You'll need gforth to build the Forth.

The Makefile builds the binary j1.bin from the Forth sources, and the 'j1' simulator.

j1 loads j1.bin, then:

 * pushes 7 on the stack
 * runs the code from location 0
 * prints the top-of stack
 * dumps memory at 0x8000 (cell 0x4000)

main.fs does some housekeeping, but the word 'main' does two things:

 * computes factorial of the top of stack
 * copies the string "Hello world" to address 0x8000

The code could be smaller if it's safe to assume that stacks will
not wrap.