CP/M/CPM_CDROM.iso/simtel/sigm/vols000/vol059/session1.doc

Text File | 1988-04-18 | 8.4 KB | 378 lines

X> % Welcome back to the next PISTOL session!
X>
X> % I shall show in this session how to create things
X> % specifically, I shall show how to define CONSTANTs
X> % VARIABLEs, WORDs, and "inline macros".
X>
X> % CONSTANTs
X> % suppose I wish to define "1K" as 1024 in decimal in
X> % order to increase the readability and decrease the size
X> % of the program.  Constants are an important tool to
X> % simplify future modifications in a program.
X>
X> 1024 '1K CONSTANT
X>
X> % we can test if the definition "took" (really unnecessary)
X> % but good for demonstration!
X>
X> 1K =
1024
X>
X> % I shall digress to the question of number bases.  We have bee
X> % been using "decimal" all along.  We could change, say to
X> % Hexadecimal (base 16) by the command:
X>
X> HEX
H>
H> % notice how the prompt is now "H>", to indicate we are
H> % now conversing with PISTOL in hexadecimal.
H>
H> % What is "1K" in hex?:
H>
H> 1K =
400
H>
H> % here is another demonstration we are in "hex":
H>
H> 8 8 + =
10
H>
H> % the "10" is sixteen in hex; here's another demo:
H>
H> 100 2 / =
80
H>
H> 100 2 /
1H> DECIMAL
1X> =
128
X> % note that the prompt helps to remind us which number base
X> % we are using.  The value of a CONSTANT is not changing
X> % when we change number bases, but writing its value "looks"
X> % different in each number base.  Let's see "1K":
X>
X> BINARY
B> 1K =
10000000000
B>
B> OCTAL
Q> 1K =
2000
Q>
Q> HEX
H> 1K =
400
H>
H> DECIMAL
X> 1K =
1024
X>
X>
X> % VARIABLEs
X> % It is possible to store items in named locations like
X> % most other higher-level languages.  For example,
X> % " X = Y + Z " is probably understood as " take what is
X> % stored at location 'X'; take what is stored at location
X> % 'Y' ; add them; store the result at the location 'Z' "
X>
X> % We must define and allocate space for variables before
X> % we can use them; we shall define the variables, Y and Z
X> % with initial values of 3 and 4 initially:
X>
X> 3 'Y variable
X> 4 'Z VARIABLE
X>
X> % to test the values of variables we use the word "?":
X>
X> Y ?
3
X> Z ?
4
X>
X> % to simulate X=Y+Z we better define X first:
X>
X> 0 'X variable
X>
X> X ?
0
X>
X> % Here goes:
X>
X> Y W@  Z W@   +    X W!
X>
X> X ?
7
X> % we did it.  We can continue to use X,Y, and Z for the
X> % remainder of the session.  To see what have been the
X> % ten most recent definitions we can use:
X>
X> TOP10
25280   X
25264   Z
25248   Y
25234   1K
25132   FINISH
25066   WRITE
25020   READ
24990   1READ
24930   DELETES
24912   REPLACE
1X>
1X> % note this command left an item on the stack. It is
1X> % used to enable us to find the names of yet earlier
1X> % definitions:
1X>
1X> NEXT10
24898   DELETE
24874   (DELETE)
24808   INPUT
24700   LI
24684   ARG#ERR
24644   1POSARG?
24610   LENTER
24596   MTDN
24570   OVERWRITE
24534   MTUP
1X>
1X> NEXT10
24488   #GETLINE
24450   LDDR
24412   LDIR
24306   LFIND
24290   ILLEGLIN
24236   LISTALL
24202   NEXTLINE
24164   NEWF
24148   EOT
24132   OLDLINE^
1X>
1X> DROP
X>
X> % the DROP is for "good housekeeping".
X>
X> % back to CONTANTs and VARIALEs:
X> % suppose we want to do:  X = '1K' + Y  :
X>
X> 1K   Y W@   +    X W!
X>
X> X ?
1027
X>
X> % looks correct.   It's about time I explain the words:
X> % W@ and W!  The first, "word-at" is used to get the
X> % contents of a location ( a VARIABLE provides a pointer
X> % to the location).  The second, "word-store", is used
X> % to place the item next to the top of stack into the
X> % location pointed to by the top of stack.
X>
X> % Let's follow the last example using the STACK utility:
X>
X> STACK
(0)
X> 1K STACK
(1)  1024
1X> Y STACK
(2)  1024  25254
2X> W@ STACK
(2)  1024  3
2X> + STACK
(1)  1027
1X> X STACK
(2)  1027  25286
2X> W! STACK
(0)
X>
X> X STACK
(1)  25286
1X> ? STACK
1027
(0)
X>
X> % the ? is equivalent to the pair of words:  W@ =
X>
X> % creating new WORDs....
X> % suppose we didn't have or didn't know the word, "?" ,
X> % we could invent it: suppose we call our word, "IS" :
X>
X> 'IS :  W@ =  ;
X>
X> % testing this new word:
X>
X> X IS
1027
X>
X> Y IS
3
X>
X> Z IS
4
X> % it appears to work. Once a word exists, we can define
X> % newer words yet, for example:
X>
X> 'XYZ? : X IS  SPACE   Y IS  SPACE  Z IS    ;
X>
X> % testing:
X>
X> XYZ?
1027 3 4
X>
X> % further testing:
X>
X> -15  X W!
X> XYZ?
-15 3 4
X>
X>
X> % DIS-assembling words:  (very useful if you forgot how
X> % you created a definition!)
X>
X> 'XYZ?  DIS
'XYZ? [:]       X       IS      SPACE   Y       IS      SPACE
Z       IS      ;
X>
X>
X> % Thus DIS shows us more or less the same thing as we typed.
X> % more examples:
X>
X> 'IS DIS
'IS [:] W@      =       ;
X>
X> '? DIS
'? [:]  W@      =       ;
X>
X>
X> % So we see that the "?" was originally defined the same
X> % way we defined "IS".
X>
X> % Sometimes we wish to observe the internal action of a
X> % definition, say, for debugging.  We use the utility
X> % "TRACE":
X>
X> 'XYZ?  TRACE
'XYZ? BEING TRACED:
(0)                                             X
(1)  25286                                      IS  -15
(0)                                             SPACE
(0)                                             Y
(1)  25254                                      IS  3
(0)                                             SPACE
(0)                                             Z
(1)  25270                                      IS  4
(0)                                             (;)
TRACE COMPLETED
X>
X>
X> % notice that the STACK utility is interposed in between
X> % each word that makes up the definition being tested.
X> % fortunately, however, the individual words, such as
X> % "IS" are not also "traced"; if TRACE showed every
X> % step we would have too much clutter.  It is then
X> % possible to do a TRACE on a suspicious subsidiary word,
X> % and so on...
X>
X> % inline macros:
X> % as the last topic of this session we shall show an
X> % alternate way to define new words:
X> % we will define "DOUBLE" in two ways:
X>
X> 'DOUBLE1 : DUP + ;
X> 'DOUBLE2 $: DUP + ;$
X>
X> % testing each:
X> 3 DOUBLE1 =
6
X> 3 DOUBLE2 =
6
X>
X> % they appear to behave at least similarly.  What about
X> % disassembling them:
X>
X> 'DOUBLE1 dis
'DOUBLE1 [:]    DUP     +       ;
X> 'DOUBLE2 dis
'DOUBLE2 [$:]   DUP     +       ;
X>
X> % Now I can try to illustrate how they really differ, by
X> % defining QUADUPLE using both:
X>
X> 'QUADUPLE :

1X:> DOUBLE1

1X:> DOUBLE2

1X:> ;
X>
X> % testing:
X>
X> 3 quaduple =
12
X> % seems reasonable (yes, I realize my spelling is bad!)
X> % let's use DIS:
X>
X> 'QUADUPLE DIS
'QUADUPLE [:]   DOUBLE1 DUP     +       ;
X>
X> % Where did DOUBLE2 go?  We see that it was repleced by
X> % " DUP + ", which is the way we defined DOUBLE2 anyway.
X>
X> % the difference between using the : ; pair for defining
X> % a word and the $: ;$ pair for the definition is the
X> % the former creates a "subroutine" and the latter defines
X> % a pattern to be substituted into whereever it will be
X> % used.  The choice of which to use is that of space
X> % verses speed.
X>
X> % Let's use TOP10 to see what monsters we have created:
X>
X> TOP10
25364   QUADUPLE
25350   DOUBLE2
25336   DOUBLE1
25310   XYZ?
25296   IS
25280   X
25264   Z
25248   Y
25234   1K
25132   FINISH
1X>
1X> drop
X>
X> % sooner or later we wish to redo definitions or discard
X> % them.  The word FORGET is used:
X>
X> 'IS  FORGET
X>
X> % testing:
X>
X> TOP10
25280   X
25264   Z
25248   Y
25234   1K
25132   FINISH
25066   WRITE
25020   READ
24990   1READ
24930   DELETES
24912   REPLACE
1X>
1X> drop
X>
X> % FORGET not only discards the chosen word (here "IS"), but
X> % also all more recent definitions.  This behavior is
X> % following a long tradition that was in FORTH and in
X> % STOIC.  At times it is convenient, at times inconvenient.
X> % Consider whether XYZ? would have been useful after IS was
X> % discarded.
X>
X> % FORGET is a word that causes designers of this kind of
X> % language sleepless nights.
X>
X> % This session has lasted long enough.... Why don't you
X> % continue experimenting and learning PISTOL more
X> % systematically by reading PISTOL.DOC?
X>
X> bye