The Interrupt Descriptor Table (IDT) is a data structure used by the CPU to manage and handle hardware interrupts. It's a table containing interrupt descriptors, one for each possible interrupt vector.

Interrupt Vectors: Each hardware device is assigned a unique interrupt vector. This vector is an index into the IDT. When an interrupt occurs, the CPU uses the interrupt vector to locate the corresponding interrupt descriptor in the IDT.

Interrupt Descriptor: The interrupt descriptor contains information about the interrupt handler, including:

• The memory address of the interrupt handler routine (ISR).
• The privilege level required to execute the ISR.
• Information about the flags that should be set when the interrupt is triggered.

When an interrupt occurs, the CPU uses the interrupt vector to fetch the interrupt descriptor from the IDT. It then loads the address of the ISR from the descriptor and transfers control to the ISR, initiating the interrupt handling process. The IDT is crucial for directing the CPU to the correct handler for each type of interrupt.

Maximum Addressable Memory: With a 32-bit address bus, the maximum amount of memory that can be directly accessed is 2³² bytes. This is equal to 4,294,967,296 bytes, or 4 GB. Therefore, the system can directly access 4GB of memory.

Impact of Data Bus Width on Performance: The data bus width significantly impacts system performance. A wider data bus allows more data to be transferred in a single clock cycle. This reduces the number of clock cycles required to complete a data transfer, leading to faster execution of instructions and improved overall system speed. For example, a 64-bit data bus can transfer twice as much data per cycle as a 32-bit data bus, assuming all other factors remain constant.

Sequence of Events for Reading Data from Memory:

1. The CPU places the memory address of the desired data onto the address bus.
2. The CPU asserts the Read signal on the control bus.
3. The memory controller receives the address and the Read signal.
4. If the memory is ready (MRD is asserted), the memory controller places the data stored at the specified address onto the data bus.
5. The CPU reads the data from the data bus.
6. The memory controller deasserts the Read signal.

Here's a breakdown of the interrupt handling process for a keyboard press:

1. Interrupt Signal: The keyboard controller generates an interrupt signal and asserts it on the CPU's interrupt line.
2. Interrupt Acknowledgment: The CPU acknowledges the interrupt signal. This typically involves the CPU finishing the current instruction and saving the current state of the processor.
3. Interrupt Vector Lookup: The CPU uses the interrupt line to determine the interrupt vector associated with the keyboard. This vector is used to index into the IDT.
4. Interrupt Descriptor Retrieval: The CPU fetches the interrupt descriptor from the IDT using the interrupt vector as the index.
5. Saving CPU State: The CPU pushes the current state of the processor (e.g., program counter, registers) onto the stack. This is crucial for returning to the interrupted program later.
6. Executing the ISR: The CPU loads the address of the ISR (obtained from the interrupt descriptor) into the program counter and transfers control to the ISR.
7. ISR Execution: The ISR performs the necessary actions, such as reading the keycode from the keyboard controller's buffer.
8. Restoring CPU State: Before returning from the ISR, the ISR restores the saved CPU state from the stack.
9. Return from Interrupt: The ISR executes a special instruction (e.g., IRET in x86) to return control to the interrupted program. This instruction pops the saved CPU state from the stack.
10. Resuming Execution: The CPU resumes execution of the interrupted program from the point where it was interrupted.

The CPU's stack is essential for saving and restoring the processor's state during interrupt handling. This ensures that the interrupted program can resume execution correctly after the interrupt has been handled.