Von Neumann Computational Model:
The first computers, like your calculator, had to be redesigned, or at least rewired, to do a different job. The idea of a general-purpose computer was developed in the 1930s and 1940s through the work of Alan Turing and John von Neumann, who proposed that the instructions for processing could be held in storage with the input data.
A computer in which the processing instructions are stored in memory with the data is called a stored-program computer or a von Neumann architecture computer.
In a von Neumann-architecture computer, the hardware device that does the processing is called the central processing unit (CPU), and the storage is called main memory or random-access memory (RAM). They are connected to the input and output (I/O) devices by a group of connecting wires called a bus.
When a computer is turned on, the CPU fetches an instruction from the memory, carries out the instruction it receives, and then fetches the next, and so on. This sequence is called the fetch-decode-execute cycle, and it carries on until the power is turned off.
RAM: RAM is referred to as random access memory; it is also called main memory. This is a temporary store that the CPU uses for data and instructions.
Writing: When the CPU saves data into memory, this is called writing. The CPU uses the bus to tell the memory what data to save and where in memory to save it.
Reading: When the CPU retrieves data from the memory to process, this is called reading.
Each memory location has a unique memory address, in the same way that your home is identified by an address. The memory addresses are just numbers, starting from zero. RAM is described as volatile, which means that its contents are lost when the power is turned off Because of this,
ROM: ROM is referred to as read-only Only Memory it is also called non-volatile memory because it stores any programs that must run when the computer is first turned on. Programs permanently stored in ROM are known as firmware. Typically, these programs are small and carry out specific tasks, such as initializing the hardware components and starting the operating system when a computer is switched on.
Writing: When the CPU saves data into memory, this is called writing. The CPU uses the bus to tell the memory what data to save and where in memory to save it.
Reading: When the CPU retrieves data from the memory to process, this is called reading.
Some important definitions:
Von Neumann architecture: computer system design in which the program is stored in memory with the data.
The central processing unit (CPU) is a hardware device that carries out the processing in a computer.
Main memory/random-access memory (RAM): a temporary store for data and instructions (programs).
Read-only memory (ROM): memory that cannot be altered and is not lost when the power is turned off.
Bus: a group of connections between devices in a computer.
Fetch-decode-execute cycle: a sequence of steps carried out repeatedly by a CPU.
Writing: when the CPU sends data to memory to be stored at a given address.
Reading: When the CPU retrieves the data stored at a given address.
Memory address: a number that uniquely identifies a (memory) storage location.
Volatile: memory that is erased when the power is turned off.
Non-volatile: memory that is not lost when the power is turned off.
Cache memory:
Most computers require a third kind of main memory; this is called cache memory (pronounced like ‘cash). Cache memory is a small amount of fast, expensive memory that is used in between two devices that communicate at different speeds, most often the CPU and RAM. In a CPU cache, frequently used code or data is loaded in chunks from the slower RAM into the cache. The CPU accesses the cache memory at its own, faster speed. This means the CPU isn’t slowed down by having to wait for data from RAM.