80286 Processor; Real-Address Mode; Protected Operating Mode - HP Vectra Reference Manual

80286 Processor
HP Vectra PC utilizes the Intel 80286 microprocessor. The 80286
has a 24-bit address bus (a 16 Mbyte physical address range), a
16-bit data bus, and a comprehensive instruction set. The 80286
supports two addressing modes: the real-address mode and the
protected mode. These modes of operation are detailed in the
following sections.
Real-Address Mode
In real-address mode the 80286 operates as a high performance
8086. It supports 1 Mbyte of contiguous system memory using 20
address bits (AO-A19). With a few differences (which are
described in the HP Vectra Technical Reference Manual Volume 2:
System BIOS the 80286 is object code compatible with the 8088
and 8086 microprocessors. In addition, the 80286 supports
several new instructions in the real-address mode not found in the
8088 or 8086. All system software (i.e., ROM BIOS, MS-DOS
Version 3.1, etc.) currently operates only in the real-address mode.
Protected Operating Mode
The protected mode allows the use of the extended features of
the 80286. These include 1 Gigabyte of virtual address space, 16
Mbytes of physical address space, on-board memory
management, and four-level memory protection. The protected
mode is designed to support multitasking/multiuser operating
systems and highly advanced languages. When the 80286 is
operating in the protected mode, it is not object code compatible
with the 8088 and 8086 software. Several new instructions are
provided in the protected mode to control these advanced
features. These instructions are not compatible with the 8088,
8086, or the 80286 operating in the real-address mode.
Processor Board 13