Private virtual memory

In the full-protection model, each process is given its own private virtual memory, which spans to 2 or 3.5 gigabytes (depending on the CPU). This is accomplished by using the CPU's MMU. The performance cost for a process switch and a message pass will increase due to the increased complexity of obtaining addressability between two completely private address spaces.

Note: Private memory space starts at 0 on x86, SH-4, ARM, and MIPS processors, but not on the PowerPC, where the space from 0 to 1 GB is reserved for system processes.
Figure 1. Full protection VM (on an x86).

The memory cost per process may increase by 4 KB to 8 KB for each process's page tables. Note that this memory model supports the POSIX fork() call.