Commodore PET User Manual page 233

}
Multiply A$ and B$
and align products
Input values for A$. B$
}
If multiplicand or
multiplier
=
0 then
answer (C$)
=
0
}sOl di.;d"
PO;"' f
At the end of step 3, the program looks Iike this:
213. INPUT A$, B$
313 IF VAL(A$)=a OR VAL(B$)=a THEN
C$:.::"a" :GOTO 1190
40 ZERO$=10a000000a0aaa000"
113013 X=LEN(A$):Y=LEN(B$)
10132 IF X>Y THEN F=X/2:GOTO 100e
1004 F='T'/2
1006 IF F>INT<F)THEN F=INT(F)+l
11308 F$=LEFT$(ZERO$,F)
1010 IF X<=F THEN AH=0:AL=VAL(A$):GOTO 1040
10213 AH=VAL<LEFT$<A$,X-F»
1030 AL=VAL(RIGHT$(A$,F»
1040 IF Y<=F THEN BH=0:BL=VAL(B$):GOTO 1070
1050 BH=VAL(LEFT$(B$,Y-F»
1060 BL=VAL(RIGHT$(B$,F»
1070 P1$=STR$(BL*AL)
1080 P2$=STR$(BL$AH)+F$
1090 P3$=STR$(BH*AL)+F$
1100 P4$=STR$(BH$AH)+F$+F$
Divide A$ and B$
into parts:
high and low
Step 4: Add the four products together. This is the most complicated part
of the "Multiple Integer Multiplication" program because parameters are passed
back and forth from the main program to an addition subroutine. We will use a
portion of the "Addition via Numeric Strings" program as a subroutine to add the
products together (page 190). Below is the portion of the addition program we will
convert into a subroutine:
2000 REM$$ADD PRODUCTS$$
2010 BLANK$="
2020 X=LEN(A$):Y=LEN(B$)
2030 IF X<Y THEN A$=LEFT$(BLANK$,Y-X)+A$
2040 IF X>Y THEN B$=LEFT$(BLANK$,X-Y)+B$
2050 D=0:N=1:C$="1
2060 FOR I=LEN(A$) TO 1 STEP-l
2070 A=VAL(MID$(A$,I,l»
2080 A=A+D:D=0
2090 B=VAL(MID$(B$, 1,1»
2100 C=A+B
2110 IF C>=10 THEN D=l
2120 IF D=l AND 1=1 THEN N=2
2130 C$=RIGHT$(STR$(C),N)+C$
2140 NEXT 1
At line 1110 the contents of P1$ and P2$ are passed to the parameters A$
and B$, which are used in the addition subroutine (Iines 2000 to 2140).
1110 A$=Pl$:B$=P2$
A$
19191918101010111
B$
1919191810101011101010101
220