The Computer Emulator

• Obtaining the software: Users of rabbit should create a new subdirectory, cd to it, and enter the command get521f. This will copy all the required files and leave the system ready to use. If your $PATH is not correctly set, you may use the full path of /usr/local/bin/get521f.

  Modifications are quite frequent (whenever a mistake is found); the get521f command also works for getting upgrades. To see if there has been an upgrade Look here.

  All components automatically check themselves to see if there is a newer version available for download. Each time exe, cmp, asm, lnk, or sys is run, if there is a newer version and you have not already been told in the last hour, you are told of its existence.
  
  

• The Compiler, command:  cmp [options] filename
  • Takes a single C source file and produces a single assembly language file as output. Unlike the unix cc command, this compiles only, it does not automatically assemble and link afterwards. If the filename has no dot in it, the system assumes ".c"; the name of the assembly file produced is the same is the C source file, but with the extension replaced by ".ass".
    
    
  • Options are available for making the compiler produce all sorts of debugging help. Look here for further information on using the compiler effectively.
    
    
  • From time to time, I have to make changes, correcting a mistake, or adding a feature. Look here for a complete list of all changes since the first release.
    
    
  • The compiler is not likely to be quite perfect. Tell me about problems. It is quite possible I've made a few mistakes (deliberately of course), or not realised that an unsupported feature might be very important to you. Tell me, and I can adjust my priorities for making additions. But remember, almost all problems can be avoided by putting inline assembly code in your C programs.
    
    
  • The compiler does not yet support all of ANSI/ISO C. Nearly all, but not quite all. Look here for a list of what is missing.
    
    
  • The compiler adds some extensions to standard C, mainly to make it more useful for operating systems programming and more helpful with debugging. Look here for details.
    
    
  • The standard libraries are extremely limited, this is because some important exercises involve writing them yourself. Look here for library information.
    
    
  • Note: I was surprised to find that freeBSD already has a cmp command. Either make sure that "." is at the beginning of your path rather than the end, or use ./cmp to run the compiler instead, or rename the compiler to something else.
  
  
  
• The Assembler, command:  asm [options] filename
  • Takes a single assembly source file and produces a single object file as output. If the filename has no dot in it, the system assumes ".ass"; the name of the object file produced is the same is the assembly file, but with the extension replaced by ".obj".
    
    
  • The compiler does not produce an object file, only an assebly file, so the assembly stage is essentail after compilation.
    
    
  • Options are available for making the assembler produce all sorts of information about what it is doing. type asm -h for information.
    
    
  • The assembler and the simulated computer share documentation. Look here for html format, or here for a word .doc.
  
  
  
• The Linker, command:  lnk [options] filename [filename...]
  • Takes any number of object files and produces a single executable file as output. If the filename has no dot in it, the system assumes ".obj"; the name of the executable file produced is the same is the object file, but with the extension replaced by ".exe".
    
    
  • The linker's job is not only to combine multiple files. The assembler never produces a proper executable file; the linker is always needed.
    
    
  • Options are available for making the linker produce all sorts of information about what it is doing. type asm -h for information.
    
    
  • Object files contain only printable ascii characters, so are human readable, but until I produce documentation for the format, it will probably require too much effort to understand what they say.
  
  
  
• The System, command:  sys [options] [filename] behaves like a normal computer with a very basic monitor (interactive command line interpreter). If a filename is provided, it is expected to be a proper executable file produced by the linker (extension ".exe" is assumed if none is specified) and is loaded into memory ready for running. If no file name is provided, you may use the load command at the monitor prompt. The monitor prompts with ">", obeys the command typed, and prompts again. The command to exit is "exit".
  
  The system has a basic on-line help system that tells you all the available commands, or gives details of an individual command. Type "help" or "help command". Interactive commands allow a user to sibgle step through or run a program, inspect and modify memory and registers and the virtual memory system, and other things.
  
  Occasionally updates are made to the emulator, as students discover the need or desire for something extra. the emulator command "version" reports the current version number; the emulator also automatically checks that it is the current version each time it is run.
  For a description of updates, Look here.
  
  Documentation is available in html format and as a word .doc file.
  
  sys command options:
  • -r means automatically Run the program after loading it, do not wait for a user command.
    
    
  • -e means automatically Exit from the system emulator after the program terminates.
    
    
  • -q means be Quiet: do not print any information while loading the program.
    so, "sys -r -e -q file" is the way to just run file.exe normally without any debugging or user intervention.
    
    
  • -o filename   redirects Output to file.
    so, "sys -r -e -q file -o outf" is the way to just run file.exe with nothing appearing on the screen, but all output going to the file outf instead.
    
    
  • -v means automatically turn on Virtual memory (or Paging) before loading the program. Normally paging is turned off until the user or a running program turns it on.
  
  
  
• The exe command, automates the process of compiling, assembling, linking, and running multi- and single- file programs.
  • The exe command takes a number of file names as parameters. If no extension is provided for a file, ".c" is assumed.
    1. All .c files are compiled and assembled.
    2. All .ass files are assembled.
    3. The resulting .obj files plus any given with the command are linked into one executable.
    4. The emulator is started, running the .exe file. If an exe file is specified in the command, it must be the only file.
    5. On termination, the emulator exits.
    Example: "exe cat.c dog.ass" has the same effect as cmp cat; asm cat; asm dog; lnk cat dog -o cat; sys; load cat; run; exit.
    
    
  • If there are any errors detected at any stage, the procedure stops.
    
    
  • Options modify the behaviour:
    • "-is" prevents execution, the emulator is not run.
    • "-ir" runs the emulator and loads the program, but does not start execution.
    • "-ie" converts into the -i option to cmp, so that compilation errors to not stop the exe procedure.
    • "-p" uses pico to edit the next-named file before compiling or assembling it.
    • "-v" is passed directly to the emulator, where turns virtual memory (paging) on before loading the program.
    • "-q" prevents progress reports from being printed after each stage.
    • "-o filename" is passed directly to the emulator, where it makes all output from the running program be redirected to the file, and not appear on the screen.
    • "-d" deletes all generated files after use.