Intel 80386 manual Virtual 8086 Environment, Executing 8086 Programs, Paging In Virtual Mode

80386

U/S	W/R	Access Type
0	0	Supervisor'Read
0	1	Supervisor Write
1	0	User Read
1	1	User Write

'Descriptortable access will fault with u/s 0, even lithe program

is executing at level 3.

Figure 4-238. Type of Access

Causing Page Fault

ing system and both 80286 and 80386 applications. Thus, in a multi-user 80386 computer, one person could be running an MS-DOS spreadsheet, another person using MS-DOS, and a third person could be running multiple Unix utilities and applications. Each person in this scenario would believe that he had the computer completely to himself. Figure 4-24 Illus- trates this concept.

4.6.2Virtual 8086 Mode Addressing Mechanism

4.5.6Operating System Responsibilities

The 80386 takes care of the page address transla- tion process, relieving the burden from an operating system in a demand-paged system. The operating system is responsible for setting up the initial page tables, and handling any page faults. The operating system also is required to invalidate (i.e. flush) the TLB when any changes are made tD any of the page table entries. The operating system must reload CR3 to cause the TLB to be flushed.

Setting up the tables is simply a matter of loading CR3 with the address of the Page Directory, and allocating space for the Page Directory and the Page Tables. The primary responsibility of th~ oper- ating system is to implement a swapping policy and handle all of the page faults.

A final concern of the operating system is to ensure that the TLB cache matches the information in the paging tables. In particular, any time the operating system sets the P present bit of page table entry to zero, the TLB must be flushed. Operating systems may want to take advantage of the fact that CR3 is stored as part of a TSS, to give every task or group of tasks its own set of page tables.

4.6 VIRTUAL 8086 ENVIRONMENT

4.6.1 Executing 8086 Programs

The 80386 allows the execution of 8086 application programs in both Real Mode and in the Virtual 8086 Mode (Virtual Mode). Of the two methods, Virtual 8086 Mode offers the system designer the most flexibility. The Virtual 8086 Mode allows the execu- tion of 8086 applications, while still allowing the sys- tem designer to take full advantage of the 80386 protection mechanism. In particular, the 80386 al- lows the simultaneous execution of 8086 operating systems and its applications, and an 80386 operat-

One of the major differences between 80386 Real and Protected modes is how the segment selectors are interpreted. When the processor is executing in Virtual 8086 Mode the segment registers are used in an identical fashion to Real Mode. The contents of the segment register is shifted left 4 bits and added to the offset to form the segment base linear ad- dress.

The 80386 allows the operating system to specify which programs use the 8086 style address mecha- nism, and which programs use Protected Mode ad- dressing, on a per task basis. Through the use of paging, the one megabyte address space of the Vir- tual Mode task can be mapped to anywhere in the 4 gigabyte linear address space ?f the 80386. Like Real Mode, Virtual Mode effeclive addresses (I.e., segment offsets) that exceed 64K byte will cause an exception 13. However, these restrictions should ~ot prove to be important, because most tasks running in Virtual 8086 Mode will simply be existing 8086 application programs.

4.6.3 Paging In Virtual Mode

The paging hardware allows the concurrent running of multiple Virtual Mode tasks, and provides protec- tion and operating system isolation. Although it is not strictly necessary to have the paging hardware enabled to run Virtual Mode tasks, it is needed In order to run multiple Virtual Mode tasks or to relo- cate the address space of a Virtual Mode task to physical address space greater than one megabyte.

The paging hardware allows the 20-bit linear ad- dress produced by a Virtual Mode program to be divided into up to 256 pages. Each one of the pages can be located anywhere within the maximum 4 giga- byte physical address space of the 80386. In ~ddi­ tion, since CR3 (the Page Directory Base Register) is loaded by a task switch, each Virtual Mode task can use a different mapping scheme to map pages to different physical locations. Finally, the paging hardware allows the sharing of the 8086 operating system code between multiple 8086 applications.

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