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3.9 XBRA procedure

The XBRA procedure ("eXtended BRAner") is based on an idea by Moshe Braner, and should be used by all programs that redirect vectors.

The principle is that each vector-redirecting program places the following structure directly before its own entry address (i.e. just before the address to which the vector was set):

typedef struct
    int8_t  xb_magic[4];   /* "XBRA" = 0x58425241         */
    int8_t  xb_id[4];      /* ID of four ASCII characters */
    int32_t xb_oldvec;     /* Old value of the vectors    */

The element xb_magic permits a positive identification of the XBRA structure; xb_id is the ID of the particular program that has redirected the vector.

With the aid of this procedure it is easy for a program to ascertain whether it is already installed (though this can be done also via the cookie jar), and to remove itself again from the vector chain. It is important, however, that all(!) programs adhere to this procedure, as otherwise the vector chain will be interrupted.

Note about MagiC: Programs that hook into any of the system vectors should also link themselves into the etv_term vector. If the program terminates abnormally, the operating system will jump first via the etv_term vector. In such a case one can then calmly draw back from all changed vectors; as MagiC uses its own etv_term vector for each application, this cannot lead to any collisions.

The XBRA procedure has won recognition over time, so that it's now considered bad programming style if one does not use it.

See also: Cookie jar   Cookie and XBRA list   Reset vector

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