|
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 3 files), and contains no standard metadata, only code and data. This simplicity exacts a price: the binary has a maximum size of 65,280 (FF00h) bytes (256 bytes short of 64 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): the entry point is fixed at 0100h. 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 time: the program loaded at 0100h was run, and no other. Although the file format is the same in MS-DOS and CP/M, .COM files for the two operating systems are not compatible; MS-DOS COM files contain x86 instructions and possibly MS-DOS system calls, while CP/M COM files contain 8080 instructions (programs restricted to certain machines could also contain additional instructions for 8085 or Z80) and CP/M system calls. It is possible to make a .COM file which will run under both operating systems. There is no true compatibility at the instruction level; the instructions at the entry point are chosen to be equal in functionality but different in both operating systems, and make program execution jump to the section for the operating system in use. It is basically two different programs with the same functionality in a single file, preceded by code selecting the one to use. Under CP/M 3, if the first byte of a COM file is C9h, there is a 256-byte header; since C9h corresponds to the 8080 instruction RET, this means that the COM file will immediately terminate if run on an earlier version of CP/M that does not support this extension. (Because the instruction sets of the 8085 and Z80 are supersets of the 8080 instruction set, this works on all three processors.) C9h is an invalid opcode on the 8088/8086, and it will cause an INT 6 exception in v86 mode since the 386. Since C9h is the opcode for LEAVE since the 80188/80186 and therefore not used as the first instruction in a valid program, the executable loader in some versions of DOS rejects COM files that start with C9h, avoiding a crash.Files may have names ending in .COM, but not be in the simple format described above; this is indicated by a magic number at the start of the file. For example, the COMMAND.COM file in DR DOS 6.0 is actually in DOS executable format, indicated by the first two bytes being ''MZ'' (4Dh 5Ah), the initials of Mark Zbikowski. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「COM file」の詳細全文を読む スポンサード リンク
|