8.1.
A sure place: the
variable
There are data that may sometimes be needed (the bottom of the stack, the
title of the dictionary, our own counters), but not always.
We can not keep them in the
vermin, because we would be embarrassed if we had to avoid these even in
the pad, which sometimes changed the title.
There are variables in FORTH
(also), which are word words like words of arithmetic or cycle
organization.
The
FORTH variables work by putting an address on the stack;
you can store the 16-bit data
at this address, the variable being created to preserve it.
|
The FORTH interpreter itself uses variables, such as so-called. System variables.
8.1. Examples of system variables
8.2.0.
S0
The word S0, which
we learned in
7.3 , is a system
variable .
8.2.1.
BASE
A BASE contains the base number of the numeric system used when scanning
and scanning. So far this was
always 10. If we want to work
in the 2-digit system, then the value of this variable is set to
2:
2 BASE (makes the variable address on the stack)
! (which we made with this operation 2)
Let's try and see what life is in the binary system:
1 1 +.
For
example, the 9th digit does not take the "stomach" of the interpreter, since
we are in a two-digit system where it makes no sense.
Surprisingly, however, the
experimental reader finds that numbers 2, 3, which are also "meaningless",
work. This is because the first
few numbers are in the dictionary;
these are often used.
It speeds up the work of the
interpreter by "finding them", without having to convert them.
The most commonly used two numbers are 10 and 16;
these are set to FORTH basic
words. Their operation is clear
from their source text:
: DECIMAL 10 BASE! ;
: HEX 16 BASE! ;
How do I know at what point is my conversion count? It's easy to say, without thinking, you just need to get the base number out of the variable and give it up:
BASE @.
We get 10 responses What do we know about this? That the base number of the numeric system in the given system is represented by a 1 and a 0, that is, it is equal to the base number of the numeric system. We will go further if we write a smaller number:
BASE @ 1-.
From
this answer we know that B, that is, 11 is the smallest one-digit number,
we are in the 12th numerical system.
Now set the default number to 5 and define a new one in the 5th numerical
system:
5 BASE!
: KIIR 10. ;
What
happens if we run the KIIR in the 10-digit system?
5 results are obtained.
There are binary numbers in the dictionary words, with the definition of
0000 0000 0000 0101, this is in the decimal system 5.
8.2.2.
OUT
We know that the FORTH parentheses on the screen write each character with
EMIT. EMIT increases the OUT
content of OUT, so if you reset the OUT then you can always find out how
many characters have been displayed since the reset.
For example, imagine that we want to write the same lines, which consist
of two (unpredictable) numbers;
we want the first number to
start at the 3rd and 3th on the screen, starting at position 13.
We assume the two numbers are
on the vermin. Write the word
that says it as above:
: BALRA-TABULAL
CR 0 OUT!
3 SPACES
.
(n1 n2 ---)
(reset the OUT variable)
(3 spaces)
(print the first number)(the OUT content now: how many characters have been written)
(already in this line)13 OUT @ -
SPACES.
;(
Add 13 to the spelled ) (number of spells, then print)
(the second number)
8.3.
Creating new
variables
Variables can be defined with VARIABLE (n ---).
The expected value of the worm
is the initial value of the variable.
The name of the variable should
be given immediately after VARIABLE:
0 VARIABLE SAJAT
With
this, the word SAJAT appears in our dictionary (as we can confirm with VLIST
at a glance). Function: Changes
the address of the variable to the stack.
For example, imagine creating a list on the printer (by switching the handset
off). How can I not know, one
sure thing: 60 lines should be on one side.
A new line in the listing program
will always start with the CR word.
Define the word CR by rolling
60 cards per line (12 EMITs), and then write down on the tab on how many
cards it is.
Since we do not know what is going on with a stack in a list, and it's simpler,
counting the rows and tabs in a variable:
0 VARIABLE LINE
0 VARIABLE LAPSZ
Both variables will have to be increased by one. To date, we know that the content of a variable would be somewhat increased by 1:
SORSZ @ (we cover the content)
1+
SORSZ! (the returned value will be returned)
FORTH Basic Word Simplifying Adds to Variables (or Any Memory Template) a
+! (n title ---)
which adds n to the one-word value on the title and stores the sum at the address. The +! Could be written in some way if it were not:
: +!
SWAP OVER
@
+
SWAP!
;
(n title ---)
(title n title)
(title n old content)
(title new content)
To increase the content of the SORSZ:
1 SORSZ +!
When calculating rows, tabs, and more, you need to increase variables by just 1. Perhaps it is worth introducing a special word that uses the 1+ action:
: INC (title ---)
DUP @
1+ SWAP!
;
The following things must be done for the printer program after the display of 60 lines:
: LAPDOB
12 EMIT
0 SORSZ!
LAPSZ @.
. "tab" CR
LAPSZ INC
;
(---)
(
countdown)
( resume counting ) (write the number of pages)
(increase the number of counters )
We will start writing the script with just a few clicks; this is the right moment to set the LAPSZ start value:
: ELSO-LAPDOB
1 LAPSZ! LAPDOB
;
The listing program will have to start listing by calling ELSO-LAPDOB. The redefined CR:
: CR
SORSZ INC (increase the count counter)
SORSZ @ 60 = (end of tab?)
IF LAPDOB
ELSE CR
ENDIF
;
8.4.
Constants
If you want to preserve a value in a dictionary that we never change, it
is simpler to use words that give the value not the title but the value itself
to the worm. These are the
constants. For example, the
words 1, 2, 3 contained in the dictionary and the 1, 2, 3 values on the
stack. This is BL, from which
we get the code for the space.
Our own constants a
CONSTANT (n ---)
so we can define it. The thing is quite similar to defining variables. For example:
42 CONSTANT CSIL-COD
we defined this constant word called CSIL-KOD. CSIL-KOD 42 is put on the stack.
What was that about?
Summary
of Chapter 8
The words learned in Chapter 8:
VARIABLE | (n ---) | A
word defining a variable. so
we use: n VARIABLE xxx. This is how we created a word dictionary called xxx. The word xxx works by putting the variable's address on the stack. At this address, when the variable is created, n is the initial value. |
CONSTANT | (n ---) | Constant
definition word. so use:
n CONSTANT yyyy. This is how we created a dictionary word yyyy. The yyyy works by stacking the constant on the stack: this is the value that was given when the constant was created on the worm. |
System Variables:
BASE | ( --- title ) | The numbers are off and on. is the base number of the concurrent versions. |
OUT | ( --- title ) | The value of this variable is increased by EMIT (each character string). So you can use it to monitor and control the number of characters you are typing. |
Other words:
HEX | (---) | The BASE value is set to 16. |
DECIMALIZE | (---) | The BASE value is set to 10. |
+! | (n title ---) | Adds n to the 16-bit content of the address, the result is stored in the address-cn. |
Examples
8.1. THE ? (title ---) source text source:
:? @. ;
Prints the 16-bit, signed value on the title. If, for example, A variable, it isTHE ?
line A is written.
8.2.
Write
a: = (title1 title2 ---) word that copies the 16 bit value on address2 to
address1
(ie if A and B are two variables, then with AB: = action A is B
value).
: = (title1 title2 ---)
@ SWAP!
;
8.3. Write a .BASE (---) word that writes BASE content decimally on the screen, but does not break it! (Or, more accurately, restoring it after it has been corrupted.)
: .BASE)
BASE @ DUP
DECIMAL
.
BASE!
;
(---)
(now
we have broken BASE) (but there is another instance of the original)
(so we can reset it)
9. Where does the variable change? Getting to know the dictionary
9.1.
The word
counter
During the FORTH programming the dictionary size is constantly changing.
When the word "top" of the
dictionary is located, the first free space after the dictionary, where the
following dictionary will be used, is interpreted by the interpreter DP
(Dictionary Pointer, say: dictator pointer).
The value of the DP variable
is used a lot, so we have a separate word for reading:
: HERE (--- title)
DP @;
A 7.1. it was a section that the pad was at a constant distance from the top of the dictionary. This phenomenon is explained by the source text of the word PAD:
: PAD HERE 68 +;
9.2.
What's in the dictionary?
A dictionary element consists of the following parts:
The parameter field - wholly arbitrarily abbreviated - is physically at the very end of the word. so after changing its definition, the word markers point exactly behind the new variable. If the value of the word counter is then set to 1, 2, etc. we increase this by multiplying the variables of the variable 1, 2, and so on. bytes. The basic word for increasing the word probe, ALLOT's source text:
: ALLOT (n ---)
DP +! ;
9.3.
Long
Variables
This hand has the option of defining double or longer
variables.
87 VARIABLE DOUBLE
88 HERE!
2 ALLOT
DUPLA
is therefore two words, the first word is the initial value of 87, and the
second the 88th.
The initial values stored in the parser and the "broadening" of the parter
are the following basic words:
:, (n ---)
HERE!
2 ALLOT
;: C, (c ---)
HERE C!
1 ALLOT
;
The following series could have been written:
87 VARIABLE DUPLA 88,
Now
we can define vectors, longer datasets.
It is important to know that
there are more convenient tools for FORTH -
we will meet them in
Chapter
15
- here we define a vector to better understand FORTH's operation (such as
Chapter 15).
For example, define a 10-element "word" vector.
Its parsing will be 20 bytes;
the
0 VARIABLE VECTOR
defined by VECTOR 2 bytes of the VECTOR by 18 bytes:
18 ALLOT
The vector is worth something if its elements can be referred to separately. Thus, we write the word IK-ELEM (n --- address) waiting for the index of the element (serial number) of the element and return its address:
: IK-ELEM
2 *
VECTOR +
2 -
;
(n-address)
(one element is 2 bytes)
(the element with 1 index at the address)
(starts with the VECTOR)
What was that about?
Summary
of Chapter 9
The words learned in Chapter 9:
DP | ( --- title ) | Changing the system. The interpreter keeps the title of the "top" of the dictionary, the first free byte after the dictionary. |
HERE | ( --- title ) | Specifies the value of the DP system variable. |
ALLOT | (n ---) | DP gives it n. |
. | (n ---) | Store n at HERE, in a word, increase the value of the word mark (DP) by 2. |
C | (c ---) | Store n at HERE by 1 byte, increasing the value of the word counter (DP) by 1. |
Examples
9.1. Write a pencil registration system! The system has three types of pencils:
0 CONSTANT RED
1 CONSTANT KEK
2 CONSTANT ZOLD
Create a 3-element variable in which each pen count can be kept. The pen count counters have an initial value of 0. Next, write the following words:
A description of the "long variable" for pencil counting:0 VARIABLE CK 0, 0,
Define a SZLO (color-tag) word that produces the pen-numbering pen for the color from the color code:
: SZLO
2 *
CK +
;
(color address)
(color code multiplied by 2)
(counters are worded)
(first counters address)
From here we have a simple business:
: CERUZA-BE (n color ---)
SZLO +!
;: CERUZA-OFF (n color ---)
SZLO
OVER OVER @
> IF "There is not enough"
DROP DROP
ELSE
SWAP MINUS SWAP +!
ENDIF
;: CERUZAK (color ---)
SZLO @.
;
9.2. Write a NULL (n ---) word that n sets the byte of 0 with the predefined variable parser. So, for example, the
0 VARIABLE HUSZ 18 NO
command line generates a variable with 20 bytes parsing; of which 2 bytes are VARIABLE, and 18 bytes 0 start bytes are NULL.
: NULL (n ---)
0 DO 0 C, LOOP;
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