The term 32-bit application came about because DOS and Microsoft Windows were originally written for the Intel 8088 and Intel 80286microprocessors. These are 16 bit microprocessors with a segmented address space. Programs with more than 64 kilobytes of code and/or data therefore had to switch between segments quite frequently. As this operation is quite time consuming in comparison to other machine operations, the application's performance may suffer. Furthermore, programming with segments is more involved than programming in a flat address space, giving rise to some complications in programming languages like "memory models" in C programming language and C plus plus programming language.
The shift from 16-bit software to 32-bit software on IBM compatible systems became possible with the introduction of the '386microprocessor. This microprocessor and its successors support a segmented address space with 16-bit and 32 bit segments (more precisely: segments with 16- or 32-bit address offset). If the base address of all 32-bit segments is set to 0, and segment registers are no more used explicitly, the segmentation can be forgotten and the processor appears as having a simple linear 32-bit address space. For compatibility reasons, however, much of the software is nevertheless written in 16-bit models.
Operating systems like Windows or OS/2 provide the possibility to run 16_bit (segmented) programs as well as 32_bit programs. The former possibility exists for backward compatibility and the latter is usually meant to be used for new software development.
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