Two listings in chapter 42 name a file that is here under two names.

Listing 42.1 is ch42/ruler.asm and listing 42.2 is ch42/clock.asm, and what you
will find in this folder is rulerntsc.asm and rulerpal.asm, clockntsc.asm and
clockpal.asm.  That is the chapter's subject showing through its own companion:
the ruler and the clock are each one program assembled for one television, and
the two versions differ only in the block of constants at the top.  The printed
listings are the NTSC ones.

Open the two side by side — diff rulerntsc.asm rulerpal.asm — and the whole of
the conversion is four numbers.  both.asm, later in the same chapter, is the
other way of doing it: one source, one flag, two cartridges.
