Higher Computing

1. Higher Computing Computer structure

2. What we need to know! • Detailed description of the purpose of the ALU and control unit • Description of the purposes of registers: to hold data being processed, instructions being executed, and addresses to be accessed • Description of the function of the data bus and the address bus • Description of the read, write and timing functions of the control lines • Identification of other control lines, including reset and interrupt lines • Simple description, referring to the appropriate buses and control lines, of the steps in the fetch/execute cycle • Description of the following elements of computer memory: registers, cache, main memory,backing storage

3. Organisation of a simple computer

4. The purpose of the ALU • The Arithmetic and Logic Unit (ALU) is the part of the Central Processing Unit (CPU) where the following take place: • Calculations • Boolean logic (AND, OR, NOT) • Comparisons (<,=, >, <=, >=)

5. The purpose of the Control Unit • The Control Unit sends out control signals: • Within the processor to move data from one register to another and to activate specific ALU functions; • To the control bus to read or write from memory; • To Input/Output modules.

6. The purpose of the registers • Registers are storage locations that are internal to the processor. • They are used to: • Hold data that is being transferred to or from memory; • Hold the address of the location in memory which the processor is accessing to read or write data; • Hold the instructions that are being carried out.

7. The function of the data bus • The lines on the data bus enable data to be transferred between the processor and the main memory. • The width of the data bus is measured by the number of lines on the bus. Each line can carry one bit. • A 32-bit data bus can transfer 32 bits at a time. • The width of the data bus is important when determining how well a system performs.

8. The function of the address bus • This holds the address of the memory location being accessed. • The more lines in the address bus the more locations the system can, in theory, access. • The maximum number of location the system can access is: • 2width of the address bus

9. The control lines • The control bus is made up of several lines. • The read line instructs the system to place data from the specified memory address on the data bus. • The write line instructs the system to take the data from the data bus and place it in the location specified by the address bus. • The Clock line provides the timing function of the system. The pulses from the clock line control when each step of the function takes place. Theses pulses control and regulate the activities of the processor.

10. The control lines (Cont.) • Other lines on the control bus include. • The reset line. A signal on this line stops all processes, clears all registers and places the system back in its original state. • The interrupt line. A signal here pauses the current process and allows another process to take control of the processor.

11. Registers Address bus Address bus MAR MAR Memory Memory Processor Processor Data bus Data bus MDR MDR MDR Control bus Control bus Data bus Address bus MDR MAR Fetch execute cycle A more detailed animation can be found here. Read line • Set up the address bus by moving the address to the Memory Address Register. • Signal the read line on the control bus. • Transfer the data from memory to the Memory Data Register. • Decode and execute the instruction. • Reset all signals.

12. What we should now know! • Detailed description of the purpose of the ALU and control unit • Description of the purposes of registers: to hold data being processed, instructions being executed, and addresses to be accessed • Description of the function of the data bus and the address bus • Description of the read, write and timing functions of the control lines • Identification of other control lines, including reset and interrupt lines • Simple description, referring to the appropriate buses and control lines, of the steps in the fetch/execute cycle

13. What we need to know! • Description of the following elements of computer memory: registers, cache, main memory, backing storage • Distinction between the above elements of memory according to function and speed of access. • The concept of addressability. • Description and evaluation of the following measures of performance: clock speed, MIPS, FLOPS, and application based tests. • Description of the effect the following factors have on system performance: data bus width, use of cache memory, rate of data transfer to and from peripherals. • Description of current trends in computer hardware, including increasing clock speeds, increasing memory and backing storage capacity.

14. Registers • Registers are storage locations that are internal to the processor. • They are used to: • Hold data that is being transferred to and from memory. • Hold the address of the memory location that the processor is reading/writing to/from. • Hold the instructions that are being carried out.

15. Cache memory • This is a small and very fast memory unit which is located close to the processor. • When the processor attempts to read from main memory, the cache is checked first. If the data is already stored there then it is transferred directly to the processor. • This saves a read from main memory which is much slower than cache memory.

16. Main Memory • This is the main internal storage area for the computers instructions. • It is divided into ROM and RAM sections. • Reading from main memory is slower than reading from registers or cache.

17. Backing Storage • This is the slowest form of data used to store data or software. • It retains its data when the power is switched off, unlike the RAM in the main memory.

18. Comparing different types of memory.

19. No.2 No.4 No.6 No.8 No.10 Addressability • In your street all the houses are given different numbers so the postman knows where to deliver his mail.

20. 11001111 11001110 11001101 11001100 Memory Addresses Memory Locations 1011……. 11001011 Data 11001010 11001001 11001000 Addressability • Computers also give each location in memory a different number so it knows where to place each piece of data, this number is known as a Memory Address. The computer can use this Memory Address to find the correct location when accessing it’s memory.

21. Addressability • The number of locations that a processor can address is, in theory, limited by the number of lines on the address bus. • If the address bus is 16 bit wide there can, in theory, be: • 216 = 65536 different memory locations. • If each memory location stores 32 bits of memory then the total memory capacity is: =65536 x 32bits = 65536 x 4bytes =262144bytes = 256KB

22. Measuring performance • There are several different way to measure the performance of a processor. You need to know about: • Clock speed. • MIPS • FLOPS • Application-based tests.

23. Clock speed • One way to compare computers is to compare the clock speed. • The clock pulses regulate the activities in the processor. • The pulses are measured in Megahertz (MHz) and Gigahertz (GHz) • 1 MHz = 1 million pulses a second.

24. Clock speed • The clock speed gives you an indication of the performance of the processor, which is at the heart of the processor, but do no over emphasize the importance of clock speed. • The performance of the processor is not dictated by the processor alone. Other factors are also important such as the data bus width.

25. MIPS • MIPS stands for Millions of Instructions Per Second. • This approach is based on the measurement of the number of machine code instructions that can be executed per second. • MIPS doe not take into account the size and complexity of the instructions being carried out and so is generally seen as a rough indication of performance.

26. FLOPS • FLOPS stands for Floating Point Operations Per Second. • This measures how many floating point operations a processor can carry out in a second. • This is generally seen as a more reliable indicator of performance, as it measures the amount of clearly definable arithmetical tasks that can be carried out in a second.

27. Application-based tests • Most computer magazines use application-based tests (benchmark tests) to compare system performance. • They set out a series of practical tasks using a range of standard application packages, award scores for the performance in each task and then use these scores to make overall comparisons.

28. Application-based tests • These are typical sets of application-based tests:

29. Application-based tests • How good are they? • Other measures of performance are reasonable but they don’t provide us with evidence of how the system will actually perform. • Application-based tests provide us with reproducible, evidence of performance when carrying out complex operations. • For this reason they are very reliable in measuring system performance.

30. How transfer rate affect performance. • All peripherals are slower than the processor. This can slow the processing down if the CPU needs to read from CD/DVD etc. • The faster the data transfer rate, the faster the overall performance of the system.

31. Transfer Rate • This difference can quickly effect the system performance.

32. Comparing CPU’s • When looking at processor developments look at key features: • Clock Speed (now measured in GHz) • Address bus • Data Bus • When looking at storage developments look at: • Capacity • Data Transfer rates