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This article describes the calling conventions used when programming x86 architecture microprocessors.

Calling conventions describe the interface of called code:

This is intimately related with the assignment of sizes and formats to programming-language types. Another closely related topic is name mangling, which determines how symbol names in the code map to symbol names used by the linker. Calling conventions, type representations, and name mangling are all part of what is known as an application binary interface (ABI).

There are often subtle differences in how various compilers implement these conventions, so it is often difficult to interface code which is compiled by different compilers. On the other hand, conventions which are used as an API standard (such as stdcall) are very uniformly implemented.

Prior to microcomputers, the machine manufacturer generally provided an operating system and compilers for several programming languages. The calling convention(s) for each platform were those defined by the manufacturer's programming tools.

Early microcomputers before the Commodore Pet and Apple II generally came without an OS or compilers. The IBM PC came with Microsoft's fore-runner to Windows, the Disk Operating System (DOS), but it did not come with a compiler. The only hardware standard for IBM PC-compatible machines was defined by the Intel processors (8086, 80386) and the literal hardware IBM shipped. Hardware extensions and all software standards (save for a BIOS calling convention) were thrown open to market competition.

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