Internal structure of Xilinx bitstream (.bit) file

Xilinx_bit_file_explained.txt

The Xilinx .bit format is pretty simple.  It uses keys and lengths to
divide the file.

Parser source code: https://gist.github.com/warmonkey/49dd574fda39f88b5eed5edc92707cf2

Here is an example.  Below is a hex dump from the beginning of a .bit file:

00000000:  00 09 0f f0 0f f0 0f f0 0f f0 00 00 01 61 00 0a  .............a..
00000010:  78 66 6f 72 6d 2e 6e 63 64 00 62 00 0c 76 31 30  xform.ncd.b..v10
00000020:  30 30 65 66 67 38 36 30 00 63 00 0b 32 30 30 31  00efg860.c..2001
00000030:  2f 30 38 2f 31 30 00 64 00 09 30 36 3a 35 35 3a  /08/10.d..06:55:
00000040:  30 34 00 65 00 0c 28 18 ff ff ff ff aa 99 55 66  04.e..(.......Uf

Field 1
2 bytes          length 0x0009           (big endian)
9 bytes          some sort of header     0f f0 0f f0 0f f0 0f f0 00

Field 2
2 bytes          length 0x0001           (key-length-value below)

Field 3
1 byte           key 0x61                (The letter "a")
2 bytes          length 0x000a           (value depends on file name length)
10 bytes         string design name "xform.ncd" end with \0

Field 4
1 byte           key 0x62                (The letter "b")
2 bytes          length 0x000c           (value depends on part name length)
12 bytes         string part name "v1000efg860" end with \0

Field 4
1 byte           key 0x63                (The letter "c")
2 bytes          length 0x000b
11 bytes         string date "2001/08/10" end with \0

Field 5
1 byte           key 0x64                (The letter "d")
2 bytes          length 0x0009
9 bytes          string time "06:55:04" end with \0

Field 6
1 byte           key 0x65                (The letter "e")
4 bytes          length 0x000c2818       (configuration length)

796696 bytes     raw bitstream starting with ffff ffff aa99 5566 sync word