Protected Mode - IBM 5170 Technical Reference

not use the full 64Kb, the unused end of the segment may be
overlayed by another segment to reduce physical memory
requirements.
Protected Mode
The protected mode offers extended physical and virtual memory
address space, memory protection mechanisms, and new
operations to support operating systems and virtual memory.
The protected mode provides a I-gigabyte virtual address space
per task mapped into a 16-megabyte physical address space. The
virtual address space may be larger than the physical address
space, because any use of an address that does not map to a
physical memory location will cause a restartable exception.
As in the real-address mode, the protected mode uses 32-bit
pointers, consisting of 16-bit selector and offset components.
The selector, however, specifies an index into a memory resident
table rather than the upper 16 bits of a real memory address. The
24-bit base address of the desired segment is obtained from the
tables in memory. The 16-bit offset is added to the segment base
address to form the physical address. The tables are
automatically referenced by the microprocessor whenever a
segment register is loaded with a selector. All instructions that
load a segment register will refer to the memory based-tables
without additional program support. The memory-based tables
contain 8-byte values called descriptors.
Following is a block diagram of the system board.
System Board 1-5