Protected (Virtual Address) Mode - IBM AT 5170 Technical Reference

All segments in the real address mode are 64K in size and may be
read, written, or executed. An exception or interrupt can occur if
data operands or instructions attempt to wrap around the end of a
segment. For example, a word with its low-order byte at offset
FFFF and its high-order byte at 0000. If, in the real address
mode, the information contained in the segment does not use the
full 64K, the unused end of the segment may be overlayed by
another segment to reduce physical memory requirements.
Protected (Virtual Address) 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.
Note: See "BIOS Programming Hints" in Section 5 for
special cautions while operating in the protected mode.
The protected mode provides a 1 G virtual address space for each
task mapped into a 16M 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 microprocessor
automatically refers to the tables 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.
System Board 1-5