3.2.4 Modifying a Parameter Packet
Some parameters passed to system calls use a packet format. The packet format parameters are of two kinds, either input parameters passing information to a system call (e.g., T_CTSK) or output parameters returning information from a system call (e.g., T_RTSK).
Additional information that is implementation-dependent can be added to a parameter packet. It is not allowable, however, to change the data types and order of information defined in the standard speci.cation or to delete any of this information. When implementation-dependent information is added, it must be positioned after the standard defined information. When implementation-dependent information is added to a packet of input information passed to a system call (T_CTSK, etc.), if the system call is invoked while this additional information is not yet initialized (memory contents indeterminate), the system call must still function normally.
Ordinarily a .ag indicating that valid values are set in the additional information is defined in the attribute .ag implementation-dependent area included in the standard speci.cation. When that .ag is set (1), the additional information is to be used; and when the .ag is not set (0), the additional information is not initialized (memory contents indeterminate) and the default settings are to be used. The reason for this speci.cation is to ensure that a program developed within the scope of the standard speci.cation will be able to run on an OS with implementation-dependent functional extensions, simply by recompiling.
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