WWIV BBS historically supports type-1 and type-2 message areas. The type-1 message area was the original type implemented in WWIV 3.21 and consists of 1 file for the header of all messages in an area, and 1 file per message containing the text of the message.
Type2 messages share the same 100 byte postrec for the headers in a SUBNAME.SUB file as type-1 message areas. The Text are contained in a single file named SUBNAME.DAT that lives in the MSGS directory.
On disk, the format is:
GAT | 2048 512-byte message blocks | [ GAT | 2048 512-byte message blocks ]*
The GAT is similar to the FAT table of a MS-DOS disk.
GAT: 2048 2-byte integers for the index of the message blocks. Let's say the message was stored in in gat #0 in blocks 1, 3, and 5, then the gat would contain:
block: 0 1 2 3 4 5 value: -1 3 -1 5 -1 -1