中科大程序设计与计算机系统lec01-intro_20141116173438874.ppt

Computer Systems:A Programmer’s Perspective程序设计与计算机系统Lecture 1Intro November 17, 2014 Wu junmin (jmwu@) Outline Course Theme Five great realities of computer systems Administrative Matters Lecture topics and assignments 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 understand and tune program performance Prepare for other “systems” classes Compilers, Operating Systems, Networks, Computer Architecture, Embedded Systems 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) -- ?? Code Security Example Typical Usage Malicious Usage 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 Great Reality #2 You’ve got to know assembly Chances are, you’ll never write program in assembly Compilers are much better more patient than you are But :Understanding assembly is key to machine-level execution model Behavior of programs in presence of bugs High-level language model breaks down Tuning program performance Understand optimizations done/not done by the compiler Understanding sources of program inef