1. CSCE 121:509-512 Simple Computer Model Spring 2015
Based on slides created by Bjarne Stroustrup and Jennifer Welch

2. HOW DATA IS REPRESENTED1
HOW DATA IS REPRESENTED

3. Fundamental unit of computer storage is a bit, a data entity that can take on one of two values, 0 or 1

4. Given a sequence of bits (01010001), what does it represent?
Depends on the code
Base 10 integer: 1,010,001
Base 2 integer: 1*20 + 1*24 + 1*26 = 81 (in base 10)
ASCII: The letter ‘Q’
Must know the code to decode a sequence
There are many other codes…

5. 2
HOW DATA IS STORED

6. Bytes Make Memory Byte: a group of 8 bits How many different values?
, , , …, ,
28 = 256 possibilities
Memory: long sequence of bytes, numbered 0, 1, 2, 3,…
byte
byte 1
byte 2
…..
byte 3

7. Addresses and Words
Address: the number of a byte in memory Contents of a byte can change Use consecutive locations to store longer sequences of information 4 bytes = 1 word
byte
byte 1
byte 2
…..
byte 3
a word

8. Limitations of Finite Data Encodings
Overflow: number is too large
suppose 1 byte stores integers in base 2, from 0 ( ) to 255 ( )
if the byte holds 255, then adding 1 to it results in 256, which is too large to be stored in the byte
CSCE : Set 2: Architecture

9. Limitations of Finite Data Encodings
Roundoff error:
insufficient precision (size of word): try to store 1/8, which is .001 in base 2, with only two bits
nonterminating expansions in current base: try to store 1/3 in base 10, which is .333…
nonterminating expansions in every base: irrational numbers such as π

10. Kinds of Storage slower and cheaper Cache: super-fast
Main memory: random access, equally fast to access any address
Disk: random access, significantly slower than main memory
slower and cheaper
Cache and main memory are where your program is executed.
Disk is where your program and data are stored.
According to the Computer History Museum, “Tape was a storage mainstay for many years and still survives, thanks to its low cost, portability, unlimited offline capacity, and standardized formats that make tapes interchangeable.”
Tape: sequential access, significantly slower than disk

11. 3
HOW DATA IS OPERATED ON

12. Main Players
Data in main memory cannot be operated on: must be copied (loaded) to special locations called registers
Once data is in registers, it can be operated on by circuitry called arithmetic logic unit (ALU)
e.g., add, multiply
Result of operation can then be copied (stored) back to main memory
Procedure is organized by control unit circuitry
figures out what the ALU should do next
transfers data between main memory and registers
Registers, ALU and control unit are in the CPU

13. CPU
ALU
registers
control unit
main memory

14. Machine Instructions
Goal: add the number stored in address 3 and the number stored in address 6; put the result in address 10.
Control unit does the following:
copies data from main memory address 3 into some register, say 1:
LOAD 3,1
copies data in main memory address 6 into some register, say 4:
LOAD 6,4
tells ALU to add the contents of registers 1 and 4, and put result in some register, say 3:
ADD 1,4,3
copies data in register 3 into main memory address 10:
STORE 3,10
LOAD, ADD and STORE are machine instructions.
How does the control unit know which instruction is next? The program!

15. 4
HOW A PROGRAM IS STORED

16. Program is stored in memory the same way data is stored!
Program: list of machine instructions using some agreed upon coding convention.
Example:
Program is stored in memory the same way data is stored!
opcode
1st operand
2nd operand
3rd operand
ADD

17. HOW A PROGRAM IS EXECUTED ON THE DATA5
HOW A PROGRAM IS EXECUTED ON THE DATA

18. How a Program is Executed
The control unit has
instruction register: holds current instruction to be executed
program counter: holds address of next instruction in the program to be fetched from memory
Program counter tells where the computer is in the program. Usually the next instruction to execute is the next instruction in memory
Sometimes we want to JUMP to another instruction (e.g., if or while)
unconditional JUMP: always jump to address given
conditional JUMP: only jump if a certain condition is true (e.g., some register contains 0)

19. CPU
ALU
registers
control unit
instruction register
program counter
main memory

20. Machine Cycle
fetch next instruction into instruction register (IR), as indicated by program counter (PC), and increment PC
decode bit pattern in IR to figure out which circuitry needs to be activated to perform instruction
execute instruction by copying data into registers and activating ALU to do the right thing
a JUMP may cause the PC to be altered

21. Diagram of Architecture
PC:
IR:
R2:
ALU
CPU:
R3:
control unit
R4:
bus
data …
main
memory:
first instr.
second instr.
third instr. … …
program

22. HOW A PROGRAM IS TRANSLATED INTO MACHINE LANGUAGE6
HOW A PROGRAM IS TRANSLATED INTO MACHINE LANGUAGE

23. Evolution of Programming Languages
High-level languages: such as Fortran, C, Java, C++
machine independent
easier for people
compiler translates high-level language programs into machine language
Assembly languages: allow symbols for operators and addresses
still machine dependent
slightly less painful
assembler translates assembly language programs into machine language
Machine languages: all in binary
machine dependent
painful for people
no translation needed

24. Why Compiling is Hard
one high-level instruction can correspond to several machine instructions
x = (y+3)/z;
fancier data structures
arrays: must calculate addresses for references to array elements
structs: similar to arrays, but less regular
fancier control structures than JUMP
while
repeat
if-then-else
case/switch
functions/subroutines/ methods: generate machine language to:
copy input parameter values
save return address
start executing at beginning of function code
copy output parameter values at the end
set PC to return address

25. Lexical Analysis Parsing Code Generation Compilation Process
break up strings of characters into logical components, called tokens, and discard comments and spaces
Ex: total = sum has 5 tokens
Lexical Analysis
decide how the tokens are related
Ex: sum is an arithmetic expression
Ex: total = sum is an assignment statement
Parsing
generate machine instructions for each high-level instruction
resulting machine language program, called object code, is written to disk
Code Generation

26. Combined code is written to disk.
Linking
Combined code is written to disk.
Linker combines results of compiling different pieces of the program separately.
If pieces refer to each other, these references cannot be resolved during independent compilation
function p …
refers to x
main
declares x
invokes p

27. Result is an executable
Loading
Result is an executable
Loader initializes PC to starting location of program and adjusts JUMP addresses
To run the program, the loader copies the object code from disk into main memory (location in main memory is determined by operating system, not programmer)

28. Source code for your program
Compiler
Object code for your program
Object code for libraries
Linker/Loader
Executable for your program

29. Credits Slide 1: “NEC APC” by Niv Singer, licensed under CC BY-SA 2.0
Slide 3: “La tecnología...” by Infocux Technologies, licensed under CC BY-NC 2.0
Slide 4:
Slide 8: “Overflow” by Braveheart, licensed under CC BY-NC-ND 2.0
Slide 9: “pumpkin pi” by Craig Damlo, licensed under CC BY-NC-ND 2.0