time step first instruction second instruction

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1 COMP Semester 1 61 Parallelism Microprocessors can use a variety of tricks to execute more instructions in a given amount of time. One of the most successful is to execute more than one instruction at a time. Two common types of parallelism are pipelining and multiple functional units. Pipelining. The execution of an instruction can be divided into several stages. For example: fetch the instruction, decode the instruction, fetch the data required, perform the operation, store the result. This can be overlapped: time step first instruction second instruction 1 fetch instruction 2 decode fetch instruction 3 fetch data decode 4 do operation fetch data 5 store result do operation 6 store result In a long stream of instructions, this technique can mean that one instruction finishes each time step, even though each instruction needs 4 times steps to be executed. In reality, a lot of complications occur. For example, if the data needed by the second instruction includes the answer from the first instruction, then the second instruction will have to wait for the first one to finish. Multiple functional units. Here we have circuits for executing several instructions at once. The difficult part is deciding when this is possible. Consider these instructions.

2 COMP Semester 1 62 A: ADD R5,R6 B: ADD R9,R10 C: ADD R6,R7 D: ADD R6,R13 Instructions A and B are completely independent and so can be executed at the same time. Similarly C and D are independent. However, C and D can t be executed until A finishes, because they need the answer from A. Thus, these 4 instructions can be executed in two steps: (1) execute instructions A and B at the same time (2) execute instructions C and D at the same time Major general-purpose microprocessors Here we will summarize a few of the major lines of microprocessors used in desk-top computers. As well as these, there are many others. x86 line (Intel and Advanced Micro Devices) These include the sequence of processors 8086 (5 MHz, 3 microns, 1978), 80286, 386, 486, Pentium, Pentium II, Pentium III, Pentium IV (up to 1500 MHz, 0.18 microns) manufactured by Intel and used in PC computers. Another current family of Intel microprocessors are called Celeron. There is a competing line of Athlon processors manufactured by AMD. The current processors are 32-bit. Intel and AMD will soon introduce lines of 64-bit processors running at over 1 GHz. PowerPC line (Motorola, IBM) These are the processors in current Macintosh computers. Main steps 601, 603, 604, G3, G4. The latest is the G4 processor, which is one of the first mass-market processors

3 COMP Semester 1 63 using copper instead of aluminium. Up to 733 MHz, 0.15 or 0.18 microns. Alpha (Compaq) These are 64-bit processors used in high-end UNIX workstations. Up to 833 MHz, with much better performance than Intel processors for scientific applications. Alpha processors are specifically designed for multiprocessing, which is the practice of having several CPUs in the one computer. Sparc (SUN Microsystems, Fujitsu) The current main family is the Ultrasparc series, up to 750 MHZ. Used mostly in SUN computers (like iwaki). Comparing microprocessor performance The clock rate is the most commonly quoted measure of processor speed, but it is only a reasonable measure for comparing similar members of the same processor family (and even then isn t the whole story). It does not even measure the number of instructions executed each second, because different instructions might need a different number of clock cycles to complete. An obsolete alternative to MHz (but arguably better) is mips (millions of instructions per second). There is a similar measure of floating-point performance: flops (floating-point operations per second). Moreover, even the concept of one processor being faster than another is somewhat dodgey. One processor might be faster for some applications and slower for others. This can easily happen, for example, if one processor has a faster CPU but the other has a faster memory bus.

4 COMP Semester 1 64 In an attempt to give more meaningful performance figures, there are a number of benchmarks that are quoted. A benchmark consists of a family of computing tasks. An overall score is determined from the time taken to perform all the tasks. The most famous benchmarks are the SPEC series (after the company that distributes them). Advances in processors mean that the criteria used for measuring performance need to change every so often, so the definition of the benchmarks changes every few years. Currently the main benchmarks for microprocessors are SPECint2000 for problems mostly involving integer arithmetic and SPECfp2000 for floating-point arithmetic. Larger numbers are better. Here are some approximate values for a few processors. SPECint2000 SPECfp2000 Alpha 21264B, 833 MHz Pentium IV, 1.5 GHz Ultrasparc 3, 750 MHz It is seen that Alpha is ahead of the others in floating-point performance even though it is equal to Pentium for integer performance. This means that Alpha is easily the fastest for scientific applications (which need much floating-point arithmetic). However, note that its clock speed is much lower than the Pentium s. Recent versions of many processors include extra instructions especially designed for the most CPU-hungry tasks of the mass market: processing of images, sound, video, and the 3-D graphics needed for computer games. For example, Pentium III has Streaming SIMD, Athlon

5 COMP Semester 1 65 has 3DNow! and G4 has Altivec. Programs have to be modified in order to use these instructions but then might run much faster. For example, the image-processing package Photoshop runs faster on a G4 than on the fastest Pentium because it makes good use of the Altivec instructions. Evolution of processor speeds over time Gordon Moore (one of the founders of Intel) predicted in 1973 that that number of transistors put on a single chip would double every 18 months until fundamental physical limits are reached. A similar statement about processor speeds is often called Moore s Law. Moore s Law has held remarkably well for more than 20 years, despite confident expert opinion every few years that the trend cannot last much longer. There currently seems little reason to doubt that Moore s Law will continue to approximately apply for the rest of this decade and maybe longer. The next few years seem assured because processors meeting the requirements are already running in the labs of several manufacturers. The required technology for continuing the trend for some years after that is fairly well understood theoretically and only needs more engineering development for practical application. Beyond the next decade, things are much less clear. The existing type of metal+silicon microprocessors will eventually reach a limit where the current flows between components cannot be reliably distinguished from the thermal motions of electrons. However, other possible technologies may be able to take over. Possibilities include replacing transistors by specially designed molecules, and circuits where the information is carried by photons (i.e.

6 COMP Semester 1 66 light) rather than by electrons. Beyond the next decade, things are much less clear. The existing type of metal+silicon microprocessors will eventually reach a limit where the current flows between components cannot be reliably distinguished from the thermal motions of electrons. However, other possible technologies may be able to take over. Possibilities include replacing transistors by specially designed molecules, and circuits where the information is carried by photons (i.e. light) rather than by electrons. There is a theoretical possibility of an entirely different type of computer based on quantum physics. It this type of computer can be built, some types of computation will be possible that are out of the question for traditional computers. However, practical investigation of these possibilities are only at the most primitive stage and it is not yet clear whether full scale quantum computers will be possible in the near future (or ever). It is also not clear whether the special abilities of quantum computers will be of sufficiently general applicability that they will make a large difference to the computing world.

7 COMP Semester 1 67 The History of Computing 1500 Leonardo da Vinci s mechanical calculator 1621 Slide rule 1623 Wilhelm Schikard s Calculating Clock 1642 Blaise Pascal s Arithmetic Machine 1670s Gottfied von Leibniz s Step Reckoner 1820 Charles Xavier Thomas de Colmar s Arithmometer 1822 Charles Babbage s Difference Engine 1834 Charles Babbage s Analytical Engine Augusta Ada Lovelace writes first computer program 1853 Scheutz & Son, working difference engine 1857 Wheatstone s use of paper tape 1885 Baldwin and Odhner, better mechanical calculator 1890 US Census tabulated using punched cards using method of Hollerith (later founder of IBM) 1935 IBM 601 punched card machine using relays (USA) 1938 Konrad Zuse, electro-mechanical computer Z1 (DE) 1939 Atanasoff and Berry, special purpose electronic computer (USA) 1941 Atanasoff and Berry, linear equation solver (USA) 1941 Konrad Zuse, Z3 programmable computer (Germany) 1943 Harvard Mark I programmable computer (USA) 1943 Thomas Flowers, Colossus, the first electronic computer (UK) 1946 ENIAC fully electronic computer (USA) 1946 Konrad Zuse s programming language Plankalkül 1946 von Neumann s paper on computer architecture 1948 First stored-program computer (UK) 1949 CSIRAC, the first Australian computer 1949 First use of magnetic tape (USA) 1950 Floppy disk invented (Japan) 1951 First commercially available computer (USA)

8 COMP Semester Magnetic core memory 1957 High level languages become significant 1959 Move from valves to transistors 1964 Move from transistors to integrated circuits 1964 IBM ARPARNET network started (USA) 1970 RAM chips appear 1971 First microprocessor appears (4004) invented 1973 First mass-market computer game (Pong) 1974 Personal computers appear 1975 UNIX marketed 1981 IBM PC released 1984 Apple Macintosh released 1989 World Wide Web invented 1997 Deep Blue beats Gary Kasparov 2000 First 1 GHz microprocessors See the comp1200 links file for much more information.

9 COMP Semester 1 69 Operating Systems Programs running on a computer can be loosely classified into two groups: Application software consists of programs that perform specific tasks requested by the human user of the computer. For example, a web browser, client, movie player, word processor, or game. System software consists of programs that manage the resources of the computer and facilitate access to those resources. For example, an assembler for translating assembly language into machine language, a compiler for translating a high-level language, programs for talking with disk drives, servicing page faults, sharing the CPU between different application programs. The boundary between application and system software is indistinct. The overall set of system software, or at least its more essential parts, comprise the operating system. Again, there is no precise definition of which components of system software belong to the operating system. Often the most essential parts of the operating system are called the kernel. Examples of operating systems are UNIX (of which Linux is a variant), Windows and MacOS.

10 COMP Semester 1 70 Processes A process is essentially a program being executed (whether or not it is actually using the CPU right now). A process consists of some machine instructions and some data areas, residing in the virtual address space of the process, and the contents of the CPU registers. Some things we think of as programs might actually be several processes cooperating with each other. For example, a web browser might have one process that communicates with the internet and another process that displays web pages on the screen. The operating system itself consists of one or more processes. Operating System tasks The most important tasks of an operating system are: To communicate with the computer user. To arrange for the execution of processes. To provide a standard interface between itself and other processes (system calls). To share the resources of the computer (memory, access to peripheral devices, CPU time, etc) between the various processes that want them. To manage page tables and paging. To communicate with storage devices, keyboards, monitors, network connections, etc. To maintain i/o caches, search directories, etc.