Games - INT 13

	Games - INT 13

INT 13 is shorthand for the 13th interrupt vector in an x86 based computer system. INT is an x86 instruction that triggers a software interrupt, and 13 is the vector passed to the instruction. The BIOS typically sets up a real mode interrupt handler at this vector that provides sector based hard disk and floppy disk read and write services using Cylinder Head Sector (CHS) addressing.

Overview

Under older operating systems, such as MS-DOS and Windows 9x, calling INT 13 will jump into the computer's BIOS code, which will carry out the read or write for the program. Newer operating systems that have DOS emulation, such as Microsoft Windows NT derivatives (e.g. NT4, 2000 XP and 2003 Server) or GNU/Linux with dosemu, will intercept the call and pass it to the operating system's native disk I/O mechanism.

The original BIOS real-mode INT 13 interface supports drives of sizes up to about 504 MiB using what is commonly referred to as Physical CHS addressing. This limit originates from the hardware interface of the x86 disk hardware. The BIOS used the CHS address given in the INT 13 call, and transmitted it directly to the hardware interface.

This interface was later extended to support addressing of up to exactly 8064 MiB using what is commonly referred to as Logical CHS addressing. This limit originates from a combination of the register value based calling convention used in the INT 13 interface, and the goal of maintaining backward compatibility. There were originally a number of BIOS that offered incompatibly versions of this interface, but eventually the computer industry standardized on the interface developed in the Award BIOS. This limit uses 1024 cylinders, 256 heads, 63 sectors, and 512 byte blocks, giving roughly 7.875 GiB of addressing (1024 * 256 * 63 * 512).

To support even larger addressing modes, an interface known as INT13 Extensions was introduced by Western Digital that uses Logical Block Addressing (LBA). The original version of this interface supports 32-bit LBAs. Newer versions support 48-bit and 64-bit LBAs. These allow addressing of 2 TiB, 128 PiB, and 8 ZiB respectively. This interface is also known as a packet interface, because it uses a pointer to a packet of information rather than the register based calling convention of the original INT 13 interface. This packet is a data structure that contains an interface version, data size, and LBAs.

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