Editing OpenVMS (section)

===Executive and Kernel===

The OpenVMS Executive comprises the privileged code and data structures which reside in the system space. The Executive is further subdivided between the ''Kernel'', which consists of the code which runs at the kernel access mode, and the less-privileged code outside of the Kernel which runs at the executive access mode.<ref name="idsm-5.2" />

The components of the Executive which run at executive access mode include the [[Record Management Services]], and certain system services such as image activation. The main distinction between the kernel and executive access modes is that most of the operating system's core data structures can be read from executive mode, but require kernel mode to be written to.<ref name="goatley-part-1" /> Code running at executive mode can switch to kernel mode at will, meaning that the barrier between the kernel and executive modes is intended as a safeguard against accidental corruption as opposed to a security mechanism.<ref>{{cite conference|url=https://www.scs.stanford.edu/nyu/04fa/sched/readings/vmm.pdf|title=A VMM security kernel for the VAX architecture|author1=Paul A. Karger|author2=Mary Ellen Zurko|author3=Douglas W. Benin|author4=Andrew H. Mason|author5=Clifford E. Kahnh|date=May 7–9, 1990|conference=Proceedings. 1990 IEEE Computer Society Symposium on Research in Security and Privacy|access-date=January 31, 2021|publisher=IEEE|doi=10.1109/RISP.1990.63834}}</ref>

The [[Kernel (operating system)|Kernel]] comprises the operating system's core data structures (e.g. page tables, the I/O database and scheduling data), and the routines which operate on these structures. The Kernel is typically described as having three major subsystems: I/O, Process and Time Management, Memory Management.<ref name="idsm-5.2" /><ref name="goatley-part-1" /> In addition, other functionality such as [[Files-11#Logical names|logical name]] management, synchronization and system service dispatch are implemented inside the Kernel.

OpenVMS allows user-mode code with suitable privileges to switch to executive or kernel mode using the <code>$CMEXEC</code> and <code>$CMKRNL</code> system services, respectively.<ref>{{cite web|url=https://vmssoftware.com/docs/VSI_SYS_SERVICES_REF_VOL_I.PDF|title=VSI OpenVMS System Services Reference Manual: A–GETUAI|publisher=VSI|date=June 2020|access-date=February 15, 2021}}</ref> This allows code outside of system space to have direct access to the Executive's routines and system services. In addition to allowing third-party extensions to the operating system, Privileged Images are used by core operating system utilities to manipulate operating system data structures through undocumented interfaces.<ref name="inside-vms">{{cite book|title=Inside VMS: The System Manager's and System Programmer's Guide to VMS Internals|url=https://books.google.com/books?id=c4UhAQAAIAAJ|isbn=0-442-00474-5|author=Wayne Sewell|year=1992|publisher=Van Nostrand Reinhold}}</ref>