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Computer Organization Lecture 1 Bryant’s Book Chapter 1 CSCE 312 1 Course Overview • Topics: – Theme – Five Great Realities of Computer Systems – Computer System Overview – Summary – NOTE: Most slides are from the textbook and the co-author Randal E Bryant of Carnegie Mellon University CSCE 312 2 Course Theme • Abstraction is good, but don’t forget reality! • Courses to date emphasize abstraction – Abstract data types – Asymptotic analysis • These abstractions have limits – Especially in the presence of bugs – Need to understand underlying implementations • Useful outcomes – Become more effective programmers • Able to find and eliminate bugs efficiently • Able to tune program performance – Prepare for later “systems” classes in CSE & ECE • Compilers, Operating Systems, Networks, Computer Architecture, Embedded Systems CSCE 312 3 Great Reality #1 • Int’s are not Integers, Float’s are not Reals • Examples – Is x2 ≥ 0? • Float’s: Yes! • Int’s: – 40000 * 40000 --> 1600000000 – 50000 * 50000 --> ?? – Is (x + y) + z = x + (y + z)? • Unsigned & Signed Int’s: Yes! • Float’s: – (1e20 + -1e20) + 3.14 --> 3.14 – 1e20 + (-1e20 + 3.14) --> ?? CSCE 312 4 Computer Arithmetic • Does not generate random values – Arithmetic operations have important mathematical properties. • Cannot assume “usual” properties – Due to finiteness of representations – Integer operations satisfy “ring” properties • Commutativity, associativity, distributivity – Floating point operations satisfy “ordering” properties • Monotonicity, values of signs • Observation – Need to understand which abstractions apply in which contexts – Important issues for compiler writers and serious application programmers – Entire courses offered on computer arithmetic (ECEN 653 Computer Arithmetic Unit Design) CSCE 312 5 Great Reality #2 • You’ve got to know assembly • Chances are, you’ll never write a program in assembly – Compilers are much better & more patient than humans • Understanding assembly is the key to understanding machinelevel execution model – Behavior of programs in presence of bugs • High-level language model breaks down – Tuning program performance • Understanding sources of program inefficiency – Implementing system software • Compiler has machine code as target • Operating systems must manage process state CSCE 312 6 Great Reality #3 • Memory Matters • Memory is not unbounded – It must be allocated and managed – Many applications are memory dominated • Memory referencing bugs especially pernicious – Effects are distant in both time and space • Memory performance is not uniform – Cache and virtual memory effects can greatly affect program performance – Adapting program to characteristics of memory system can lead to major speed improvements CSCE 312 7 Memory Referencing Errors • C and C++ do not provide any memory protection – Out of bounds array references – Invalid pointer values – Abuses of malloc/free • Can lead to nasty bugs – Whether or not bug has any effect depends on system and compiler – Action at a distance • Corrupted object logically unrelated to one being accessed • Effect of bug may be first observed long after it is generated • How can I deal with this? – – – – Program in Java, Lisp, or ML Understand what possible interactions may occur Use or develop tools to detect referencing errors Use debugged library routines CSCE 312 8 Great Reality #4 • There’s more to performance than asymptotic complexity • Constant factors matter too! – Easily see 10:1 performance range depending on how code written – Must optimize at multiple levels: algorithm, data representations, procedures, and loops • Must understand system to optimize performance – How programs compiled and executed – How to measure program performance and identify bottlenecks – How to improve performance without destroying code modularity and generality CSCE 312 9 Great Reality #5 • Computers do more than execute programs • They need to get data in and out – I/O system critical to program reliability and performance • They communicate with each other over networks – Many system-level issues arise in presence of network • • • • Concurrent operations by autonomous processes Coping with unreliable media Cross platform compatibility Complex performance issues CSCE 312 10 Course Overview • Topics: – Theme – Five great realities of computer systems – Computer System Overview – Summary – NOTE: Most slides are from the textbook and the co-author Randal E Bryant of Carnegie Mellon University CSCE 312 11 Terms • bit, byte, word • KB, MB, GB, TB, PB, EB, ZB, YB… – https://en.wikipedia.org/wiki/File_size • sec, ms, us, ns, ps Hardware Organization CPU Register file ALU PC System bus Memory bus Main memory I/O bridge Bus interface I/O bus USB controller Mouse Keyboard Graphics adapter Disk controller Display Disk Expansion slots for other devices such as network adapters hello executable stored on disk Inside Pentium 4 Intel Core i7 950 Processor Hardware Terms • • • • CPU: Central Processing Unit ALU: Arithmetic/Logic Unit PC: Program Counter USB: Universal Serial Bus Main Memory • A temporary storage device that holds both a program and the data it manipulates. • Consists of a collection of dynamic random access memory (DRAM) chips. • Logically a linear array of bytes. Processor • The engine that interprets instructions stored in main memory. • At any point in time, the PC points at some machine language instruction in main memory. • A processor repeatedly executes the instruction pointed at by the PC. • A processor operates according to a very simple instruction execution model, defined by its instruction set architecture (ISA). Abstraction • Both hardware and software consists of hierarchical layers (abstraction). – To cope with the complexity of computer systems. • The interface between the hardware and lowlevel software: Instruction Set Architecture (ISA) ISA • Includes anything programmers need to know to make a binary machine language program work correctly. • Includes instructions, I/O devices, and so on. • Modern ISAs: – 80x86/Pentium, PowerPC, DEC Alpha, MIPS, SPARC, HP, … What is Computer Architecture? Computer Architecture = Instruction Set Architecture + Machine Organization What Is Computer Architecture? Application Operating System Compiler Instr. Set Proc. Firmware I/O system Datapath & Control Digital Design Circuit Design Layout Instruction Set Architecture Register File, ALU • The register file is a small storage device that consists of a collection of word-sized registers, each with its own unique name. • The ALU computes new data and address values (add, subtract, multiply, divide, or, and, xor, etc.) Memory Hierarchy L0: Smaller, faster, and costlier (per byte) storage devices Registers L1: L2: L3: Larger, slower, and cheaper (per byte) storage devices L5: L4: CPU registers hold words retrieved from cache memory. On-chip L1 cache (SRAM ) Off-chip L2 cache (SRAM) L1 cache holds cache lines retrieved from the L2 cache. L2 cache holds cache lines retrieved from memory. Main memory (DRAM) Local secondary storage (local disks) Remote secondary storage (distributed file systems, Web servers) CSCE 312 Main memory holds disk blocks retrieved from local disks. Local disks hold files retrieved from disks on remote network servers. 24 Memory Hierarchy Faster REGISTERS Smaller CACHE MEMORY Slower DISK Larger Cache Memory CPU chip Register file L1 cache (SRAM) ALU Cache bus L2 cache (SRAM) System bus Memory bridge Bus interface CSCE 312 Memory bus Main memory (DRAM) 26 Memory Hierarchy • Storage at one level serves as a cache for storage at the next lower level. • The register file is a cache for the L1 cache, L1 is a cache for L2, and so forth. DRAM vs. SRAM • DRAM stands for Dynamic Random Access Memory. It is a type of semiconductor memory in which the memory is stored in the form of a charge. Each memory cell in a DRAM is made of a transistor and a capacitor. The data is stored in the capacitor. Capacitors loose charge due to leakage and hence DRAM's are volatile devices. To keep the data in the memory, the device must be regularly refreshed whereas SRAM is static, so it will retain a value as long as power is supplied. SRAM is typically faster than DRAM since it doesn't have refresh cycles. Since each SRAM memory cell is comprised of 6 Transistors unlike a DRAM memory cell, which is comprised of 1 Transistor and 1 Capacitor, the cost per memory cell is far greater in an SRAM compared to a DRAM. With similar reasoning, it can be asserted that DRAMs come in larger densities than SRAMs given a fixed area. Example : SRAMs are used in Caches because of higher speed and DRAMs are used for main memory in a PC because of higher densities. Smartphone RAM vs. Internal Memory • “My cellphone has 4GB RAM and 16GB memory (or internal storage).” • RAM is … • The internal memory (or storage) is flash memory. • Flash memory is an electronic non-volatile computer storage medium that can electrically erased and reprogrammed. – Cellphone storage – USB drive – Solid-state drive (SSD) as a replacement for hard drives Operating System • A layer of software interposed between the application program and the hardware. • All attempts by an application program to manipulate the hardware must go through the OS. • Two primary purposes of OS – Protect HW from misuse by runaway applications. – Provide applications with simple and uniform mechanisms for manipulating complicated and often wildly different lowlevel hardware devices. OS abstracts HW Application programs Software Operating system Processor Main memory Hardware I/O devices Processes Virtual memory Files Processor Main memory CSCE 312 I/O devices 31 Process • The operating system’s abstraction for a running program. • Multiple processes can run concurrently on the same machine. – Traditional systems (uniprocessor) can execute only one program at a time. – Multicore processors can execute several programs simultaneously. Process • In either case, a single CPU can execute multiple processes concurrently by having the processor switch among them. => Context switch • The OS keeps track of all the state information (context) that the process needs in order to run. • Context: PC value, register file values, … Process • At any point in time, a uniprocessor system can only execute the code for a single process. • When the OS decides to transfer control from the current process to another process, it performs a context switch by saving the context of the current process, restoring the context of the new process, and passing control to the new process. Thread • A process can actually consists of multiple execution units, called threads, each running in the context of the process and sharing the same code and global data. Virtual Memory • An abstraction that provides each process with the illusion that it has exclusive use of the main memory. • Each process has the same uniform view of memory, called virtual address space. Memory Performance Example • Implementations of Matrix Multiplication – Multiple ways to nest loops /* ijk */ for (i=0; i<n; i++) { for (j=0; j<n; j++) { sum = 0.0; for (k=0; k<n; k++) sum += a[i][k] * b[k][j]; c[i][j] = sum; } } /* jik */ for (j=0; j<n; j++) { for (i=0; i<n; i++) { sum = 0.0; for (k=0; k<n; k++) sum += a[i][k] * b[k][j]; c[i][j] = sum } } CSCE 312 37 Matmult Performance (Alpha 21164) Too big for L1 Cache Too big for L2 Cache 160 140 120 ijk 100 ikj jik 80 jki kij 60 kji 40 20 0 matrix size (n) CSCE 312 38 Amdahl’s Law • When we speed up one part of a system, the effect on the overall system performance depends on both how significant this part was and how much it sped up. Told: original execution time Tnew: execution time after improvement α: fraction of time that will be improved k: improvement factor Tnew = (1 - α) Told + (α Told )/k = Told [ ( 1 - α) + α /k] Speedup S = Told / Tnew = 1 / [ (1 - α) + α /k] CSCE 312 39 Example • A new CPU is 10 times faster in the Web serving application than the original processor. Assuming that the original CPU is busy with computation 40% of the time and is waiting for I/O 60% of the time, what is the overall speedup gained by the enhancement? CSCE 312 40 Example • Suppose you work as a truck driver, and you have been hired to carry a load of potatoes from Boise, Idaho to Minneapolis, Minnesota, a total distance of 2,500 kilometers. You estimate you can average 100 Km/hr driving within the speed limits, requiring a total of 25 hours for the trip. CSCE 312 41 Example • You hear on the news that Montana has just abolished its speed limit, which constitutes 1,500 Km of the trip. Your truck can travel at 150 Km/hr. What will be your speedup for the trip? • How fast must you travel through Montana toget an overall speedup for your trip of 1.67x? CSCE 312 42 Course Perspective • Most Systems Courses are Builder-Centric – Computer Architecture • Design pipelined processor in Verilog – Operating Systems • Implement large portions of operating system – Compilers • Write compiler for simple language – Networking • Implement and simulate network protocols CSCE 312 43 Course Perspective (Cont.) • Our Course is Programmer-Centric – Purpose is to show how by knowing more about the underlying system, one can be more effective as a programmer – Enable you to • Write programs that are more reliable and efficient • Incorporate features that require hooks into OS – E.g., concurrency, signal handlers – Not just a course for dedicated hackers • We bring out the hidden hacker in everyone – Cover material in this course that you won’t see elsewhere CSCE 312 44 Summary • The computer system is more than just hardware! • We need to understand both the hardware and system interfaces to properly use a computer • We shall focus on more details to such concepts through out this course. CSCE 312 45