what is a computer
an electronic device which takes an input, processes data and delivers an output
what is a computer program
a set of instructions for the computer to follow
where is the computer program stored
in the memory
what does the CPU do
carries out instructions from the memory
what happens during ‘fetch’
in the fetch part of the cycle the CPU fetches the next instruction from the computers main memory (RAM) and brings it back to the CPU
what happens during ‘decode’
in the decode stage of the cycle the CPU insects the instruction and breaks it down to work out what it needs to do
what happens during ‘execute’
during the execute stage of the cycle the CPU carries out the instruction, this could cover many things like:
- going back to main memory and grabbing some data
- performing a calculation
- storing information back into main memory
how often does this FDE cycle happen on a modern processor
3 billion cycles per second (3Ghz)
what is clock speed and what is it measured in
clock speed is how many times the CPU carries out the FDE cycle per second and its measured in hertz (Hz)
what does the Arithmetic Logic Unit (ALU) do
it is responsible for carrying out arithmetic calculations and making logical decisions
what does the Control Unit (CU) do
the brains of the CPU, it sends signals to controls how data moves around the CPU and coordinates the CPU’s operations
what is the cache
a small amount of very fast memory which is typically located close to the CPU or even inside it, it provides fast access to frequently used instructions and data
what are registers
tiny, super fast pieces of memory inside the CPU each with they’re own specific purpose
what are the 4 registers that we need to know about
- accumulator
- program counter (PC)
- memory address register (MAR)
- memory data register (MDR)
what components does the CPU consist of (name 4)
- ALU
- control unit
- cache
- registers
what is a fixed program computer
a computer that only has one purpose and cannot do any other others, to change its program would require rewiring or redesigning the machine
what is a stored program computer
a computer that has changeable programs
what did John von Neumann do
he described the first design for modern computers that had stored programs, this went on to be called Von Neumann architecture
what are the key elements of Von Neumann architecture (name 5)
- data and instructions are both stored as binary digits
- data and instructions are both stored in primary memory
- instructions are fetched one at a time and in order
- the processor has to decode and execute an instruction before fetching another one
- the cycle continues until no more instructions are available
what does the program counter do
hold the address of the next instruction to be executed in memory
what does the MAR do
hold the address of where data is to be fetched or stored
what does the MDR do
holds the data fetched from, or to be written to memory
what does the accumulator do
holds the results of calculations done by the ALU
describe the FDE cycle in detail, including registers
- the PC is checked as it holds the address of the next instruction
- this address is then used by the MAR to fetch the instruction needed
- the instruction is then bought back to the MDR
- the address in the program counter is then incremented to point at the next instruction
- once the instruction is in the CPU, the CU decodes it to see what has to be done
- now that we know what do to the instruction can be executed
