Demand Paging MS-DOS-Based Applications (47125)

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Part 1: Windows 3.0, and Windows 3.1 Without 32-Bit Disk Access

Under enhanced mode Windows version 3.0, it is not possible to allow MS-DOS-Based applications to use virtual (demand paged) memory (VM) when they are active (they are the foreground application, or have the background execution option set). This is because the paging mechanisms of the system use either:

• MS-DOS plus an MS-DOS device plus INT13
• Just INT13

In other words, to access the paging file (virtual memory that is currently demand-paged out) Windows must call code that resides in the VM along with the application (BIOS, MS-DOS, and so on).

The problem this causes is best illustrated by an example:

1. Run an MS-DOS-based application that hooks INT 13h.
2. Arrange to "page out" the page that has part of the application's INT 13h hook in it.
3. Touch a "not present" page.
   
   
   1. This generates a page fault.
   2. The system calls "page this page in."
   3. This calls INT 13h.
   4. This page faults because the INT 13h hook of the application is currently paged out.
   5. The system calls "page this page in."
   6. This calls INT 13h.
   7. This page faults because the INT 13h hook of the application is currently paged out.

This continues forever, causing a deadlock situation, and the system halts.

To prevent this problem, make sure every page of an MS-DOS-based application is always present when the application's VM is active. Thus, MS-DOS-based application VMs are always present, and therefore, do not use demand-paged memory except when they are not active.

Given the way that system paging works, there is no way to work around this limitation, partly because the installed MS-DOS-based application base is so diverse. Note also that this problem with INT 13 is only part of the picture. Almost all hardware interrupts can cause exactly the same problem.

Part 2: Windows version 3.1 with 32-Bit Disk Assess

One solution is to change the way system paging works so that to access the paging file, the system paging does not have to use any code that resides in the VM with the application. 32-bit disk access for Windows version 3.1 performs this type of functionality.

32-bit disk access addresses the problem that prevents the demand paging of MS-DOS-based application VMs. 32-bit disk access allows the paging component of the system to access the paging file without having to access anything that resides in the VM (the application, MS-DOS, the BIOS, and so on). This prevents the paging deadlock problem discussed above because all of the code and data needed to access the paging file is inside enhanced mode Windows and is designed such that access to the paging file will never cause a page fault to occur.

As a side benefit, this is also much faster than calling MS-DOS or some other component in the VM because calling VM code requires that the 80386 CPU change modes between protected mode and virtual mode. The transitions between CPU modes are quite expensive in terms of CPU cycles. Therefore, the other benefit of 32-bit disk access is that there is an improvement in "paging throughput" because the "CPU mode transitions" required by access to the paging file through MS-DOS or INT 13h are eliminated.