In a lot of FORTH implementations, constant numbers like 0, 1, -1 and others are hard coded, not just for speed but also for space: a call to a code word only takes one cell, instead of using two cells with a LIT [value].

Here's some Breshenham line code I first wrote in FORTH (Mitch Bradley's SunForth on a 68k Sun-2 with cg2 graphics board), then translated to 68k code (using the rpn FORTH assembler) -- the FORTH code is commented out before the corresponding assembly code:

https://donhopkins.com/home/archive/forth/cg/line.f

The cg2 board wasn't directly memory mapped -- it had a really weird way of selecting and accessing one row and one column at a time, which was kinda convenient for drawing lines and curves, and ropping blocks around, but nowhere as convenient as direct memory mapped access would have been.

https://donhopkins.com/home/archive/forth/cg/cg.f

I had to reverse engineer it from the map of the registers in this helpful include file:

https://donhopkins.com/home/archive/forth/cg/cg2reg.h

    * To save on virtual address space, we don't map the plane or pixel mode
    * memory (first two megabytes).  However, when calling mmap the user has
    * to add 2M to the desired offset anyway (goofy, huh?).