int13A3

Int13A3 is a designation found in some retrocomputing documentation and emulator source code to refer to a particular variant or patch of the BIOS interrupt 13h disk service routines. It is not part of official IBM PC BIOS documentation, and its exact meaning can vary between projects. The term often appears in discussions of legacy disk I/O, firmware analysis, and low-level emulator development, where developers label specific code paths or patch sets for reference.

BIOS interrupt 13h provides disk input/output services to read and write sectors on floppy and hard disks. In most traditional PC architectures, software issues an interrupt with a function number in the high byte of the accumulator and uses various registers to specify drive, cylinder, head, sector, and transfer address. Because int13h has many documented functions and numerous undocumented quirks across different hardware revisions, contributors sometimes assign informal labels such as int13A3 to denote a particular implementation detail, patch, or disassembly region that corresponds to a recognizable pattern within a given project.

In practice, int13A3 commonly appears as a label in disassembly, patch notes, or emulator source to identify a specific routine that handles disk initialization, sector addressing, or a specialized read/write path. The label is primarily of interest to researchers, hobbyists, and developers working with BIOS-level disk I/O in historical software, firmware dumps, or emulation environments.

See also: BIOS interrupt 13h, INT 13h disk services, disk I/O in legacy PCs, firmware emulation. Given its informal nature, the exact definition of int13A3 should be interpreted in the context of the source where it appears.