27.1.3. Supervisor Mode and User Mode

So, programmer cannot set or reset those particular bits ok. These flag bits will be always affected by the result of an ALU, but ... is your interrupt enable and disable.

In computing, certain bits in the processor are designated as flag bits, which are automatically altered by the Arithmetic Logic Unit (ALU) during operations. Unlike regular variables, programmers cannot manually set or reset these bits. One such crucial flag bit is the interrupt enable flag. When this flag is set, it allows the processor to respond to interrupts from hardware devices. If set to disable, the processor will ignore any incoming interrupts until the current task is completed.

Think of the interrupt enable flag like a do-not-disturb sign on a hotel room. When the sign is on, hotel staff cannot enter to clean or attend to needs (i.e., ignore interrupts). When you take the sign off, they can come in and check on you (i.e., allow interrupts).

Now, what will happen if whenever you are doing, if some device is going to interrupt? We are bound to give the service to the interrupted devices...

When a device sends an interrupt signal to the processor, the processor must finish executing the current instruction before it can service the interrupt. This is crucial in scenarios where the processor is performing critical tasks. For instance, in aviation systems, it might be handling data from various aircraft sensors. During such operations, certain interrupts should be disregarded to maintain focus on the priority task. That's where the interrupt enable and disable flags come into play - they help in managing whether the processor can be interrupted.

Imagine a chef in a busy kitchen. If he is prepping a meal for a VIP customer, he won't want anyone interrupting him with trivial requests. He can set up a 'do not disturb' sign while he focuses on the important task. Once he's completed the meal, he can then take care of other requests.

So, responsibility lies with the programmer who is going to write the interrupt service routine...

Programmers are responsible for ensuring that they handle interrupts correctly within their code. This means they must enable interrupts before exiting an interrupt service routine after they have completed their necessary operations. If a programmer fails to enable the interrupt flag again, the processor will continue to ignore any interrupts, which may lead to system responsiveness issues.

Think of a traffic officer directing cars at a busy intersection. If the officer finishes a task but forgets to signal the next flow of traffic, vehicles will remain stalled, causing backups. Similarly, if a programmer neglects to re-enable interrupts, the system will not respond to important signals from other devices.

So, similarly we are having one particular flag bits which is the supervisor mode...

The concepts of supervisor mode and user mode pertain to how a processor manages different levels of access for executing programs. In supervisor mode, typically granted to root or admin users, the software can perform any operation and access all system resources. Conversely, in user mode, regular users run programs with restricted privileges, meaning they cannot affect critical system settings. This dual mode safeguards against unauthorized access and unintentional harm to the system's integrity.

Consider a building where only the janitor and management have keys to all rooms. This represents supervisor mode, where they can enter any area and manage resources. However, regular employees can only enter their respective offices and common areas, reflecting user mode. This hierarchy prevents mishaps and maintains order within the building.

So, some bits are set by the programmer and some bits cannot be set by the programmer...

The program status word (PSW) is a key aspect of processor architecture that includes important status bits about the operation of the processor. Some of these bits, like the interrupt enable/disable flag and supervisor/user mode bits, can be modified by programmers, allowing for control over how a program interacts with the system. Other bits are set and changed automatically by the processor during execution and play crucial roles in managing the state and flow of program execution.

Think of the PSW like a control center dashboard for an airplane. Some controls can be altered by the pilot (programmer), like adjusting the altitude or speed, while other indicators (automatically changing lights and alerts) are managed by the aircraft's systems without the pilot's input.

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