Kernel Mode
In the context of IBM z/OS, **Kernel Mode** refers to a highly privileged execution state of the CPU where the operating system's core components and trusted system programs run. It grants unrestricted access to all system resources, including memory, I/O devices, and privileged CPU instructions, enabling the operating system to manage and control the entire system. This mode is essential for maintaining system integrity, security, and stability.
Key Characteristics
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- Privileged CPU State: The CPU operates in a special mode (often referred to as Supervisor State in z/Architecture) that allows execution of all instructions, including those restricted from user programs.
- Full System Resource Access: Programs running in kernel mode have direct and unrestricted access to all hardware resources, such as memory, I/O channels, control registers, and CPU features.
- Operating System Core: The z/OS kernel, including the Supervisor, I/O Supervisor, Virtual Storage Manager, and other foundational components, executes in kernel mode.
- Protection Mechanism: It acts as a fundamental security and stability mechanism, preventing user applications from directly manipulating critical system resources or interfering with other programs.
- PSW Control: The Program Status Word (PSW) contains a control bit (the
Pbit for Problem/Supervisor state) that determines if the CPU is in Problem State (user mode) or Supervisor State (kernel mode).
Use Cases
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- Executing Supervisor Calls (SVCs): When a user program needs an operating system service (e.g., allocating memory, performing I/O, creating a task), it issues an
SVCinstruction, which causes a switch to kernel mode for the OS to handle the request. - I/O Operations: All direct interaction with I/O devices (disk, tape, network) is performed by the operating system's I/O supervisor, which runs in kernel mode to manage channels, control units, and devices.
- Memory Management: The Virtual Storage Manager (VSM) and other memory management routines operate in kernel mode to allocate, deallocate, and protect virtual and real storage across different address spaces.
- Task and Dispatcher Management: The z/OS dispatcher, which manages CPU allocation among tasks and address spaces, runs in kernel mode to perform context switching and manipulate task control blocks.
- System Initialization and Recovery: During system IPL (Initial Program Load) and in response to system errors, critical recovery routines execute in kernel mode to initialize hardware, load the OS, and restore system integrity.
- Executing Supervisor Calls (SVCs): When a user program needs an operating system service (e.g., allocating memory, performing I/O, creating a task), it issues an
Related Concepts
Kernel Mode is synonymous with Supervisor State in z/Architecture, contrasting sharply with Problem State (user mode), where application programs execute with restricted privileges. Transitions between these states are typically initiated by Supervisor Call (SVC) instructions or Program Call (PC) instructions, allowing user programs to request OS services. The Program Status Word (PSW) is crucial, as its state bit (P bit) defines the current execution mode, enforcing the separation and protection of system resources from user applications within an Address Space.
- Minimize Privileged Code: Design system components to perform only necessary operations in kernel mode, returning to problem state as quickly as possible to reduce the attack surface and potential for system instability.
- Thorough Testing and Review: Any code intended to run in kernel mode must undergo rigorous testing and code review due to its potential to impact the entire system and its critical nature.
- Secure Coding Practices: Implement robust error handling