Secondary Bitmap
What it is
Bitmap indicating the presence of data elements 65–128. Present only when any of those elements is used.
The first data element is itself a bitmap. Every ISO 8583 message carries a primary bitmap, right after the MTI, covering DEs 1–64; if bit 1 of the primary bitmap is set, DE 1 — a second 64-bit map covering DEs 65–128 — immediately follows.
Bitmaps are transmitted as 8 binary bytes or 16 hex characters depending on the interface. Reading a message always starts here: the bitmap tells the parser exactly which elements are present and in what order.
Example values
8000000000000008Secondary bitmap present with DE 65 and DE 125 flagged (bit 1 and bit 61 of the secondary map).Specification reference
Defined in ISO 8583:1987 (base). Card networks publish their own dialect tables (Visa BASE I / V.I.P., Mastercard CIS) that refine formats, lengths, and — for private-use elements — the entire layout. Always confirm against the interface specification you implement.