fetch-execute cycle

The fetch-execute cycle is the sequence of actions that a central processing unit performs to execute each machine code instruction in a program.

A program counter(PC) is a register in the central processing unit that contains the addresss of the next instruction to be executed. The PC is automatically incremented after each instruction is fetched to point to the following instruction. It is not normally manipulated like an ordinary register but instead, special instructions are provided to alter the flow of control by writing a new value to the PC, e.g. JUMP, CALL, RTS.

An address bus is the connections between the CPU and memory which carry the address from/to which the CPU wishes to read or write. The number of bits of address bus determines the maximum size of memory which the processor can access.

At the beginning of each cycle the CPU presents the value of the program counter on the address bus. The CPU then fetches the instruction from main memory (possibly via a cache and/or a pipeline) via the data bus into the instruction register.

From the instruction register, the data forming the instruction is decoded and passed to the control unit which sends a sequence of control signals to the relevant function units of the CPU to perform the actions required by the instruction such as reading values from registers, passing them to the ALU to add them together and writing the result back to a register.

The program counter is then incremented to address the next instruction and the cycle is repeated.

The fetch-execute cycle was first proposed by John von Neumann.

Other sites

http://cairns.cs.jcu.edu.au/teaching/Subjects/cp1300/1999/systems/

http://www.cs.ius.indiana.edu/LZ/rwisman/web_docs/C335/html/Right.htm

http://www.mkdata.dk/click/

RAM is our working memory storage. All the data, which the PC uses and works with during operation, are stored here. Data are stored on drives, typically the hard drive. However, for the CPU to work with those data, they must be read into the working memory storage, which is made up of RAM chips.

First, let us look back in time. Not too many years ago, Bill Gates said, that with 1 MB RAM, we had a memory capacity, which would never be fully utilized. That turned out to be untrue.

Back in the 80s, PCs were equipped with RAM in quantities of 64 KB, 256 KB, 512 KB and finally 1 MB. Commodore 64 had 64 KB RAM.

The first Windows operated PCs could address 2 MB RAM, but 4 MB soon became the standard. The race has continued through the 90s, as RAM prices have dropped dramatically.

Today. it would be foolish to consider less than 32 MB RAM in a PC. Many have much more. 128 MB is in no way too much for a "power user" with Windows 95/98. Windows 98 is a little better at handling memory, but still a lot of RAM is a good thing.

RAM speed is measured in ns (nano seconds). The fewer ns, the faster is the RAM. Years ago, RAM came in 120, 100 and 80 ns. Today, we are talking about 60 ns and faster.