A block of storage from which entries of a specified, fixed size can be allocated and deallocated quickly. Callers of the Ex(ecutive) Support zone routines must synchronize their allocation and deallocation of entries within their zones.
For example, the I/O Manager uses zones to implement lookaside lists for fast allocation and deallocation of IRPs and MDLs. As another example, some of the system-supplied SCSI class drivers use zones to allocate and release memory for SRBs.
A set of entry points parallel to the executive’s system services. A call to a ZwXxx entry point from kernel-mode code (including calls from other system services or NT drivers) supplies the corresponding system service, except the caller’s access rights and the arguments to the Zw “alias” are not checked for validity, and the call does not cause the previous mode to be set to user mode. It is the responsibility of a kernel-mode caller to check all user-supplied arguments for validity before such a caller passes them on to a ZwXxx entry point. See also system services, probe, and SEH.