARPA (01/27/86)
>> 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 > >What? If physical memory is limited to 4 meg, why is DEC selling >8 meg boards for these machines? > > -neville The uVAX-II has a memory bus that is separate from the QBUS. The QBUS is limited to 4Mbytes, but the memory bus has a larger address space (I haven't looked at the schematics to find out exactly how large). I know DEC supports a 9Mbyte configuration. The 8192 QBUS map regs are used to map the 4Megabyte (8192 pages) of QBUS into the larger physical address space on the memory bus. BTW. DEC is supposed to announce some new VAXen this week! Art@ACC.ARPA ------