A Chess Program in PL/I

I had different version of a chess program back in the 80's when I was teaching PL/I. It is a good example of doing something complex (chess) fairly simply in code. This version looks pretty clean, maybe because of the Fortran root.

I'm going to guess that the 5000 / -7500 is an error in the translation, a typo. If I find an hour to kill I may compile it up with -5000 to see what it does.

Why is the white king represented by 5000 but the black king by -7500? All the other white and black pieces have the same absolute value.

(This really brought back memories. I haven't written a PL/I program since 1978.)

Relatively short -- the Fortran version is 500 lines.