The document discusses CPU registers, instruction execution, and interrupts in operating systems, detailing various types of registers and their roles during instruction execution cycles. It explains the instruction execution process, including fetching and executing instructions, as well as distinguishing between hardware and software interrupts. Additionally, it covers different classes of interrupts and the sequence of events triggered by them.

1. Unit -1
CPU REGISTER, INSTRUCTION EXECUTION AND INTERRUPTS IN
OPERATING SYSTEM BY RAM K PALIWAL
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2. CPU Register
CPU registers
The registers vary in number and type, depending on the computer architecture.
◦ Accumulators
◦ Index registers
◦ Stack pointers
◦ General-purpose registers
◦ condition-code information registers.
When any process is running along with the program counter, this state
information (CPU register information) must be saved when an interrupt occurs,
to allow the process to be continued correctly afterward
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3. Instruction Execution
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All computers have an instruction execution cycle. A basic instruction
execution cycle can be broken down into the following steps:
1. Fetch cycle
2. Execute cycle

4. Instruction Execution
The fetched instruction is loaded into the instruction register (IR). The
instruction contains bits that specify the action the processor is to take. The
processor interprets the instruction and performs the required action. In
general, these actions fall into four categories:
1. Processor-memory: Data may be transferred from processor to memory or
from memory to processor.
2. Processor-I/O: Data may be transferred to or from a peripheral device by
transferring between the processor and an I/O module.
3. Data processing: The processor may perform some arithmetic or logic
operation on data.
4. Control: An instruction may specify that the sequence of execution be
altered.
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5. Interrupts
Virtually all computers provide a mechanism by which other
modules (I/O, memory) may interrupt the normal sequencing of the
processor.
Interrupts are provided primarily as a way to improve processor
utilization.
For example, most I/O devices are much slower than the processor.
Types of Interrupts:
Hardware
Software
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6. Interrupts
Hardware Interrupts: If the signal for the processor is from external
device or hardware is called hardware interrupts.
Example: from keyboard we will press the key to do some action this
pressing of key in keyboard will generate a signal which is given to
the processor to do action, such interrupts are called hardware
interrupts.
Hardware interrupts can be classified into two types they are
◦ Maskable Interrupt: The hardware interrupts which can be delayed when a
much highest priority interrupt has occurred to the processor.
◦ Non Maskable Interrupt: The hardware interrupts which cannot be delayed
and should process by the processor immediately.
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7. Interrupts
Software Interrupts: Software interrupt can also divided in to two
types.
◦ Normal Interrupts: the interrupts which are caused by the software
instructions are called software instructions.
◦ Exception: unplanned interrupts while executing a program is called
Exception. For example: while executing a program if we got a value
which should be divided by zero is called a exception.
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8. Interrupts
Classes of Interrupt
Program: Generated by some condition that occurs as a result of an
instruction execution, such as arithmetic overflow, division by zero,
attempt to execute an illegal machine instruction, and reference outside a
user’s allowed memory space.
Timer: Generated by a timer within the processor. This allows the
operating system to perform certain functions on a regular basis.
I/O: Generated by an I/O controller, to signal normal completion of an
operation or to signal a variety of error conditions
Hardware Failure: Generated by a failure, such as power failure or
memory parity error.
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9. Interrupt Processing
An interrupt triggers a number of events, both in the processor
hardware and in software. Figure shows a typical sequence. When
an I/O device completes an I/O operation, the following sequence of
hardware events occurs:
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10. Interrupt
Processing
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11. References
1. Operating System Concept ninth Edition by Abraham Silberschatz and Galvin
2. Operating Systems, internals and Design Principles, sixth edition by William Stalling
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