Computer Systems An Introducton

Computer Systems: Introduction Programmable machines Hardware + Software (program) Hardware Program 5/31/2019 Computer Systems: Introduction

Computer Systems: Introduction What does a program do? Take input data from users or world Process data Output the result Take input from keyboard and mouse Determine if the input is a click at button “save”, or a click at button “slide show”, or a press on keyboard “K”. Then, do it! Display the message “saved”, or show slide, or letter “K”. 5/31/2019 Computer Systems: Introduction

Representation of Data and Programs Bits: 0 or 1 Bytes: string/sequence of bits e.g. 10010110 Normally, a byte is composed of 8 bits. Words: sequence of bytes A word can be composed of 4 or 8 bytes. There are standard formats of data stored in a computer. ASCII code, Unicode 5/31/2019 Computer Systems: Introduction

Languages and Programs A program is a sequence of instructions for computer. A format of instructions = a language Computer languages High-level languages Java, Python, C, ASP, HTML, … Low-level languages Assembly languages Machine languages 5/31/2019 Computer Systems: Introduction

Computer Systems: Introduction Examples of Languages High-level languages C int main(void) { int n, m, sum=0; for (n=0; n<7; n++) { scanf(“%d”, &m); sum+=m; } return 0; Assembly languages 6800 assembly ldx i ldaa ar,x staa max ; max = ar[0] lp:ldaa i+1 cmpa #2 bgt ex ; while i <= N cmpa max ; if max<ar[i] ble sp staa max ; then max=ar[i] sp:inx ; i=i+1 stx i jmp lp ex: Easier for human to understand Easier for machines to understand (Also, for machine languages) 5/31/2019 Computer Systems: Introduction

Computer Systems: Introduction Language Translation A compiler is a program which translates a program in one language into another language. Source program compiler Target program Source language Target language 5/31/2019 Computer Systems: Introduction

Computer Systems: Introduction Compiler Usually, the source language is a high-level language, and the target language is a low-level language. Assembler is a compiler which translates from an assembly language to a machine language. 5/31/2019 Computer Systems: Introduction

Where are programs/data stored? In memory Components of computers Input Unit Processor Output Unit Memory 5/31/2019 Computer Systems: Introduction

Computer Systems: Introduction Memory Linear storage Addressable in words “Random access” Data/program can be read from/ write to memory, one word at a time. To write data d to memory addressed a , data d and address a must be specified. To read data from memory addressed a , address a must be specified. 5/31/2019 Computer Systems: Introduction

Components of a computer: More detail processor memory registers ALU data program Control units Instruction register data address Program counter 5/31/2019 Computer Systems: Introduction

Components of Processors Arithmetic-logic unit (ALU) Performs arithmetic and logic functions: add, substract, multiply, divide, and, or, not, etc. Registers Fast-access memory used by a processor to store intermediate results. Program counter (PC) Keeps track where the current instruction is. Normally, incremented to point to the next instruction. Changed by instructions that alter the flow of control of a program (if-then-else, loop, and function call). 5/31/2019 Computer Systems: Introduction

Components of Processors Instruction register (IR) Store the current instruction fetched from memory. Its content (the instruction) signals the control unit to initiate the execution of that instruction. Control unit Most complex part of a processor. Send control signals to all parts in the processor to co-ordinate their activities. A large finite state machine. 5/31/2019 Computer Systems: Introduction

How a computer system works Fetch-execute cycle Fetch: get an instruction (addressed by PC) from memory and store in IR, and update PC. Execute: work according to the instruction in IR If the instruction causes PC changed, PC is updated again. 5/31/2019 Computer Systems: Introduction

Instruction Execution Cycle processor memory registers ALU data program Control units Instruction register data address Program counter increment 5/31/2019 Computer Systems: Introduction

Computer Systems: Introduction Processor Control unit Data path Logic Multiplexor Registers Bus 5/31/2019 Computer Systems: Introduction

Computer Systems: Introduction Data Path Logic is a combinational function, out = f(in1 ... inn) where f is a Boolean function {not, and, or}. out = x1 + x2 · x3 + x1 ·x2 f x1 x2 x3 out 5/31/2019 Computer Systems: Introduction

Computer Systems: Introduction Data Path Registers are storage elements out(t+1) = in(t) control signal “load” width of a register defines the number of bits that can be stored register n n in out load 5/31/2019 Computer Systems: Introduction

Computer Systems: Introduction Data Path Muliplexors have n inputs (of width m) and select one input to be the output, called n:1 multiplexor. The control signal to determine the output is called the select signal. x0 x1 x2 x3 MUX out selector 5/31/2019 Computer Systems: Introduction

Computer Systems: Introduction Data Path Bus consists of wires and buffers. Wires carry data (signal). Buffer controls the traffic of data from any element to a bus. A bus can be shared to reduce the number of wire within a circuit. A bus can broadcast data to many receivers limited by the fan-out electrical characteristic of the bus, the ability to drive other circuits. 5/31/2019 Computer Systems: Introduction

Levels of descriptions of computer systems Applications Operating systems Instruction set Functional units Finite state machine Logic gates Electronics 5/31/2019 Computer Systems: Introduction

Levels of descriptions of computer systems Applications Application level is what a user typically sees a computer system, running his/her application programs. An application is usually written in a computer language which used many system functions provided by the operating system. Operating systems Instruction set Functional units Finite state machine Logic gates Electronics 5/31/2019 Computer Systems: Introduction

Levels of descriptions of computer systems Applications An operating system is abstraction layers that separate a user program from the underlying system-dependent hardware and peripherals. Operating systems Instruction set Functional units Finite state machine Logic gates Electronics 5/31/2019 Computer Systems: Introduction

Levels of descriptions of computer systems computer architecture begins at the instruction set. An instruction set is what a programmer at the lowest level sees of a processor one instruction set with different level of performance for many models, based on the implementation of a control unit via “microprogram”. Present day processor designs converge, their instruction sets become more similar than different. Applications Operating systems Instruction set Functional units Finite state machine Logic gates Electronics 5/31/2019 Computer Systems: Introduction

Levels of descriptions of computer systems Units are divided by their functions, e.g. ALU, control unit, etc. The interconnection between units are described. Applications Operating systems Instruction set processor registers ALU Control units Instruction register Program counter Functional units data Finite state machine memory Logic gates address Electronics 5/31/2019 Computer Systems: Introduction

Levels of descriptions of computer systems Applications Mathematical description of the behavior of a system. Important tool for verification of the correct behavior of the hardware. Operating systems Instruction set Functional units Finite state machine Logic gates Electronics 5/31/2019 Computer Systems: Introduction

Levels of descriptions of computer systems Applications Logic gates (e.g. and, or, not gates) are used to build functional units. Operating systems Instruction set Functional units Finite state machine Logic gates Electronics 5/31/2019 Computer Systems: Introduction

Levels of descriptions of computer systems Applications Electronic circuits are used to build logic gates. Operating systems Instruction set Functional units Finite state machine Logic gates Electronics 5/31/2019 Computer Systems: Introduction