0. Department of Computer Science Faculty of Science RU.
COS2014 Basic Concepts
Department of Computer Science
Faculty of Science
RU.

1. Assembly Language for Intel-Based Computers, 5th Edition
Kip Irvine

2. Why study assembly language?
Learn about computers
Computer architecture
Operating systems
Data representation
Hardware devices
Learn about assembly languages
Learn about compiling
Learn how to write embedded programs
Learn the assemble language for Intel 80x86

3. Welcome to Assembly Language: Definitions
Assembly language: machine-specific language with a one-to-one correspondence with the machine language for that computer
Machine language: The language a particular processor understands
Assembler: converts programs from assembly language to machine language

4. Welcome to Assembly Language: Example
High-Level Language:
x = a + b;
Assembly Language:
MOV AX, a ADD AX, b MOV x, AX
Machine language:
A A3 0000

5. Welcome to Assembly Language: Problems with assembly language
Provides no structure
Is not portable
Applications can be very long
“Hard” to read and understand
Lots of detail required

6. Assembly Language Machine language: Assembly language:
Machine instructions: direct instructions to the processor, e.g. to be encoded to control the datapath
A numeric language understood by the processor
Assembly language:
Statements (instructions) in short mnemonics and symbolic reference, e.g. ADD, MOV, CALL, var1, i, j, that have a 1-to-1 relationship with machine instructions
Understood by human

7. CPU, memory, I/O, Operations on variables, …
When you are writing a C program，
how does a computer look like？
CPU, memory, I/O,
Operations on variables, …

8. A Model of Computer for C
i = i + j;
xfloat = 1.0;
if (A[0]==0) …
Program
Counter
Memory
i, j, k;
xfloat, yfloat;
&& + 
CPU
A[0], A[1], … if
for
Typed storage

9. Why a C program code can run on your computer?
This model is quite different from what the hardware in a computer does when you run your program！
Why a C program code can run on your computer?
Obviously, someone does some translation for you！

10. We have known … C program Assembly program C compiler x = (a+b) * b
MOV AX, a
ADD AX, b
MUL c
MOV x, AX
C compiler
Input and output of a compiler

11. We can see that Assembly Lang. is closer to real computer hardware!
From the angle of Assembly Lang., how does a computer look like?

12. A Model of Computer for ASM
MOV AX, a
ADD AX, b
MOV x, AX …
Memory a
CPU AX b BX PC
... JX x
+ -

13. We need one more level of translation！
Assembly program/machine code
still have some distance to the real computer hardware.
e.g. Multi-core CPU、hyperthreading
We need one more level of translation！

14. Computer Model of a Lower Layer
(from Computer Architecture textbook)

15. A Layered View of Computer+  if
for
&&
CPU
Memory
i, j, k;
xfloat, yfloat;
A[0], A[1], …
i = i + j;
if (A[0]==0) …
Program
Counter
i = i + j;
xfloat = 1.0;
if (A[0]==0) …
CPU
Memory
ADD AX, b
MOV x, AX … a b x AX BX JX
...
+ - PC
MOV AX, a
ADD AX, b
MOV x, AX …
xxxxxx
xxxxx

16. From Assembly to Binary
MOV AX, a
ADD AX, b
MUL c
MOV x, AX
Assembler
Input and output of an assembler
Machine code

17. Different Levels of Abstractions
temp = v[k];
v[k] = v[k+1];
v[k+1] = temp;
High Level Language Program
Compiler
MOV AX, a
ADD AX, b
MOV x, AX
Assembly Language Program
more elaborated later in Sec. 1-2: Virtual Machine Concept
Assembler
Machine Language Program
Machine Interpretation
Control Signal
ALUOP[0:3] <= InstReg[9:11] & MASK

18. What’s Next? Virtual machine concept (Sec. 1-2)
Data representation (Sec. 1-3)

19. Virtual Machine Concept
Purpose of this section:
Understand the role of assembly language in a computer system
Side product:
The principle of layered abstraction for combating complexities, e.g. OSI 7-layer protocol

20. Virtual Machine Concept
A layered abstraction of computers proposed by A. Tanenbaum
Each layer provides an abstract computer, or virtual machine, to its upper layer
Virtual machine:
A hypothetical computer that can be constructed of either HW or SW
What is a computer?

21. Simplest Model of Computers
Program
Instructions
Input data
Compute engine
Memory
Output data
c.f., y = f(x)
Layered abstraction: A computer consists of layers of such virtual machine abstractions

22. Why Layered Abstraction?
Big idea: layered abstraction to combat complexities
A strategy of divide-and-conquer
Decompose a complex system into layers with well-defined interfaces
Each layer is easier to manage and handle
Only need to focus on a particular layer, e.g. to make it the best
Also, it makes interaction clear
Particularly if one layer is realized in hardware and the other in software

23. Layered Abstraction of Computer
Each layer as a hypothetical computer, or virtual machine, that runs a programming language
Can be programmed with the programming language to process inputs and outputs
Program written in Li can be mapped to that Li-1 by:
Interpretation: Li-1 program interprets and executes Li instructions one by one
Translation: Li program is completely translated into Li-1 program, and runs on Li-1 machine
Compute engine
Memory
Input data
Output data
Program
Instructions

24. Layered Abstraction of Computer
Virtual Machine +  if
for
&&
CPU
Memory
i, j, k;
xfloat, yfloat;
A[0], A[1], …
i = i + j;
if (A[0]==0) …
Program
Counter
i = i + j;
xfloat = 1.0;
if (A[0]==0) … Li
CPU
Memory
ADD AX, b
MOV x, AX … a b x AX BX JX
...
+ - PC
MOV AX, a
ADD AX, b
MOV x, AX …
Li-1
xxxxxx
xxxxx

25. Languages of Different Layers
English: Display the sum of A times B plus C.
C++: cout << (A * B + C);
Assembly Language:
mov eax,A
mul B
add eax,C
call WriteInt
Intel Machine Language: A F E

26. High-Level Language Level 5
Application-oriented languages, e.g., C, C++, Java, Perl
Written with certain programming model in mind
Variables in storage
Operators for operations
Programs compiled into assembly language (Level 4) or interpreted by interpreters
What kind of computer does C see?

27. Assembly Language Level 4
Instruction mnemonics that have a one-to-one correspondence to machine language
Based on a view of machine: register organization, addressing, operand types and locations, functional units, …
Calls functions written at the OS level (Level 3)
Programs are translated into machine language (Level 2)
What kind of computer does it see?

28. Operating System Level 3
Provides services to Level 4 programs as if it were a computer
Programs translated and run at the instruction set architecture level (Level 2)

29. Instruction Set Architecture
Level 2
Known as conventional machine language
Attributes of a computer as seen by assembly programmer, i.e. conceptual structure and functional behavior
Organization of programmable storage
Data types and data structures
Instruction set and formats
Addressing modes and data accessing
Executed by Level 1 program (microarchitecture)

30. Microarchitecture Level 1
Can be described by register transfer language (RTL)
Interprets conventional machine instructions (Level 2)
Executed by digital hardware (Level 0)
Register
Memory
Control Signals
Controller
ALU N Z
clock IR PC

31. Digital Logic Level 0 CPU, constructed from digital logic gates
System bus
Memory

32. What’s Next? Virtual machine concept (Sec. 1-2)
Data representation (Sec. 1-3)

33. Data Representation Purpose of this section
Assembly program often needs to process data, and manage data storage and memory locations  need to know data representation and storage
Binary numbers: translating between binary and decimal
Binary addition
Integer storage sizes
Hexadecimal integers: translating between decimal and hex.; hex. subtraction
Signed integers: binary subtraction
Character storage

34. Binary Numbers Digits are 1 and 0 MSB – most significant bit
1 = true
0 = false
MSB – most significant bit
LSB – least significant bit
Bit numbering:

35. Binary Numbers Each digit (bit) is either 1 or 0
Each bit represents a power of 2:
Every binary number is a sum of powers of 2

36. Integer Storage Sizes Standard sizes: Why unsigned numbers?
What is the largest unsigned integer that may be stored in 20 bits?

37. Signed Integers
The highest bit indicates the sign 1 = negative, 0 = positive
If the highest digit of a hexadecimal integer is > 7, the value is negative. Examples: 8A, C5, A2, 9D

38. Forming Two's Complement
Negative numbers are stored in two's complement notation
Complement (reverse) each bit
Add 1
Why?
Note that =

39. Binary Addition
Starting with the LSB, add each pair of digits, include the carry if present.

40. Hexadecimal Integers
All values in memory are stored in binary. Because long binary numbers are hard to read, we use hexadecimal representation.

41. Translating Binary to Hexadecimal
Each hexadecimal digit corresponds to 4 binary bits.
Example: Translate the binary integer to hexadecimal:

42. Converting Hexadecimal to Decimal
Multiply each digit by its corresponding power of 16:
dec = (D3  163) + (D2  162) + (D1  161) + (D0  160)
Hex 1234 equals (1  163) + (2  162) + (3  161) + (4  160),
or decimal 4,660.
Hex 3BA4 equals (3  163) + (11 * 162) + (10  161) + (4  160),
or decimal 15,268.

43. Data representation: Number systems (bases)
Number systems used
Binary: The internal representation inside the computer. Externally, they may be represented in binary, decimal, or hexadecimal.
Decimal: The system people use.
ASCII representations of numbers used for I/O:
ASCII binary
ASCII octal
ASCII decimal
ASCII hexadecimal

44. Data representation: Hex Addition & Multiplication
Hex addition and multiplication tables are large
We can still do simple calculations by hand B852h Ah A65h * 100h
(Your turn) ABCh B3h E F 0 h * 102h

45. Data representation: Converting to decimal
= 1 * * * * *100
(Human) conversions: hex to decimal ABCDh = 10* * * * = 10* * * = =

46. Data representation: Converting to decimal
(Human) conversions to decimal ABCDh = (((10*16+11)*16+12)*16+13 = (easier on calculator)

47. Data representation: Your Turn: Conversion problems
1234 base or (1234)5= _________ 10

48. Data representation: Conversion from decimal
(Human) conversions from decimal = ??? In hex
2748 = 171 *
171 = 10 *
10 = * so value is ABCh
How do we know this is the Hex representation?
2748 = 171 *
= (10* ) *
= 10 * * * 160
= ABCh

49. Data representation: Your Turn: Conversion problems
Write decimal 58 in binary.
Write decimal 194 in base 5

50. Learn How To Do the Following:
Form the two's complement of a hexadecimal integer
Convert signed binary to decimal
Convert signed decimal to binary
Convert signed decimal to hexadecimal
Convert signed hexadecimal to decimal

51. Ranges of Signed Integers
The highest bit is reserved for the sign. This limits the range:
Practice: What is the largest positive value that may be stored in 20 bits?

52. Character Storage Character sets Null-terminated String
Standard ASCII (0 – 127)
Extended ASCII (0 – 255)
ANSI (0 – 255)
Unicode (0 – 65,535)
Null-terminated String
Array of characters followed by a null byte
Using the ASCII table
back inside cover of book

53. Numeric Data Representation
pure binary
can be calculated directly
ASCII binary
string of digits: " "
ASCII decimal
string of digits: "65"
ASCII hexadecimal
string of digits: "9C"

54. Character Representation
ASCII (Table of ASCII Codes)
American Standard Code for Information Interchange
Standard encoding scheme used to represent characters in binary format on computers
7-bit encoding, so 128 characters can be represented
0 to 31 & 127 are "control characters" (cannot print)
Ctrl-A or ^A is 1, ^B is 2, etc.
Used for screen formatting & data communication
32 to 126 are printable (see the last page in textbook)

55. ASCII Character Codes CHAR DECIMAL HEX BINARY '9' 57d 39h 0011 1001b
'A' 65d 41h b
'Z' 90d 5Ah b
'a' 97d 61h b
'z' 122d 7Ah b

56. Binary Data
Decimal, hex & character representations are easier for humans to understand; however…
All the data in the computer is binary
An int is typically 32 binary digits
int y = 5; (y = 0x ;)
In computer y =
int z = -5; (y = 0xFFFFFFFB;)
In computer, z =

57. Binary Data A char is typically 8 binary digits
char x = '5'; (or char x = 53, or char x = 0x35)
In computer, x =
char x = 5; (or char x = 0x05;)
In computer, x =
Note that the ASCII character 5 has a different binary value than the numeral 5
Also note that 1 ASCII character = 2 hex numbers

58. Storage Size Terminology
Byte
8 bits (basic storage size for all data)
Word
16 bits (2 bytes)
Doubleword
32 bits (4 bytes)
Quadword
64 bits (8 bytes)