Question

At the initial stage of each instruction cycle, the processor fetches an instruction from
memory. The processor interprets the instruction and performs the required action.
Provide illustration of the process with a thorough explanation on the rudimentary
instruction cycles the processor follows to execute fundamental instructions. Discuss
how this processor interacts with the cache memory to give output speedily.

Answer 1

The instruction cycle (also known as the fetch–decode–execute cycle, or simply the fetch-execute cycle) is the cycle that the central processing unit (CPU) follows from boot-up until the computer has shut down in order to process instructions. It is composed of three main stages: the fetch stage, the decode stage, and the execute stage.

Generic CPU Instruction Cycle

The generic instruction cycle for an unspecified CPU consists of the following stages:

1. Fetch instruction: Read instruction code from address in PC and place in IR. ( IR ← Memory[PC] )
2. Decode instruction: Hardware determines what the opcode/function is, and determines which registers or memory addresses contain the operands.
3. Fetch operands from memory if necessary: If any operands are memory addresses, initiate memory read cycles to read them into CPU registers. If an operand is in memory, not a register, then the memory address of the operand is known as the effective address, or EA for short. The fetching of an operand can therefore be denoted as Register ← Memory[EA]. On today's computers, CPUs are much faster than memory, so operand fetching usually takes multiple CPU clock cycles to complete.
4. Execute: Perform the function of the instruction. If arithmetic or logic instruction, utilize the ALU circuits to carry out the operation on data in registers. This is the only stage of the instruction cycle that is useful from the perspective of the end user. Everything else is overhead required to make the execute stage happen. One of the major goals of CPU design is to eliminate overhead, and spend a higher percentage of the time in the execute stage. Details on how this is achieved is a topic for a hardware-focused course in computer architecture.
5. Store result in memory if necessary: If destination is a memory address, initiate a memory write cycle to transfer the result from the CPU to memory. Depending on the situation, the CPU may or may not have to wait until this operation completes. If the next instruction does not need to access the memory chip where the result is stored, it can proceed with the next instruction while the memory unit is carrying out the write operation.

An example of a full instruction cycle is provided by the following VAX instruction, which uses memory addresses for all three operands.

            mull    x, y, product
            

1. Fetch the instruction code from Memory[PC]
2. Decode the instruction. This reveals that it's a multiply instruction, and that the operands are memory locations x, y, and product.
3. Fetch x and y from memory.
4. Multiply x and y, storing the result in a CPU register.
5. Save the result from the CPU to memory location product.

• How this processor interacts with the cache memory to give output speedily

Cache memory works by taking data or instructions at certain memory addresses in RAM and copying them into the cache memory, along with a record of the original address of those instructions or data.

This results in a table containing a small number of RAM memory addresses, and copies of the instructions or data that those RAM memory address contain.

Memory Cache "Hit"

When the processor requires instructions or data from a given RAM memory address, then before retrieving them from RAM it checks to see if the cache memory contains a reference to that RAM memory address. If it does, then it reads the corresponding data or instructions from the cache memory instead of from RAM. This is known as a "cache hit". Since the cache memory is faster than RAM, and because it is located closer to the CPU, it can get and start processing the instructions and data much more quickly.