1) What is a processor?
The processor is the core of any computational device. This little chip is capable of carrying out all of the instructions for the computer. It does this by performing basic arithmetic operations (add, subtract...) and logical operations (and, or...) on input functions that are all specified by the instructions. There are three main stages or steps to any successful processor: Fetch, Decode, and Execute. These stages are about retrieving (Fetching) and reading (Decoding) instructions that are located on the central memory then actually running them (Execute).
The inputs to a processor are the instructions sent to the processor to execute. In a general purpose computer, these signals can come from devices such as a keyboard, a mouse, a CD, etc.
The outputs of a processor are the end result of each instruction (or set of instructions). In a general purpose computer, these outputs can be a monitor display, an LED, a printer, etc.
The control unit is where the control signals are all sent from. It includes Program Counter (PC) and Instruction Register (IR) which hold hold the address in memory for the next instruction to be executed and the instruction currently being executed respectively. The control unit will determine the appropriate control signals to send out based upon the instruction in the IR.
The processing unit is where the arithmetic and logical operations are actually executed by the processor. It typically consists of one or more Arithmetic Logic Units (ALUs), and a set of registers to store temporary data.
The Memory of a processor is typically broken into 3 parts:
- The instruction set, or list of commands to execute.
- The data, or information available for the processor to access as well as a location for the processor to save new data to.
- The stack, or list of functions to execute. For out project, knowledge of the stack was not necessary as our processor was very simple and had no need for a stack.
The Memory will also typically have two dedicated registers associated with it:
- The memory address register (MAR), which stores the address (location) to look into in the memory.
- The memory data register (MDR), which acts as the gate for data entering and leaving the memory.
The MAR will accept the address to store from either the PC or from an instruction being executed (if data needs to be read from or written to the memory), while the MDR will store the data that is to be read from or written to memory. (While the instructions will usually have a dedicated space in the memory, they will be seen as data being read from memory by the MDR, since "instructions" and "data" are abstractions of what the machine is actually reading. To the machine, it is all just a series of LOW and HIGH signals, or 0's and 1's.)