Weird code

[justice at isucs1.UUCP]

Ok you wizards out there, why does this program do what it does?
Here are a couple of hints:
	1) It only works on Vaxes
	2) The same idea is used to initialize the UNIBUS adapters
--------------Here is a script showing the details------------------------
Script started on Mon Oct 14 14:08:05 1985
% cat reg.c
main()
{
        register int x, y;
        int i;
        int func();

        fix((int *)func);
        x = 1; y = 5;
        for (i = 1; i <= 10; i++) {
                func(1,5);
                printf("x = %d, y = %d\n",x,y);
        }
}

func(x,y)
int x,y;
{
        register int a,b;

        a += a; b *= 5;
        x--; y++;
}

fix(f)
int *f;
{
        *f &= ~0xc00;
}
% cc reg.c -N -o reg
% reg
x = 2, y = 25
x = 4, y = 125
x = 8, y = 625
x = 16, y = 3125
x = 32, y = 15625
x = 64, y = 78125
x = 128, y = 390625
x = 256, y = 1953125
x = 512, y = 9765625
x = 1024, y = 48828125
% exit
% 
script done on Mon Oct 14 14:08:29 1985
--------------------------------------------------------------------------
Good luck!
Brian Justice, Iowa State University
UUCP: {okstate||umn-cs||csu-cs}!isucs1!justice	 CSNET: justice at iowa-state