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.Īlthough the file format is the same in DOS and CP/M. 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. In the Intel 8080 CPU architecture, only 65,536 bytes of memory could be addressed (address range 0000h to FFFFh). This was not an issue on 8-bit machines since they can address 64k of memory max, but 16-bit machines have a much larger address space, which is why the format fell out of use. 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.
OPNE .MELLEL FILE IN WINDOWS CODE
This simplicity exacts a price: the binary has a maximum size of 65,280 (FF00 h) bytes (256 bytes short of 64 KB) and stores all its code and data in one segment. 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. The COM format is the original binary executable format used in CP/M (including SCP and MSX-DOS) as well as DOS. However, this similarity in name has been exploited by malware writers. com (for "commercial") top-level Internet domain name. COM file name extension has no relation to the. Even when complemented by the more general EXE file format for executables, the compact COM files remained viable and frequently used under DOS. This convention was later carried over to DOS. With the introduction of Digital Research's CP/M (a microcomputer operating system), the type of files commonly associated with COM extension changed to that of executable files. COM was used as a filename extension for text files containing commands to be issued to the operating system (similar to a batch file). On the Digital Equipment Corporation (DEC) VAX operating systems of the 1970s. A COM file is a type of simple executable file.
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