ebcdic-overpunch-decoding

EBCDIC overpunch (zoned decimal sign)

On classic mainframe tapes, signed numeric amounts are stored as N-1 digits followed by a single byte where the high-order bits carry the sign and the low-order bits carry the final decimal digit. When the file is converted to ASCII / printable form, the last byte is rendered as a letter (or a brace).

Sign and digit table

Char	Sign	Last digit		Char	Sign	Last digit
{	+	0		}	−	0
A	+	1		J	−	1
B	+	2		K	−	2
C	+	3		L	−	3
D	+	4		M	−	4
E	+	5		N	−	5
F	+	6		O	−	6
G	+	7		P	−	7
H	+	8		Q	−	8
I	+	9		R	−	9

Positive zero is {; negative zero is }. Both decode to magnitude 0 — keep the sign attached so downstream arithmetic doesn't drop a debit's polarity.

A common defect

The positive half of the table ({, A–I) is the visible-looking case and is often handled first. The negative half (}, J–R) looks like ordinary letters and is easy to forget. If only the positive letters are decoded, debits and credits both come through as positive — the totals look plausible but signs are wrong on every negative row.

Decoding pattern (placeholder names)

Split the field into the leading digits and the overpunch byte, look up the sign + units digit from the table above, recombine, then build the signed working-storage value. The shape below uses generic identifiers — substitute your own field names:

       *> placeholders: digits-buf holds the recombined unsigned digits,
       *> op-char is the overpunch byte, sgn / lo-digit are scratch.
       split raw-amount into (leading-digits, op-char)
       EVALUATE op-char
           WHEN "{"  sgn := "+"  lo-digit := 0   *> positive zero
           WHEN "A" THRU "I"  sgn := "+"  lo-digit := 1..9
           WHEN "}"  sgn := "-"  lo-digit := 0   *> negative zero
           WHEN "J" THRU "R"  sgn := "-"  lo-digit := 1..9
       END-EVALUATE
       digits-buf := leading-digits with lo-digit appended
       signed-value := (sgn = "-")  ?  -digits-buf  :  +digits-buf