To load the ASSEMBLER vocabulary I usually give 

  NEEDS ASSEMBLER 

or, using the old fashion blocks way

  100 LOAD  

that in turn, loads many screens between 100 and 160.

The above Forth listing creates a new word GREY-SCREEN that XORs whatever is on Layer-0 screen with a checkered-pixel pattern.

You will notice how the Assembler notation is slightly different from the usual. This is due to avoid Vocabulary conflicts, both op-codes and arguments that have often specialized, e.g.  LDX HL| n NN,  to load HL register-pair, or INCX HL| to increment it. There is almost no need for labels and DJNZ loops can be enclosed by HERE ... BACK,  to resolve the displacement.

In any case, BC register-pair must be preserved since it contains the current Forth Instruction-Pointer. Also IY since it is used in standard ROM calls. All other registers can be used.

In this Forth implementation I carefully avoided the use of alternate registers, unless interrupts are disabled. This allows fast-switch context using EXX for an Interrupt Routine - in the future.

I have to write a separate document for the ASSEMBLER vocabulary. Still, you can find some clues by reading the complete source available under ./SRC directory. Also screens # 150 to # 159 provide some test examples.

In this table:

You can use (IX+ operand wherever you can use (IY+ operand.

N.B. : in Forth spaces matter.: