27.2.5. Software Poll Mechanism

The processor will complete the execution of the current instruction and give service to the devices by indicating with another signal called acknowledgment signal. If the processor is engaged in important work and receives an interrupt, we may choose to disallow further interrupts by using a flag bit called interrupt enable. Setting it enables interrupts, while disabling it prevents the processor from servicing interrupted devices until the current work is finished.

When the CPU is executing instructions and receives an interrupt signal from a device, it finishes the current instruction and prepares to service the interrupt. This is done using an acknowledgment signal. However, if the CPU is busy with critical tasks, we can modify how it handles interrupts using a flag bit called 'interrupt enable'. If this flag is set, it allows the processor to respond to interrupts even during execution; if it's not set, the processor will ignore interrupts until it completes its current task. This mechanism provides control over when the CPU can be interrupted.

Imagine a chef cooking a delicate dish in a restaurant. If a waiter brings an important order, the chef must finish the current dish before taking any new orders (current instruction). However, if the chef can’t be interrupted for a more urgent order (an important task), they can choose to temporarily ignore new requests (disabling interrupts) until the dish is done.

The programmer is responsible for enabling and disabling interrupts. If an interrupt service routine begins with an 'interrupt disable' command, that command needs to be followed with an 'interrupt enable' command before returning to the main program. Failing to re-enable interrupts will mean that additional interrupts will not be serviced.

In programming, when an interrupt service routine is executed, it's crucial to manage interrupt flags correctly. If the routine starts by disabling interrupts (using 'interrupt disable'), it is the programmer's duty to ensure that interrupts are enabled again before returning from the routine. If the interrupts are left disabled, the processor will not respond to any new interrupts, which could lead to significant issues, such as missing important signals from devices.

Think of a teacher (programmer) in a classroom (processor) who decides not to allow any students to ask questions (disabling interrupts) while teaching a complex topic. If the teacher forgets to allow questions again before starting the next topic, the students can't ask crucial questions, which could lead to misunderstandings later.

Flag bits also include a supervisor mode to determine the level of access a user has in the operating system. In UNIX or Linux systems, a root user can perform many system operations that regular users cannot. The supervisor mode flag controls these permissions.

In computer systems, particularly in operating systems like UNIX or Linux, there are different user levels of access. A user operating in 'root' or supervisory mode has elevated permissions and can execute a wider range of commands compared to regular users, who operate in user mode. The permission levels are managed through flag bits, ensuring that only authorized users can modify critical system settings.

Consider a building where only the building manager (root user) can access all rooms and control the utilities, while regular occupants (regular users) have keys to their apartments but cannot enter rooms for maintenance. This access control ensures security and proper management within the building.

To determine which device sent an interrupt when multiple devices are connected, a software polling mechanism is employed. The CPU runs a routine to check each I/O module in turn and identify the source of the interrupt by querying their status.

In scenarios where several devices could generate interrupts, the CPU cannot immediately know the source of the interrupt. To address this, a software polling routine is executed. This routine systematically checks each device to see if it sent the interrupt signal. The CPU queries each device sequentially; if a device responds affirmatively, it can proceed with servicing that device. This method is a way to manage multiple interrupts effectively.

Imagine a restaurant where the chef (CPU) has multiple waiting tables (I/O devices). When customers signal for service (send interrupts), the chef walks to each table in order, asking if anyone needs help (polling). Only when a table responds will the chef provide service, ensuring no customer is missed.

In hardware polling, devices connected in a chain pass an interrupt acknowledgment signal down the line. The device that triggered the interrupt responds with its identity, enabling the CPU to find out which device requires servicing. The bus master system is another method, where the device controlling the bus can raise an interrupt.

Hardware polling involves a physical structure where devices are connected in a daisy chain. An interrupt acknowledgment signal is sent down this chain, allowing each device to respond if it raised an interrupt. This approach allows the CPU to identify which device requires service without extensive software logic. Additionally, in bus master systems, only the device that controls the bus can raise interrupts, making identification more streamlined.

Think of a train system (hardware polling) where cars are linked together. When one car (device) has an issue, it sends a signal to the next car to inform the conductor (CPU) which car needs attention. In a bus master system, imagine only the train engine (bus master) can signal issues to the conductor, simplifying communication and ensuring that only one source of a problem is relayed at a time.

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