1.1 Systems Architecture Flashcards
The fetch-decode-execute cycle
The complete process of retrieving an instruction from storage, decoding it and carrying it out.
The Fetch step
-The program counter holds the address of the next instruction
-The address is copied in the MAR
-the address bus goes to the value in the MAR
-the data in the location is sent along the data bus and into the MDR
-data is copied into the instruction register
The Decode step
-Works out what the instructions mean
The Execute step
-A signal is sent along an address bus to the memory location
-data is transferred along a data bus to the accumulator
-the program counter increments by 1
CPU
The Central Processing Unit
-The internal hardware component of the computer that is responsible for executing the instructions of programs and to control the rest of the computer system
-consists of six main components: CU, ALU, registers, cache, buses, clock
ALU
Performs calculations and logical comparisons in the CPU
CU
The Control Unit
-It decodes instructions and sends signals to control how data moves around the CPU.
Cache
Memory in the processors that provides fast access to frequently used instructions and data
-speeds up processing time as the data and instructions don’t have to be fetched directly from RAM.
Registers
Tiny areas of extremely fast memory located in the CPU, normally designed for a specific purpose where data or control information is stored temporarily. Includes the program counter, MAR, MDR, accumulator, instruction register
-8-bit CPU has 8 bit registers
How do registers increase processing speed
-used to hold temporary data while a software program is running
-as the CPU processes the data, the software program will shift data in and out of the registers as it is easier than in and out of cache or RAM - speeds up processing time
Program counter
Holds the address of the next instruction. Step 1 of the fetch-execute cycle
-as each instruction is processed, the software that is running updates the program counter with the address of the next instruction to fetch.
MAR
Memory Address Register. Holds the address of data ready to be used by the MDR or the address of an instruction passed from the program counter.
-Every instruction data in main memory is located at a unique address
MDR
Memory Data Register. Holds data fetched from or to be written to memory. Step 2 of the fetch-execute cycle.
Accumulator
Holds the result of calculations.
Clock
The clock is the electronic unit that synchronizes related components by generating pulses at a constant rate.
-single tick - CPU processing one instruction
Clock speed
Clock speed is the frequency at which the internal clock generates pulses.
-Clock speed of 3 GHz = 3 billion instructions per second.
Bus
Data bus - carries data around the system
Address bus - carries info on where the data needs to go/ where the data comes from
Control bus - carries signals round the system to control components (e.g. DVD drive, hard disks, graphic cards)
Common characteristics that affect CPU performance
-Clock speed
-Cache size
-No. of cores
How clock speed affects CPU performance
The higher the clock rate, the faster the computer may work
How cache size affects CPU performance
The larger the cache, the more data can be stored without having to go back to main memory
How the no. of cores affects CPU performance
A CPU can have one or more processing units. Each unit is called a core, which contains an ALU, CU and registers
-CPUs with multiples cores have more power to run multiple programs at the same time
-having two cores will not double a computer’s speed, they have to communicate with each other through channels, which uses up some speed
No. of cores - multitasking
With 2 cores, a CPU can run 2 tasks at the same time
No. of cores - parallel processing
When a single task (program) is split in 2 or more parts and each part is processed at the same time.
-however, it is not always twice the speed as some programs don’t neatly split into 2
Embedded systems
A computer built to solve a highly specific problem and not easy to change (A computer system that is built into another device)
-e.g. operating system placed inside a washing machine, microwave, traffic lights
Advantages of embedded systems
-Small
-Built on a single circuit board so when they break, easy to replace
-use less power than general purpose computer
-designed with 1 task in mind - efficient
Disadvantages of embedded systems
-if embedded computer goes wrong, equipment most likely stop working
-if fails, might have to go to a specialist supplier for replacement
Van Neumann Architecture
Traditional computer architecture that forms the basis of most digital computer systems.
Instructions are fetched, decoded and executed.
