The First PC Utility: boot.exe

main() {
int fi;
struct exec hdr;

    fi = open ("pgm", O_RDONLY);
    read (fi, &hdr, sizeof (hdr));
    read (fi, (char *) 0, hdr.a_text +  hdr.a_data);
    (* (void * () 0) ();
    /* NOTREACHED */
}

There are some niggling little gotchas, however:

Bill Gates thought 640KB would be big enough for anyone - well, its not for us.

• Programs are frequently larger than is considered "convenient" in the PC world. On the PC, 64K or less is considered adequate, while the UNIX kernel we must load averages about 280 Kbytes in size, so we will have to manage the so-called "far" pointers in a large model 8086 program.
• The bottom (address 0) of PC memory contains a critical portion of the MS-DOS operating system. We will need to use MS-DOS itself to load the program, so we can't touch this area until after we read in the entire program. We will therefore have to allocate a pool of memory space large enough to temporarily hold a copy of the program we are loading until it is safe to overwrite location 0.
• Once we enter into protected mode, we can't easily go back and enter MS-DOS again, so we must do all our checks and anticipate needs prior to taking that last giant step.

Now we can implement the utility:






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Tags: operating system | porting unix to the 386 | porting unix | three initial pc utilities | protected mode |