1. CSCE Fall 2012
Prof. Jennifer L. Welch

2. Simplified Model of a Computer
How data is represented and stored
How data is operated on
How a program is stored
How the program is executed on the data
How programs are translated into machine language
CSCE

3. How Data is Represented
Fundamental unit of computer storage is a bit, data entity that can take on two values, 0 and 1
Given a sequence of bits ( ), 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
Other codes:
Negative numbers (e.g., 2’s complement)
Numbers with fractional parts (floating point)
CSCE

4. How Data is Stored Byte: a group of 8 bits; 28 = 256 possibilities
, , …, ,
Memory: long sequence of locations, each big enough to hold one byte, numbered 0, 1, 2, 3,…
Address: the number of the location
Contents of a location can change
Use consecutive locations to store longer sequences
e.g., 4 bytes = 1 word
…..
byte 0
byte 1
byte 2
CSCE

5. 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 0, not 256
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
CSCE

6. Kinds of Storage
The computer reads and writes the data stored in it. From fastest to slowest:
cache: super-fast
main memory: random access, equally fast to access any address
disk: random access, but significantly slower than main memory
tape: sequential access, significantly slower than disk
CSCE

7. How Data is Operated On Central processing unit (CPU) consists of:
arithmetic/logic unit (ALU):
performs operations on data (e.g., add, multiply)
registers: special memory cells, hold data used by ALU, even faster than cache
control unit:
figures out what the ALU should do next
transfers data between main memory and registers
CSCE

8. 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!
CSCE

9. How a Program is Stored
Program: list of machine instructions using some agreed upon coding convention.
Example:
Program is stored same way data is stored!
ADD
opcode
1st operand
2nd operand
3rd operand
CSCE

10. 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)
CSCE

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

12. Diagram of Architecture
PC:
IR:
R1:
R2:
R3:
R4:
ALU
control
unit
CPU:
bus
data …
main
memory:
first instr.
second instr.
third instr. … …
program
CSCE

13. Evolution of Programming Languages
Machine languages: all in binary
machine dependent
painful for people
Assembly languages: allow symbols for operators and addresses
still machine dependent
slightly less painful
assembler translates assembly language programs into machine language
High-level languages: such as Fortran, C, Java, C++
machine independent
easier for people
compiler translates high-level language programs into machine language
CSCE

14. Compilation Challenges faced by a compiler:
one high-level instruction can correspond to several machine instructions
Ex: x = (y+3)/z;
fancier data structures, such as
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
CSCE

15. Compilation Challenges faced by a compiler:
functions (a.k.a. procedures, subroutines, methods): must generate machine language code 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
CSCE

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

17. Linking
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.
Combined code is written to disk.
function p …
refers to x
main …
declares x
invokes p
CSCE

18. Loading
Loader: when it’s time to run the program, the loader copies the object code from disk into main memory
location in main memory is determined by the operating system, not the programmer
Loader initializes PC to starting location of program and adjusts JUMP addresses
Result is an executable
CSCE