Signed overpunch

In computing, a signed overpunch is a coding scheme which stores the sign of a number by changing (usually) the last digit. It is used in character data on IBM mainframes by languages such as COBOL, PL/I, and RPG.^[1] Its purpose is to save a character that would otherwise be used by the sign digit.^[2] The code is derived from the Hollerith Punched Card Code, where both a digit and a sign can be entered in the same card column. It is called an overpunch because the digit in that column has a 12-punch or an 11-punch above it to indicate the sign. The top three rows of the card are called zone punches,^[3] and so numeric character data which may contain overpunches is called zoned decimal.

In IBM terminology, the low-order four bits of a byte in storage are called the digit, and the high-order four bits are the zone.^[4] The digit bits contain the numeric value 0–9. The zone bits contain either 'F'x, forming the characters 0–9, or the character position containing the overpunch contains a hexadecimal value indicating a positive or negative value, forming a different set of characters. (A, C, E, and F zones indicate positive values, B and D negative).

The PACK instruction on IBM System/360 architecture machines converts the sign of a zoned decimal number when converting to packed decimal, and the corresponding UNPK instruction will set the correct overpunched sign of its zoned decimal output.^[5]