Introduction to Computer Systems

1. Introduction to Computer Systems 15-213 “The Class That Gives CMU Its Zip!” Topics: • Theme • Four great realities of computer systems • Chap 1 in “Computer Systems” book And gives Ithaca??

2. Course Theme • Abstraction is good, but don’t forget reality! Courses to date emphasize abstraction • Abstract data types (e.g., Comp 220) • Asymptotic analysis (e.g., Comp 311) These abstractions have limits • Especially in the presence of bugs • Need to understand underlying implementations • Don’t believe what Ali says

3. Course Theme Useful outcomes • Become more effective programmers • Able to find and eliminate bugs efficiently • Able to tune program performance • Prepare for later “systems” classes • Programing Languages, Operating Systems, Computer Networks, Complex Systems

4. The Changes in Computing

5. Great Reality #1 Information is Bits + Context Examples Source Program: #include <stdio.h> int main() { printf(“hello world\n”); } ASCII representation of the source program (text file): # i n c l u d e <sp> 105 110 99 108 117 100 101 32 < s t d i o . h > 115 116 100 105 111 46 104 62 Etc…. Actual representation on the disk:

6. Great Reality #1 Information is Bits + Context Examples Source Program: #include <stdio.h> int main() { printf(“hello world\n”); } ASCII representation of the source program (text file): # i n c l u d e <sp> 105 110 99 108 117 100 101 32 < s t d i o . h > 115 116 100 105 111 46 104 62 Etc…. Actual representation on the disk: 00100011 01101001 01100011 01101100 01110101 01100100 …

7. All information is represented as bits Including disk files, programs in memory, user data in memory, data transferred across the internet, … The only thing that distinguishes data objects is context • Same bits may represent an integer, floating-point number, character string, or machine instruction.

8. Great Reality #1 cont. 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 --> 3.1 • 1e20 + (-1e20 + 3.14) --> ?? see Student/comp210/examples/testInt.c see Student/comp210/examples/testFloat.c

9. Great Reality #1 cont. 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 --> 3.1 • 1e20 + (-1e20 + 3.14) --> ?? see Student/comp210/examples/testInt.c -1794967296 0.0 see Student/comp210/examples/testFloat.c

10. 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

11. Computer Arithmetic • Observation • Need to understand which abstractions apply in which contexts • Important issues for compiler writers and serious application programmers