Intel 80287, 80286 manual Modes of Operation, Advanced Features, Memory Management

INTRODUCTION TO THE 80286

1.2 MODES OF OPERATION

The 80286 can be operated in either of two different modes: Real Address Mode or Protected Virtual Address Mode (also referred to as Protected Mode). In either mode of operation, the 80286 represents an upwardly compatible addition to the 8086 family of processors.

In Real Address Mode, the 80286 operates essentially as a very high-performance 8086. Programs written for the 8086 or the 80186 can be executed in this mode without any modification (the few exceptions are described in Appendix C, "Compatibility Considerations"). Such upward compatibility extends even to the object code level; for example, an 8086 program stored in read-only memory will execute successfully in 80286 Real Address Mode. An 80286 operating in Real Address Mode provides a number of instructions not found on the 8086. These additional instructions, also present with the 80186, allow for efficient subroutine linkage, parameter validation, index calculations, and block 1/0 transfers.

The advanced architectural features and full capabilities of the 80286 are realized in its native Protected Mode. Among these features are sophisticated mechanisms to support data protection, system integ- rity, task concurrency, and memory management, including virtual storage. Nevertheless, even in Protected Mode, the 80286 remains upwardly compatible with most 8086 and 80186 application programs. Most 8086 applications programs can be re-compiled or re-assembled and executed on the 80286 in Protected Mode.

1.3 ADVANCED FEATURES

The architectural features described in section 1.1 of this chaper are common to both operating modes of the processor. In addition to these common features, Protected Mode provides a number of advanced features, including a greatly extended physical and logical address space, new instructions, and support for additional hardware-recognized data structures. The Protected Mode 80286 includes a sophisti- cated memory management and multilevel protection mechanism. Full hardware support is included for multitasking and task switching operations.

1.3.1 Memory Management

The memory architecture of the Protected Mode 80286 represents a significant advance over that of the 8086. The physical address space has been increased froml megabyte to 16 megabytes (224 byies), while the virtual address space (i.e., the address space visible to a program) has been increased from 1 megabyte to 1 gigabyte (230 bytes). Moreover, separate virtual address spaces are provided for each task in a multi-tasking system (see the next section, 1.3.2, "Task Management").

The 80286 supports on-chip memory management instead of relying on an external memory manage- ment unit. The one-chip solution is preferable because no software is required to manage an external memory management unit, performance is much better, and hardware designs are significantly simpler.

Mechanisms have been included in the 80286 architecture to allow the efficient implementation of virtual memory systems. (In virtual memory systems, the user regards the combination of main and external storage as a single large memory. The user can write large programs without worrying about the physical memory limitations of the system. To accomplish this, the operating system places some of the user programs and data in external storage and brings them into main memory only as they are needed.) All instructions that can cause a segment-riot-present fault are fully restartable. Thus, a not- present segment can be loaded from external storage, and the task can be restarted at the point where the fault occurred.

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