   a com file is a type of simple executable file. on the digital equipment operating systems of the 1970s, .com was used as a filename extension for text files containing commands to be issued to the operating system (similar to a batch file).this site has a reference book for the rt-11 operating system running on the pdp-11 minicomputer, which shows in section 5.3 that .com is used to refer to a command file with the introduction of cp/m (a microcomputer operating system), the type of files commonly associated with com extension changed to that of executable files. this convention was later carried over to ms-dos. even when complemented by the more general .exe file format for executables, the compact com files remain viable and frequently used in ms-dos. the .com file name extension has no relation to the .com (for "commercial") top-level internet domain name. however, this similarity in name has been exploited by malicious computer virus writers. ==ms-dos binary format== the com format is the original binary executable format used in cp/m and ms-dos. it is very simple; it has no header (with the exception of cp/m&nbsp;3 files), and contains no metadata, only code and data. this simplicity exacts a price: the binary has a maximum size of 65,280 (ff00h) bytes (256&nbsp;bytes short of 64&nbsp;kb) and stores all its code and data in one segment. since it lacks relocation information, it is loaded by the operating system at a pre-set address, at offset 0100h immediately following the psp, where it is executed (hence the limitation of the executable's size). this was not an issue on early 8-bit machines because of how the segmentation model works, but it is the main reason why the format fell into disuse soon after the introduction of 16- and then 32-bit processors with their much larger, segmented memory. in the intel 8080 cpu architecture, only 65,536 bytes of memory could be addressed (address range 0000h to ffffh). under cp/m, the first 256 bytes of this memory, from 0000h to 00ffh were reserved for system use by the zero page, and any user program had to be loaded at exactly 0100h to be executed. com files fit this model perfectly. note that before the introduction of mp/m and concurrent cp/m, there was no possibility of running more than one program or command at a 
CP/M_technology DOS_files DOS_technology Executable_file_formats Filename_extensions