COMP-3 / packed decimal storage

USAGE COMP-3 (a.k.a. PACKED-DECIMAL) stores a signed numeric field as binary-coded-decimal nibbles: each byte holds two decimal digits, except the last byte whose low nibble is the sign:

PIC S9(11)V99 USAGE COMP-3
+--+--+--+--+--+--+--+
|D1|D2|D3|D4|D5|D6|D7|   7 bytes total for 13 digits
+--+--+--+--+--+--+--+
              ^^-- sign nibble in the LOW half of the last byte

Bytes-on-disk = ceil((digit_count + 1) / 2). For PIC S9(11)V99 that is ceil(14 / 2) = 7 bytes. The high nibble of the first byte holds the first decimal digit; the implied decimal point is not stored.

Sign nibbles:

Nibble	Meaning
C	positive (some shops emit F for unsigned, but C is standard for SIGNED)
D	negative
F	unsigned / non-S PIC

Declaring COMP-3 in an FD

The record layout must match the byte budget. If the data in the file is packed but you declare the field as PIC X(n) or as an unpacked decimal, the COBOL runtime will not decode it and the value will look like binary garbage when displayed.

FD  MAST-FILE
    RECORD CONTAINS 80 CHARACTERS.
01  MAST-REC.
    05  M-ACCT     PIC X(6).
    05  M-TYPE     PIC X.
    05  M-STATUS   PIC X.
    05  M-CCY      PIC X(3).
    05  M-CARRY    PIC S9(11)V99 USAGE COMP-3.   *> 7 bytes
    05  FILLER     PIC X(62).                    *> remainder of 80

Use a runtime check: cobc -x -free -o prog PROG.cbl && ./prog and DISPLAY one record's COMP-3 field before relying on it. Common mistakes: