A unique feature of the mainframe is its processor architecture.
On the mainframe, you can address up to 2G in the 31-bit architecture. This is less than on other architectures. However, you do not have the kernel included in the user address space as you might have on other architectures (see Figure 20-1).
Figure 20-1. On the mainframe architecture, with 31-bit addressing, the kernel is in its own address space
The separation of the kernel from the user space results in additional security for the kernel. It is impossible for the user to access the kernel address space other than by system calls.