Fetch Execute Cycle Travis Griffiths

Naming Conventions and Disclaimer Individual registers in a particular CPU will have different names depending on the documentation used. Due to differences in design the exact specifics of any particular processor will be different, possibly even within the same processor Example: large addresses may require multiple passes to fully decode.

Steps of Fetch Execute Cycle Fetch –The next instruction must be located, moved across at least one system bus to the processor Execute –The instruction must be carried out, this often requires information be moved around in one or more registers and system memory.

Registers Instruction Pointer (IP) –The location of the next instruction. –Sometimes called the Program Counter (PC) Memory Access Register (MAR) Current Instruction Register (CIR) or (IR) –The current instruction. Accumulator (AX) –Used for short term storage, and in many instructions

Busses Address Bus –Moves locations of data to different registers, particularly between the Instruction Pointer, Memory Address Register, and Memory Data Bus –Moves contents of memory addresses

What is a Bus Anyway? Explicitly connecting every component in a system results in an exponential growth of connections. Bus interconnection solves this. (n(n-1))/2 connections A bus connection

Fetching an Instruction 0001 Instruction Pointer Memory location contents 0FFF 0FA0 010D 00C Address Bus

Fetching an Instruction 0001 Instruction Pointer Memory location contents 0FFF 0FA0 010D 00C Address Bus Contents of the Program Counter are passed across the Address Bus

Fetching an Instruction 0001 Instruction Pointer Memory location contents 0FFF 0FA0 010D 00C Address Bus 0001 Memory Access Register The address moves over the address bus to the Memory Access Register

Fetching an Instruction Memory location contents 0FFF 0FA0 010D 00C Memory Access Register The memory location of the next instruction is located.

Fetching an Instruction Memory location contents 0FFF 0FA0 010D 00C Data Bus The contents of memory at the given location are moved across the data bus

Fetching an Instruction Memory location contents 0FFF 0FA0 010D 00C FFF Instruction Register Data Bus Into the instruction register (IR)

Fetching an Instruction 0FFF Instruction Register With the instruction loaded from memory into the Instruction Register the fetch portion of the cycle is complete.

Notes on Fetch This model assumes a simple fetch as our memory length all fits as a single unit in our registers. Some processors make use of a queue to “preload” instructions. This speeds up execution as memory is slower than the CPU and many cycles are wasted waiting for instructions to arrive.

Execute The specifics of an Execute cycle are particular to the instruction that has just been loaded. Instruction sets are particular to a processor and will often make use of other registers within the system.

Questions?