bcw (12/13/82)
From: Bruce C. Wright @ Duke University Re: PDP-11 architecture It depends on how you count it whether the PDP-11 architecture has a 64 KB address space. It is at least arguable that the address space is 192 KB, and it is also arguable whether this makes much difference anyway. First of all, you have to consider the fact that PDP-11's (at least of the large variety like the PDP-11/70 and presumably the Micro-J/11) have separate Instruction and Data (I/D) space, each of which can be 64 KB long. Also, the larger PDP-11's have Supervisor space, into which library subroutines can be placed; this is also 64 KB long (64 KB * 3 = 192 KB). Secondly, it is quite possible to use (and at least some PDP-11 operating systems allow) more data space than 64 KB, by the use of "virtual" arrays and similar objects. This means that you can access segments of the array immediately, and make a fast system call to switch the mapping to other parts of the array. Some language processors support this transparantly to the user - I know that at least DEC's Fortran under RT-11 or RSX will do so, and I think that there are others. This does imply some overhead - analogous to paging overhead in a paging machine - which may be too much for some environments, but for most uses of *personal* computers, this is quite satisfactory. Probably the only cases where this overhead is really critical are things like medical imaging, heavy-duty process or experiment control, and the like (though I know of at least some medical imaging systems which are able to make do with PDP-11 architecture ...) Bruce C. Wright @ Duke University