Registers and the Fetch-Execute Cycle in Computer Architecture

This report delves into the core concepts of computer architecture, specifically focusing on the role and function of various processor registers within the fetch-execute cycle. It begins by defining key registers such as the Program Counter, Current Instruction Register, Memory Address Register, and Memory Data Register, outlining their individual purposes in storing addresses, instructions, and data. The report then explains the fetch-decode-execute cycle, illustrating how each register contributes to instruction processing. It covers the loading of instructions, decoding operations, and the execution phase using accumulators for arithmetic and logical operations. The report also discusses polling and interrupts, explaining how these mechanisms handle device access and signal urgent events. It provides a comprehensive overview of register functionality, emphasizing their importance in the overall performance and efficiency of computer systems. The document includes references to relevant academic resources.

Computer architecture and
performance

Registers and role in architecture
• Following are the some of the registers that are used in the
computers
• Program Counter - This register is helpful in storing the address of
the next instruction that will be executed by the processor.
• Current Instruction Register – This register holds current
instruction being executed.
• Memory Address Register - It stores RAM address to which next
read or write operation is to be done.
• Memory Data Register : The register holds the data which is read
from RAM or is to be written to RAM.

Registers and Processors
• Accumulators - These type of Registers holds data on which any
process will work on as well as the results of logical or arithmetical
operations.
• Status Register - It holds information about last carried out
operation by the processor such as whether last sum operation
produced a positive or negative result.
• Interrupt Register - This register is responsible for holding the
information about any kind of interrupt that may happen at the time
of processing.

Processing cycle
• FETCH
• CPU reads contents from PC(Program Counter) in order to get the
address of next process instruction which needs to be executed.
After it is fetched, decoded. For instance it is 3264.
• As soon as the address if the next instruction is read value of the
PC increases and becomes PC = PC + 1(3265)
• Contents of 3264 are loaded on MAR.
• Addresses in MAR register is then placed in RAM.

Decode
• In the next stage contents of the address are in the next
stage are moved to MDR.
• Instruction from MDR register is copied to CIR
register. The reason behind this can be stated as MDR
is often used for completing the execution of certain
instruction.

Decoding process
• Contents of CIR register divided in parts. As there may be part of
instructions which includes operations such as ADD, SUB and other parts
of the instruction are fetching data/address where data can be found.
• ADD, SUB etc are denoted as the OPERATORs and remaining data part
denoted as OPERAND.
• Operators like ADD, SUB are decoded by control unit in the CPU, so that
the processor knows is to be completed.
• Operand placed on the MAR.
• Contents of MAR is found in RAM and after this they are put on the MDR.

Execute
• Stored instruction are executed in the next stage. The
arithmetic, logical operations are carried by using
Accumulators.
• For the aforementioned operations signal is sent to different
registers or parts of CPU in order to execute instructions
such as ADD.

Polling
• Polling:
In computing the Polling method is helpful in order to confirm
that microcontrollers access the devices by their own and collect
the information of the processes that is required processing.
• In case of Polling method, external devices are not considered as
independent systems. The devices depend on processor and only
the microcontrollers can access to obtain the information it
needs.

Drawback of the method
• Drawback of Polling method is while trying to write then some times
it leads to waste of time. As the microcontroller that needs to wait in
order to find out whether the required new information is available.

Interrupts
• In computing an Interrupt is signal that is sent to processor/
microcontroller in order to mark an event which requires
immediate consideration to complete all the processes
optimized way.
• Interrupt can be considered as a request to the processor to
stop execution of the current thread or process and attend or
make time to address the urgent execute of a special code.
• Every time a device/process requires processes or service and
notifies the processor by sending an interrupt.
• •

Interrupts
• Common example of interruption in computing environment is
key press or mouse click which leads to sending an interrupt
signal to microcontroller and perform the request or read
information for key pressed.
•
• Even processors can send interrupts to itself while executing a
program. This type of scenario occurs in case of an error in code.
such as division by 0.
• In multi-processor environment, processors use and send
interrupts in order to communicate with each other.

References
• Ramsay, J.D. and Sodhi, I., Apple Inc, 2019. Method and Apparatus for
Indicating Interrupts. U.S. Patent Application 15/700,838.
• Liu, F., Ki, Y.S. and Xiling, S.U.N., Samsung Electronics Co Ltd,
2018. Method for application-aware interrupts management. U.S. Patent
9,965,412.
• Han, K., Diefenbaugh, P. and Sharp, S., Intel Corp, 2017. Polling
determination. U.S. Patent 9,619,411.
• Hammarlund, P., Crossland, J.B., Kaushik, S.D. and Aggarwal, A., Intel
Corp, 2015. Inter-processor interrupts. U.S. Patent 8,984,199. Tsirkin, M.,
Red Hat Israel Ltd, 2015. Application-driven shared device queue polling
in a virtualized computing environment. U.S. Patent 9,009,702.