A 32-bit application is software that runs in a 32-bit flat address space (a flat memory model).
In computer architecture, 32-bit integers, memory addresses, or other data units are those that are at most 32 bits (4 octets) wide. Also, 32-bit CPU and ALU architectures are those that are based on registers, address buses, or data buses of that size. 32-bit is also a term given to a generation of computers in which 32-bit processors were the norm.
The term 32-bit application came about because DOS and Microsoft Windows were originally written for the Intel 8088 and Intel 80286 microprocessors. 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 and C++.
The shift from 16-bit software to 32-bit software on IBM compatible systems became possible with the introduction of the '386 microprocessor. 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 not 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.
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