Virtual Memory: Introduction

Paging and/or segmentation allows us to use non-contiguous memory but requires special hardware

Relative sizes of logical and physical memory

One question that is of interest is the relative size of the two address spaces. For the schemes we have discussed thus far, it would necessarily have to be the case that

(Recall that an attempt to use a logical address for which there is no corresponding physical address leads to abortion of the process.) This means that several entire processes can be resident in physical memory at the same time. (Recall the example of the PDP-11/70: logical space is 64K due to the 16-bit architecture of the CPU; but physical space can be as big as 4 Meg since physical addresses are 22 bits long.