Commodore PET User Manual page 207

If C is greater than or equal to 10. the carry variable 0 is incremented to 1 at
line 1070; otherwise it remai ns 0:
1070 IF C)=10 THEN D;1
A
~
+B
~
C llij]-15>=10-D~
or
A ~
+B
~
C
~ -3<10-D~(nochange)
o
will be either
a
or 1. but never greater than 1 because the maximum possi-
ble sum of any two single-digit numbers is 18. thus the maximum tens value that
can be carried over is 1.
To prevent losing the carry in D. line 1040 resets the value of A to A
+
0 on
the next loop iteration:
1040 A;A+D:I1=0
If this statement were omitted. the carry would never be carried out. and the value
of A would be incorrect. When 0 is added to A. 0 is reset to
a
in preparation for
the next loop iteration.
Step 4: Link the individual
sums
(C) and convert the total sum into a
string. Just as the augend and addend were entered as strings to avoid the 9-
digit length Iimit, the sums must be converted back into a string to avoid the
length Iimit again during printing. This step converts the numbers back to string
form.
Line 1090 links the individual sums of C and converts the final answer back
into string form.
The STR$(C) function converts Cinto a one-digit string. The RIGHT$ func-
tion extracts the rightmost N characters from STR$(C). N is set to 1 at line 1000 to
indicate that we want only the rightmost character to be extracted; the leftmost
character of C is unnecessary because it is the sign value
("~"
if positive and "-"
if negative) and would be concatenated between each number of C$ if we were
not to exclude it.
11::101::1 N=1
N[ill]
1060 C=A+:B
C~=A~+B~
1090 C$=RIGHT$(STR$(C).N)+C$
C$=RIGHT$(STR$(C}, 1)+C$
C$=RIGHT$(!MlID.l )+C$
C$=1êI + C$
194