The infinitely profitable program

On Sat, 2 Jul 2016, Eric Christopherson wrote: Clever? yes. Genius? no, or you would have to count too many of us. NO. A .COM file is a pure image of memory, starting at location 100h. The "header" or "zero page" or "program segment prefix" is generated on the fly during the loading of the program. (There is a lot more to know about it) The Zero Page is 256 bytes of data structures. Among those are a copy of the command line (placed at 80h), and, if there was a filename as an argument on the command line, a fully parsed File Control Block for that filename (placed at location 5Ch) (There is a lot more to know about it) The first thing that I did in CP/M was to create a zero length program. it seemed intuitively obvious as a needed capability. Didn't everybody realize that and do it? It never occurred to me to try to SELL it! In those days, there was almost an assumption that price would be proportional to program length. For those who wanted "source code" for it: SAVE 0 A:JMP100.COM (SAVE was one of the five CP/M internal commands, ERA,REN,DIR,TYPE) Since program loading created the data structures from 0 through 0FFh, but 0 bytes did NOT alter the TPA (Transient Program Area), it provided a way at the command line to do a jump to location 100h, which could sometimes restart or regain control from some kinds of program crashes. Headers for executable came along with MS-DOS. (already present in many/most operating systems other than CP/M & MS-DOS) In MS-DOS, a .COM file is STILL a pure image of memory, starting at location 100h. But, an MS-DOS .EXE file is the form of MS-DOS executable that DOES have a header. The .EXE header starts with "MZ". Having a fixed beginning lets the computer recognize which files are .COM and which are .EXE without resorting to filename. 'Course that does prevent you from creating a .COM file that starts with DEC BP POP DX since that will be indistinguishable from the marker of a .EXE file. Why "MZ", instead of something else, maybe even something that would NEVER be used in a .COM? Initials of Mark Zbikowski. Who was he? As little is known about him as is about Howard Fullmer ("Parasitic Engineering" and chief engineer for Morrow) The result is that at the MS-DOS command line, if you type something, the command processor (COMMAND.COM) checks its list of internal commands. If found, it executes it. If not found, it looks for that a matching filename with .COM, and executes it. If not found, it looks for .EXE and executes it. If not found, it looks for .BAT and executes it. If not found, it gripes. ("BAD COMMAND OR FILENAME") (There is a lot more to know about it) If what was found was .COM or .EXE, then it starts by looking at the first two bytes of the file. It actually DOES NOT MATTER if you rename or MIS-name a .COM as .EXE, or MIS-name a .EXE as .COM! The differentiation is done by whether the first two bytes are "MZ". MIS-naming a .EXE .COM will cause it to be found (and run) before one with otherwise same name with .EXE extension. Do you need examples of why you might want to? (There is a lot more to know about it) If it is not "MZ", then it creates a program segment prefix, loads the file into RAM starting at 100h, and jumps to 100h. If it IS "MZ", then the .EXE loader processes the header of the file. .EXE was present in MS-DOS since before release of 1.00, I seriously doubt that his earliest ports to MS-DOS were "before the EXE file format existed". -- Grumpy Ol' Fred cisin at xenosoft.com