Jet Set Willy II: The Final Frontier (Amstrad CPC)

Jet Set Willy: Expanded Edition

Developer Message#

A little message was hidden by the developer of this game, which can be viewed with a hex editor. This has been altered slightly for formatting purposes. The message begins around 4F1 on the original tape.

Well, here is a little message to those of you 
who dare to HACK into other peoples programs! 
I cannot be bothered to put line feeds or carriage 
returns into all this, so there. Interesting 
thing, the Z80. Did you know, for example, about
the conditional RST 38 instruction? Just try doing 
a JR Z,-1. Any condition will work. Not much use on 
the Amstrad! What do you mean you use it all 
the time!?!?!? Bye, and good luck with the game 
(tee hee!)

On the Z80, JR Z,-1 is equivalent to

    jr z,label-1
label:

and is encoded as 28 FF. If the zero flag is set, jr z will jump to one byte before the next instruction, which is the one-byte opcode FF, which is rst 38h, the instruction to perform a software interrupt jump to Z80 physical address 38h. What the Amstrad CPC BIOS uses 38h for is unknown (it appears to be the hardware IRQ handler, but I (andlabs) am not entirely sure).