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).DE 1 on the wire
Bitmap position
DE 1 is bit 1 of the primary bitmap: byte 1, mask 0x80. A primary bitmap with only DE 1 present reads 8000000000000000 in hex.
Length
DE 1 is fixed length: always 8 bytes, with no length prefix. A parser reads exactly that many and moves on.
Size in the message
Exactly 8 bytes of binary data. Interfaces that carry binary data as text send each byte as two hex characters, so the value takes 16 characters there.
Format b
Format b means binary: raw bytes, often shown or sent as hexadecimal text.
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.