Xz77gt

variable being created it checks
the syntax of the variable name.
Then an area of RAM is used to
store the value along with the
variable name.
If a reserved word has been
found the position of the token
value in the ROM list
determines where the program
jumps to in the Basic ROM to
verify the correct syntax of any
characters which follow the
word until a colon or zero byte
is discovered, For example, if
the line was PRINT "HELLO",
the PRINT token would be 99
Hex, The system would then
expect a variable name, a
quotation mark, a colon or a
zero. In this case it finds a
space. which is ignored, followed
by a quotation mark.
The system then takes the
ASCII code from between the
quotes one by one. The ASCII is
converted into a screen POKE
value and this places the letters
which spell HELLO on the next
available screen location.
When the end of a line is
found the line link is retrieved
from memory and the next line
link is stored along with the
new line number. The line is
then copied into the Basic
buffer and the line is scanned
and interpreted as before.
This process continues until
there are no more lines left or
an END or STOP command is
located.
Obviously with all this
memory movement, evaluation
and syntax checking going
on the program runs a lot more
slowly than a pure machine
code program written
specifically to create the same
effect but it should be remembered
that all programs are
executed by the microprocessor
in machine code, Basic is
only a figment of the ROM's
memory. When a language
such as Logo or Pascal is loaded
the Basic ROM is switched out
in favour of the new machine
code routines which interpret
the syntax of the new language.
Any language on a
computer is a language of
convenience rather like
Margaret Thatcher and
President Mitterand discussing
policies through the medium
of an interpreter. As we all
know this not only lengthens
the time taken for the
discussion but can also lead to
misunderstandings unless
great care is taken.
Next month we shall see
how the processor helps to
store numerical and string
variables.
400 FORL=1T04:60SU8690:PRINT
RES(11):60SU81150:NEXT:REM LD
907:LDA1120
410 FORBL=7T00 STEP-I:YRS."(
LEFT)-':RESTORE:BLS=RIGHTS(
STRS(BL),I)
420 IF8L=6THENXX=0:PA=3
430 IFBL=5THENPA=0
440 FORL=0102:605U8690:PR:NT
RES(01):60SU81150:NEXT:REM SY
A 0800,Y
450 60SUB710ta="(R6H1)(R6HT
)0a+BLS
460 P0S=ACS(6)
,0 "+CS:60SUB1190
(LEFT)(LEFT)
470 READW:606U81150:PRINTRES
(0)
480 PRINTMS(2):POS=MDP'080"
+BLI:60SUB1190
490 CS=
1439313532
20 (RVS) 1
"+MI0 , 500 TS="(HOME)10804RISHTI(S
S("13 81.12+1,2)
TROL),1)4"(0001N)(LEFT)(LEF
19133 :606U811
THLEFTI(LEFT)(LEFT)a+RISHTS
(51S,BL4.3)
510 PRINT4BS:PO3=AES(4)+"20"
:60SUB1190
520 60SUB1000:PRIN
-SUB10001READCS:60SUB1120
RES(0):60
530 PRINTa(HOMEHDOWN)
540 P0S=1:606U81190
550 PRINT"(HOME) (DOWNI(DOWN)
POISHTS(ACS,BL+2);"
560 609U8690:PRINTRES(I):605
UB1150
570 YPS="1(DOWN)(LEFT) 1(DOWN
)(LEFT)1(DOWN)(LEFT)(LEFT)--
1•:60SUB710
580 CS="0
1,1):IFEL=0THENC$
+RIGHT 590 0 READW:60SUB1150:PR!NTRES
S(STRII -1" (0)
(BL-1) 600 FORIAT01:605U8690:PRINT
RES(W):60SUB11601NEXT
610 IFBL=OTHENREADAS,AS:XX=5
014=7:60T0630
620 READCS,0:606U81160
630 606UB960:PRINTDPS:FORD=0
TOXX310:NEXTIPRINTRES(0):NEX
TBL
640 60SU8690:PRINTRES(W):60S
UB1150
650 PRINT"(HOME)(DOWN)(DOON)
(DONN)(DONN)(R6HT) (RGHT)READ
Ya:XX=250
660 POW(HOME)(DOWN)(DOWN)(
DOWNI(DOWN)(DOWN)(R6HT)(R6HT
I 1
:
6
0
6
U
12
670 PRINTa(CLR)"
680 END
690 60SU81060:60SUB1100:READ
CS:60SUE11120:FOROOT01:60SUB
990:NEXT:READC$01
700 RETURN
710 PRINTYWYPS:6OSUB960:RE
TURN
720 PRINTIOBOO (C64) ______
730 PRINT' I
I
740 . PRINT" ISYS491521 HRVS)
Y RES (OFF)I"
750 PRINT"
760 PRINT' 1
1 a
I I
770 PRINT' 1 I HRVS)
X RES (OFF)la
780 PRINT'
a
1
770 PRINT" 1
1'
I I
800 PRINT" 1
ACC (OFF)1
I IERVS)
810 PRINT' L(RVS)
----
(OFF 820 PRINT" HRVS)SCREEN(OFF
I RRVS) C000 (OFF)!
830 PRINT' HRVSIP
RDSRAM(OFF)I°
840 PRINT' __ 1111VS)C
OUNTER(OFF)1"
850 PRINT" I 1
a
860 PRINT" 1 1 I
la
870 PRINT' (RVS) (OFF)
----ERVS)ADDRESSIOFFI"
880 PRINT'
a
890 PRINT' (RVS) (OFF)____
900 PRINT' (RYS) DATA(OFF)I
RRVS) DECODER (OFF)1
910 PRINT' (RVS) BUS(OFFII
920 PRINT" (RVS) (OFF) (R
VS) (OFF) (RVS) (OFF)
(RVS) (OFF)"
930 PRINT (RVS) (OFF) (R
VS) (OFF) (RVS) (OFF)
(RVS) (OFF)"
940 RETURN
950 REM 11111 DELAY
960 FORDOTOXX:NEXT
970 RETURN
980 REM IS1: DATA BUS
990 FOR 8=1T03:60T01010
Memory
1000 FOR B=3TOISTEP-1
1010 PRINTRW(R6HTI(RVS)
CB) 1
1020 0 FORC=1102:PRINT"(RSHT)(
RVS)18$(8)D8S(B)D8S(B)140(8
):NEXT
1030 605U89601NEXT
1040 RETURN
1050 REM MI PC VALUE
1060 READPS
1070 POS=ACS(3)+
+":60SUB11901PRINTN(RVS)"
6
(LEFT) PDS "+PS
1080 PRINT/ISM:RETURN
1090 REM 1111 MEMLOC
1100 605U8960:PRINTMDSPS:RET
URN
1110 REM 3111 MEMCONT
1120 POSOID14."(DOON)(R6HT)1
M60901190 4.
1130 PRINTMS(0):RETURN
1140 REM MI DECODER
1150 IFOITHENPRINTACS(10"
1160 P0S=ACS(11)43:60SU81190
1170 PRINTRECOMETURN
1180 REM Mt FLASH
1190 FORA=OTOPAIPRINT"(RVS)"
P01"(0FFIN:60SUB960:PRINTPOS
:609U0960:NEXT:RETURN
1200 REM litt DATA
1210 DATA0004,99,"STA 0
11,0005,00,
8,2 , 1
1220 00', DATA2,20
1230 2,C0 DATAC007,88,1EY " ,
1,6 06,0
1240 8,0 DATAC008,10,BPL,14009,
FA,"(R6HT)(R6HTIORSHTI(R6HT)
SFA
1004'0
,1 1250 DATA000A,60,"RTS a,
1
"(R 1260 DATACPU DEMONSTRATION,"
6HT :2
HOW I(R - SOW
6HT !"
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