Track 1 Data
What it is
Magnetic-stripe Track 1 (ISO 7813): format code, PAN, cardholder name, expiry, service code, discretionary data.
DE 45 on the wire
Bitmap position
DE 45 is bit 45 of the primary bitmap: byte 6, mask 0x08. A primary bitmap with only DE 45 present reads 0000000000080000 in hex.
Length
DE 45 is variable length (LLVAR): a 2-digit prefix gives the length of the value in characters, from 0 to 76. A 12-character value is sent as 12 followed by the value. In ASCII the prefix is the bytes 31 32; dialects that pack lengths as BCD send 12 instead.
Size in the message
At most 78 bytes when the message travels as text (ASCII or EBCDIC), or 77 bytes in binary dialects that pack the length prefix as BCD.
Format an
Format an means alphanumeric: letters and digits.
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.