Editing Fat binary (section)

===Combined COM and SYS files===
DOS [[device driver]]s (typically with file extension [[.SYS]]) start with a file header whose first four bytes are [[hexadecimal|FFFFFFFFh]] by convention, although this is not a requirement.<ref name="Paul_2002_COMSYS"/> This is fixed up dynamically by the operating system when the driver [[loader (computing)|loads]] (typically in the [[DOS BIOS]] when it executes [[DEVICE (CONFIG.SYS directive)|DEVICE]] statements in [[CONFIG.SYS]]). Since DOS does not reject files with a .COM extension to be loaded per DEVICE and does not test for FFFFFFFFh, it is possible to combine a COM program and a device driver into the same file<ref name="Paul_2002_CTMOUSE"/><ref name="Paul_2002_COMSYS"/> by placing a jump instruction to the entry point of the embedded COM program within the first four bytes of the file (three bytes are usually sufficient).<ref name="Paul_2002_COMSYS"/> If the embedded program and the device driver sections share a common portion of code, or data, it is necessary for the code to deal with being loaded at offset +0100h as a .COM style program, and at +0000h as a device driver.<ref name="Paul_2002_CTMOUSE"/> For shared code loaded at the "wrong" offset but not designed to be [[position-independent]], this requires an internal address fix-up<ref name="Paul_2002_CTMOUSE"/> similar to what would otherwise already have been carried out by a [[relocating loader]], except for that in this case it has to be done by the loaded program itself; this is similar to the situation with [[self-relocating driver]]s but with the program already loaded at the target location by the operating system's loader.