appendixd.rst

Appendix D: Answers to "Further Thinking” Problems

Chapter Three

1. The answer depends on whether you believe that other components will need to know the numeric code associated with each key. Usually this would not be the case. The simpler, more compact form is therefore preferable. Also in the first version, to add a new key would require a change in two places.

2. The problem with the words RAM-ALLOT and THERE are that they are time-dependent : we must execute them in a particular order. Our solution then will be to devise an interface to the RAM allocation pointer that is not dependent on order; the way to do this is to have a single word which does both functions transparently.

   Our word’s syntax will be

: RAM-ALLOT   ( #bytes-to-allot -- starting-adr)
    ... ;

This syntax will remain the same whether we define it to allocate growing upward:

: RAM-ALLOT  ( #bytes-to-allot -- starting-adr)
    >RAM @  DUP ROT +  >RAM ! ;

or to allocate growing downward:

: RAM-ALLOT  ( #bytes-to-allot -- starting-adr)
    >RAM @  SWAP -  DUP >RAM ! ;

Chapter Four

Our solution is as follows:

\ CARDS Shuffle                              6-20-83
52 CONSTANT #CARDS
CREATE DECK  #CARDS ALLOT   \ one card per byte
: CARD ( i -- adr) DECK + ;
: INIT-DECK  #CARDS 0 DO  I  I CARD  C!  LOOP ;
INIT-DECK
: 'CSWAP  ( a1 a2 -- )  \  swap bytes at a1 and a2
   2DUP C@  SWAP C@  ROT C!  SWAP C! ;
: SHUFFLE   \  shuffle deck of cards
   #CARDS 0 DO  I CARD  #CARDS CHOOSE CARD  'CSWAP
   LOOP ;

Chapter Eight

: DIRECTION  ( n|-n|0 -- 1|-1|0)  DUP  IF  0< 1 OR  THEN ;