comp3-packed-decimal

1# COMP-3 / packed decimal storage
2 
3`USAGE COMP-3` (a.k.a. PACKED-DECIMAL) stores a signed numeric field as
4binary-coded-decimal nibbles: each byte holds two decimal digits, except the
5last byte whose low nibble is the sign:
6 
7```
8PIC S9(11)V99 USAGE COMP-3
9+--+--+--+--+--+--+--+
10|D1|D2|D3|D4|D5|D6|D7|   7 bytes total for 13 digits
11+--+--+--+--+--+--+--+
12              ^^-- sign nibble in the LOW half of the last byte
13```
14 
15Bytes-on-disk = `ceil((digit_count + 1) / 2)`. For `PIC S9(11)V99` that is
16`ceil(14 / 2) = 7` bytes. The high nibble of the first byte holds the first
17decimal digit; the implied decimal point is not stored.
18 
19Sign nibbles:
20 
21| Nibble | Meaning |
22|--------|---------|
23| `C`    | positive (some shops emit `F` for unsigned, but `C` is standard for SIGNED) |
24| `D`    | negative |
25| `F`    | unsigned / non-`S` PIC |
26 
27## Declaring COMP-3 in an FD
28 
29The record layout must match the byte budget. If the data in the file is
30packed but you declare the field as `PIC X(n)` or as an unpacked decimal,
31the COBOL runtime will not decode it and the value will look like binary
32garbage when displayed.
33 
34```cobol
35FD  MAST-FILE
36    RECORD CONTAINS 80 CHARACTERS.
3701  MAST-REC.
38    05  M-ACCT     PIC X(6).
39    05  M-TYPE     PIC X.
40    05  M-STATUS   PIC X.
41    05  M-CCY      PIC X(3).
42    05  M-CARRY    PIC S9(11)V99 USAGE COMP-3.   *> 7 bytes
43    05  FILLER     PIC X(62).                    *> remainder of 80
44```
45…