Writing a new driver

Hello mates,
I am trying to write a simple ODBC driver for Windows. It can also be OLE DB or DAO driver but the final aim is to connect to a non-standard DBMS normally through ADO, DAO, OLE DB or ODBC etc. I tried to find any documentation o nline but it is very little. There are a few SDKs but they are not free so I cannot use any of them.
In case any of you, who have developped the psqlODBC driver, know any source, have any tutorial, or can explain how can someone can write a simple read (and maybe after this read/write) driver please send it to me by email.
I imagine it will be sort of a program implementing a standart interface which o n its back end will interact customly with a DBMS, files etc.
Any help will be appreciated.
Regards,
Atanas Atanasov

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