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

2. Course Theme Courses to date emphasize abstraction
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>
< s t d i o . h >
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>
< s t d i o . h >
Etc….
Actual representation on the disk: …

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 * >
50000 * > ??
Is (x + y) + z = x + (y + z)?
Unsigned & Signed Int’s: Yes!
Float’s:
(1e e20) > 3.1
1e20 + (-1e ) --> ??
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 * >
50000 * > ??
Is (x + y) + z = x + (y + z)?
Unsigned & Signed Int’s: Yes!
Float’s:
(1e e20) > 3.1
1e20 + (-1e ) --> ??
see Student/comp210/examples/testInt.c
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