Multiply two extremely large numbers

' This is an example program written in QB45 to
' multiply two numbers using the handwritten way.
'
'  ie:
'   12
'   12
' -----
'   24
'  12
' -----
'  144
'
' This routine does not work for all numbers, though it
'  will calculate 2 numbers that are larger than a calculator
'  can handle.
' Remember, this is for learning purposes only.  The resultant
'  number can not always be trusted as the correct number.
'
'   Forever Young Software (1997)
'
'
DIM TcA AS STRING * 1, TcB AS STRING * 1
DIM ALen AS INTEGER, BLen AS INTEGER

A$ = "14395613774750208"     ' both must by same length
B$ = "05974790569203456"     ' add 0's at first if needed
ALen = LEN(A$)
BLen = LEN(B$)

REDIM AddA(BLen, ALen * 2 * 2)  AS INTEGER

CLS
PRINT
PRINT STRING$(BLen, " "); "   "; A$
PRINT STRING$(BLen, " "); " X "; B$
PRINT STRING$(BLen, " "); "---"; STRING$(ALen, "-")

BPos% = BLen
T% = BLen
FOR R% = 1 TO ALen
  TcB = MID$(B$, BPos%, 1)
  Carry% = 0
  AddA(R%, T% + 1) = 0
  APos% = ALen
  FOR T% = BLen TO 1 STEP -1
    TcA = MID$(A$, APos%, 1)
    Temp% = (VAL(TcB) * VAL(TcA)) + Carry%
    IF Temp% > 9 THEN
      Temp1% = Temp% MOD 10
      AddA(R%, T% + 1) = Temp1%
      Carry% = (Temp% - Temp1%) \ 10
    ELSE
      AddA(R%, T% + 1) = Temp%
      Carry% = 0
    END IF
    APos% = APos% - 1
  NEXT T%
  AddA(R%, 1) = Carry%
  BPos% = BPos% - 1
NEXT R%

FOR R% = 1 TO BLen
  PRINT STRING$(BLen + 3 - R%, " ");
  FOR T% = 1 TO ALen + 1
    PRINT RIGHT$(STR$(AddA(R%, T%)), 1);
  NEXT T%
  PRINT
NEXT R%
PRINT "   "; STRING$(BLen * 2, "-")


'  start to print answer

DIM Ans((BLen * 2) + 1) AS STRING * 1
AnsP% = (BLen * 2) + 1
Ans(AnsP%) = RIGHT$(STR$(AddA(1, ALen + 1)), 1)

Carry% = 0
FOR T% = ALen TO 1 STEP -1
  Temp2% = 0
  Temp% = 0
  FOR Tt% = 1 TO BLen
    Temp% = Temp% + AddA(Tt%, T% + (Tt% - 1))
  NEXT
  Temp% = Temp% + Carry%
  IF Temp% > 9 THEN
    Temp1% = Temp% MOD 10
    Temp2% = Temp1%
    Carry% = (Temp% - Temp1%) \ 10
  ELSE
    Temp2% = Temp%
    Carry% = 0
  END IF
  AnsP% = AnsP% - 1
  Ans(AnsP%) = RIGHT$(STR$(Temp2%), 1)
NEXT T%

FOR Rr% = 2 TO BLen
  T% = 0
  Temp% = 0
  FOR R% = Rr% TO BLen
    T% = T% + 1
    Temp% = Temp% + AddA(R%, T%)
  NEXT
  Temp% = Temp% + Carry%

  IF Temp% > 9 THEN
    Temp1% = Temp% MOD 10
    Temp2% = Temp1%
    Carry% = (Temp% - Temp1%) \ 10
  ELSE
    Temp2% = Temp%
    Carry% = 0
  END IF
  AnsP% = AnsP% - 1
  Ans(AnsP%) = RIGHT$(STR$(Temp2%), 1)
NEXT

PRINT "=>";
FOR R% = 1 TO (BLen * 2) + 1
  PRINT Ans(R%);
NEXT