Windows 3.11 Temporary vs Permanent Swap Files

In Microsoft Windows 3.11, virtual memory was managed using either temporary or permanent swap files to simulate additional RAM via hard drive space. The main differences between the two lie in how disk space was allocated, how the operating system accessed the hard drive, and the overall performance impact on the system.

Disk Space Allocation and Persistence

A permanent swap file consisted of a hidden, contiguous block of disk space that was pre-allocated on an uncompressed drive. It created a hidden file named 386SPART.PAR alongside a pointer file named SPART.PAR. Because the space had to be completely contiguous, Windows 3.11 required users to defragment their drive prior to creating it. Once configured, this file permanently occupied hard drive space, remaining on the disk even after exiting Windows to MS-DOS.

In contrast, a temporary swap file (named WIN386.SWP) was created dynamically only when Windows 3.11 started in 386 Enhanced Mode and was automatically deleted when the user shut down Windows. It did not require contiguous clusters and could be broken up across fragmented sectors of the disk, making it easier to set up on drives with limited free space.

Performance and 32-Bit Disk Access

The primary technical advantage of a permanent swap file was performance. Because its physical location on the disk was fixed and contiguous, Windows 3.11 could bypass MS-DOS and the system BIOS entirely. This enabled "32-Bit Disk Access" (via FastDisk technology), allowing protected-mode drivers to communicate directly with supported Western Digital-compatible disk controllers. Bypassing real-mode DOS interrupts significantly reduced latency and kept the CPU in protected mode during memory swaps.

A temporary swap file could not use 32-Bit Disk Access. Because the file could be fragmented across different sectors, Windows had to rely on standard MS-DOS file system routines and BIOS interrupts (INT 13h) to read and write data. This process forced the processor to continually switch between protected mode and real mode, introducing substantial performance overhead.

Drive Compatibility

Permanent swap files had strict drive requirements: they could only reside on a local, uncompressed FAT partition. They could not be placed on network drives, RAM disks, or drives compressed with utilities like DoubleSpace or Stacker. Temporary swap files offered greater flexibility and could be used on compressed drives, though they still could not reside on read-only or network media.