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

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. 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 Understanding assembly key to machine-level execution model • Behavior of programs in presence of bugs • High-level language model breaks down • Link-time errors hard to find • Tuning program performance • Understanding sources of program inefficiency • Avoiding security holes • Example: buffer overflow bugs • Implementing system software • Compiler has machine code as target • Operating systems must manage process state

5. Compilers Programs are translated by other programs into different forms • A C program in text-file format must be translated in a low-level binary format • Humans can read the text-file • Computers can read the binary • Assembly is (basically) a human readable form of binary • Translate source to binary in Unix by compiler driver: unix> gcc –o hello hello.c

6. Compilers Translation phases: • Preprocessing phase. The preprocessor (cpp) modifies the original C program according to directives that begin with #hello.chello.i • Compilation phase. The compiler (ccl) translates the text file into a different text file containing an assembly level file.hello.i hello.s • Assembly phase. The assembler (as) translates the assembly file into a machine language (binary) format. hello.s  hello.o • Linking phase. The linker (ld) merges separately compiled files, libraries (like stdio), etc. into an executable object file that can be loaded into memory and run. hello.0 printf.o  hello

7. Compilers Translation phases: • figure 1.3 printf.o Pre- processor (cpp) Compiler (cc1) Assembler (as) Linker (ld) hello.c hello.i hello.s hello.o hello Source program (text) Assembly program (text) Modified source program (text) Relocatable object programs (binary) Executable object program (binary)

8. Executing programs Processors only understand binary programs • The hello.c program has been translated to the executable object file hello and stored on disk • To run, we type the program’s name: unix> ./hello hello world unix> • A shell loads and runs the program.

9. Executing programs • Organization • To understand how the program is run, must understand how the hardware is organized.

10. Executing programs • Motherboard • Contains the processor, RAM, cache, bus interface

11. Executing programs CPU • The “brains”

12. Executing programs • Organization • We create abstract models of the hardware.

13. Executing programs • Buses. Carry a number of bytes of information between components. • The number is a fundamental system parameter called a word. • Example: Pentium word = 4 bytes; i7core = 8 bytes; embedded controllers = 1 or 2 bytes.

14. USB controller Graphics adapter Disk controller Disk Executing programs I/O Devices. The system’s connection to the external world. • Example: keyboard, mouse, display, disk drive • Connected to the I/O bus by either a controller or an adapter. • Controllers are chip sets in the device or on the motherboard. • Adapter is a card that plugs into a slot on the motherboard.

15. Executing programs • 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 (RAM) chips. • Logically organized as a linear array of bytes, each with a unique address. • Instructions and data will take up a variable number of bytes in RAM. Main memory

16. Executing programs • The central processing unit (CPU). The engine that interprets (or executes) instructions stored in main memory. • Consists of many electronic devices and small pieces of memory (registers): • Control unit (CU) • Arithmetic/Logic Unit (ALU) • Program counter (PC) register that points at next instruction in main memory. • Other general and special purpose registers. CU

17. Executing programs The central processing unit (CPU). Continued. • Performs the same basic tasks over and over again in a cycle (called the execution cycle) • Fetch the next instruction from main memory • Decode (or interpret) the bits in the instruction • Executes the instruction • Update the PC to point at the next instruction • There are only a few simple operations (or instructions) that a processor can execute

18. CPU Register file ALU PC System bus Memory bus Main memory "hello" Bus interface I/O bridge Expansion slots for other devices such as network adapters I/O bus USB controller Graphics adapter Disk controller Mouse Keyboard Display Disk User types "hello" Executing programs Putting it all together: running the hello program • When we type “./hello” the shell program reads each character and stores each into memory.

19. CPU Register file ALU PC System bus Memory bus Main memory "hello,world\n" Bus interface I/O bridge hello code Expansion slots for other devices such as network adapters I/O bus USB controller Graphics adapter Disk controller Mouse Keyboard Display hello executable stored on disk Disk Executing programs Putting it all together: running the hello program • When we hit the enter key, the shell loads the executable hello file by executing a sequence of instructions that copies the code and data from the disk to the memory. Uses DMA to bypass the CPU

20. Executing programs Putting it all together: running the hello program • Now the CPU begins executing the instructions in the hello program. • The program instructions copy the bytes in the string “hello world\n” from memory to the register file, • then from there to the display device. • The device displays the string on the screen. • See next slide.

21. Running the hello program CPU Register file ALU PC System bus Memory bus Main memory "hello,world\n" Bus interface I/O bridge hello code I/O bus Expansion slots for other devices such as network adapters USB controller Graphics adapter Disk controller Mouse Keyboard Display Disk hello executable stored on disk "hello,world\n"

22. Example See: http://courses.cs.vt.edu/csonline/MachineArchitecture/Lessons/CPU/index.html Much simpler than the IA32 architecture. Does not make the register file explicit. Uses an accumulator (not in IA32).