1. Instructor: Dr. Lynn Ziegler
Assembly & Machine Languages
CSCI130
Instructor: Dr. Lynn Ziegler

2. Layered Architecture LAYER Order High-order P.L.: Visual Basic 1
System SW: O.S. 3
Data Representation 5

3. Higher-level Programming Languages
(3GL) High-level languages are in the middle
Use English-like phrases and mathematical notation
x = x+1;
A limited vocabulary with precise meaning
Make us think at a higher-level of abstraction
No attention to technical and hardware details
Like O.S.
Much larger instruction set for programmers
Multiplication
Don’t worry about what circuits are available 3

4. Higher-level Programming Languages (PLs)
(1GL) Machine language programming
limited set of instructions
binary numbers:
opcodes, absolute memory addresses and data
Memory addresses
(2GL) Assembly language
English mnemonics for binary opcodes
Decimal data
Labels for memory addresses and variables
limited instruction set
Use human languages to program
Large vocabulary (space and time to search) … opcode lookup
500,000 words - 1,000,000 words (including scientific words)
Synonyms
Context

5. CPU - Connects to main memory (MM) thru a bus - Bus = bundle of wires
- Transfers data between CPU and memory
- Width of bus determines speed
- e.g. 32-bit bus (Higher  faster)
- Where is data stored when copied to CPU?

6. CPU Divided into CU and ALU CU (Control Unit)
Orchestra organizer
Oversees transfer of data between MM and CPU
Accesses info in ROM (e.g. during reboot)
Has a bunch of memory registers
ALU (Arithmetic Logic Unit)
Contains circuits to do computations
Built for each basic/hardwired operation (+,-, x, /, =,>,<, NOT)
Includes a special register accumulator AC to hold results

7. The History of Computing
Computers are made up of units called gates and storage units. Gates are used to process data while storage units are used to store data. Vacuum tubes were used for both.
ENIAC (February 14, 1946)
weighed over 30 tons, and consumed 200 kilowatts of electrical power 7

8. An ALU in 1957
This was an arithmetic unit you sit back and admire. It was part of Honeywell's Datamatic 1000 computer. (Image courtesy of Honeywell, Inc.)

11. Modern CPUs

12. Programs & Algorithms Characteristics of an algorithm:
List of steps to complete a task
Each step is PRECISELY defined and is suitable for the machine used
Increase the value of X
Jump!
Add 5 to variable X
The process terminates in a finite amount of time
No infinite loops
Written in an English-like language (Pseudocode) 12

13. Programs & Algorithms
Program: A formal representation of a method for performing some task
Written in a programming language understood by a computer
Detailed and very well-organized (computers just do what they are told)
Follows an algorithm … method for fulfilling the task
Plan to do something VS the actual performance

14. Combining Limited Instructions
Computing an answer to an equation:
5* – 7
Assume our computer can’t directly multiply, subtract, or raise to power
Multiplication task:
1: Get 1st number, Number_1
2: Get 2nd number, Number_2
3: Answer = 0
4: Add Number_1 to Answer
5: Subtract 1 from Number_2
6: if Number_2>0, go to step 4
7: Stop
Shows you in bold what functionality is needed to accomplish the task at hand 14

15. Assembly Language Machine code consists of the
binary instructions, data & addresses
can directly be executed by the CPU
We as humans are not good in working with sequences of 1’s and 0’s
We prefer to use an English-like style (English mnemonics and decimal numbers)
This is called the assembly language
It has 1-to-1 correspondence to machine code
one instruction for every 3-bit Opcode and decimal numbers for addresses and data
Slight other changes

16. Assembly Language Additional problems addressed:
Most commands like LOAD, STOR, JUMP and ADD require memory addresses
How do we know which address in memory is free to use?
We also assumed that PC will contain 0 initially
Our program will be loaded to first memory location…is this always true?
What if we have multiple executing programs on the same machine?
Which will get the 0 address then?

17. Assembly Language
In reality, when we program, we don’t concern ourselves with such low level details This is handled by the operating system.
From now on, we use variables/labels instead of addresses and totally forget about memory
 ADD Counter which must initialized

18. clock
MHz (~3.0 GHz)

19. Assembly Language of a Computer
Instruction
Description
HALT
Stop program execution
JUMP loc
Go to location loc to continue program execution (unconditional)
JZER loc
Go to location loc to continue program execution but only if AC=0 (i.e. if zflag=1) (conditional)
JPOS loc
Go to location loc to continue program execution but only if AC>0 (i.e. if pflag=1) (conditional)
LOAD loc
Copy contents of location loc into AC (zflag, pflag might be affected)
STOR loc
Copy contents of AC into location loc
ADD loc
Add contents of location loc to AC (zflag, pflag might be affected)
SUB loc
Subtract contents of location loc from AC (zflag, pflag might be affected)
READ
Put user’s input inside AC (zflag, pflag might be affected)
WRITE
Output contents of AC to user – e.g. on screen 19

20. Simple VB-like program 1
Private Sub cmdSimple ()
Dim X As Integer
X = inputbox(…)
Msgbox x
End Sub
READ
STOR X
LOAD X
WRITE
HALT
X: 0

21. Simple VB-like program 2
Private Sub cmdSimple ()
Dim X As Integer
X = inputbox(…)
If X>0 then
Msgbox x
End If
End Sub

22. Assembly Version for program 2
READ
STOR X
LOAD X
JPOS Disp
HALT
Disp: WRITE
X: 0

23. Simple VB-like program 3
Private Sub cmdSimple ()
Dim X As Integer
Dim Y As Integer
Y = 0
X = inputbox(…)
If X>0 then
Msgbox x
Else
Msgbox y
End If
End Sub

24. Assembly Version for program 3
READ
STOR X
LOAD X
JPOS Disp
LOAD Y
WRITE
HALT
Disp: WRITE
HALT
X: 0
Y: 0

25. VB-like program 4 Private Sub cmdSimple () Else End Sub
Dim X As Integer
Dim Y As Integer
Dim Z As Integer
X = inputbox(…)
Y = inputbox(…)
Z = 0
If X>Y then
Msgbox X
ElseIf X=Y then
Msgbox Z
Else
Msgbox Y
End If
End Sub

26. Assembly Version for program 4
READ
STOR X
STOR Y
LOAD X
SUB Y
JPOS MaxX
JZER Equal
LOAD Y
WRITE
HALT
MaxX: LOAD X
WRITE
HALT
Equal: LOAD Z
X: 0
Y: 0
Z: 0

27. VB-like program 5 Msgbox Sum Private Sub cmdSimple () End Sub
Dim Sum As Integer
Dim N As Integer
N = inputbox(…) Do
Sum = Sum + N
N = N -1
Loop While N>0
Msgbox Sum
End Sub

28. Assembly Version for program 5
READ
STOR N
Loop: LOAD Sum
ADD N
STOR Sum
LOAD N
SUB One
JPos Loop
LOAD Sum
WRITE
HALT
Sum:0
N: 0
One:1

29. VB-like program 6 Msgbox Sum Private Sub cmdSimple () End Sub
Dim Sum As Integer
Dim N1 As Integer
Dim N2 As Integer
N1 = inputbox(…)
N2 = inputbox(…) Do
Sum = Sum + N2
N2 = N2 -1
Loop While N2>=N1
Msgbox Sum
End Sub

30. Assembly Version for program 6
READ
STOR N1
STOR N2
Loop: LOAD Sum
ADD N2
STOR Sum
LOAD N2
SUB One
SUB N1
JPOS Loop
JZER Loop
LOAD Sum
WRITE
HALT
Sum:0
N1: 0
N2: 0
One:1

31. VB-like program 6 VB-like: Dim Sum As Integer Dim N As Integer
Program to print a 1 if an input number is even (multiple of 2) and 0 otherwise
Try it out in SimHymn: Applications.htm
VB-like:
Dim Sum As Integer
Dim N As Integer
N = InputBox(…) Do
N = N - 2
Loop While N > 0
VB-like:
If N = 0 Then
MsgBox 1
Else
MsgBox 0
End If
Branch example 31

32. Dec: 2 N: 0 One: 1 Zero: 0 READ STOR N LOOP: LOAD N SUB Dec STOR N
JPOS Loop
JZER Even
LOAD Zero
WRITE
HALT
Even: LOAD One
Dec: 2
N: 0
One: 1
Zero: 0

33. A Simple 8-bit Computer
Use a simplified version of real computers to understand machine language programs
32 8-bit main memory registers
5 bits (25 registers) to represent an address
00000 to 11111
PC holds memory addresses  5-bit PC
For simplicity, assume only positive and negative integers (2’s complement notation)
8-bit AC

34. clock
MHz (~3.0 GHz)

35. A Simple 8-bit Computer Two 1-bit flags
zflag: 1 if AC zero, 0 otherwise
pflag: 1 is AC is positive, 0 otherwise
ALU supports the following basic operations
Addition, subtraction, if AC=0, if AC>0
8 instructions (next slide)
3 bits (8 instructions)
000 to 111
8 bits per memory location  rest 5 bits of the instruction contain the address of the input data to the instruction
III AAAAA
8-bit IR
Memory holds 8-bit data or instructions

36. The Machine Language Instruction set depending on CPU
Hardwired
Binary code for each instruction (Opcode)
Different CPUs might have different operations for which circuits exit in the ALU
Programs can only use those opcodes
The set of all opcodes (i.e. instructions) together is known as the machine language

37. Assembly Language Computers can only understand machine code
Programs written in assembly must be translated to machine code to be executable by CPU
The assembler is responsible for that
Stored in memory and executed before any assembly program can be executed
(english-like) assembly source code  (binary) machine code
Does a lookup for each word (Opcodes)
Other things (find empty memory locations for my variables)
The result is called object code

38. Machine Language of a Computer
Instruction
Opcode
Address
Description
HALT: 000xxxxx
000 (i.e. 0)
NOT USED
Stop program execution (Halt does not require an address)
JUMP: 001xxxxx
001 (i.e. 1)
xxxxx
Place address in PC (unconditional)
JZER: 010xxxxx
010 (i.e. 2)
Place address in PC only of AC=0 (i.e. if zflag=1) (conditional)
JPOS: 011xxxxx
011 (i.e. 3)
Place address in PC only of AC>0 (i.e. if pflag=1) (conditional)
LOAD: 100xxxxx
100 (i.e. 4)
Copy contents of address into AC (zflag, pflag might be affected)
STOR: 101xxxxx
101 (i.e. 5)
Copy contents of AC into address
ADD: 110xxxxx
110 (i.e. 6)
Add contents of address to AC (zflag, pflag might be affected)
SUB: 111xxxxx
111 (i.e. 7)
Subtract contents of address from AC (zflag, pflag might be affected)

39. A Simple 8-bit Computer Input/Output (from user perspective)
LOAD 30:  register 30 in memory is designated as an input cell
A LOAD with address 30 causes machine to access an input device
The user is asked to provide an input (input box)
Program waits/pauses meanwhile
The input is saved in address 30 and loaded to AC
STOR 31:  register 31 in memory is designated as an output cell
The contents of AC is saved in address 31
Machine also copies data to output device
User sees the output on his/her screen (message box)

40. VB-like program 6 Msgbox Sum Private Sub cmdSimple () End Sub
Dim Sum As Integer
Dim N1 As Integer
Dim N2 As Integer
N1 = inputbox(…)
N2 = inputbox(…) Do
Sum = Sum + N2
N2 = N2 -1
Loop While N2>=N1
Msgbox Sum
End Sub

41. Assembly Version for program 6
READ
STOR N1
STOR N2
Loop: LOAD Sum
ADD N2
STOR Sum
LOAD N2
SUB One
SUB N1
JPOS Loop
JZER Loop
LOAD Sum
WRITE
HALT
Sum:0
N1: 0
N2: 0
One:1

42. Machine-like Version for program 6
0: LOAD 30 ( )
1: STOR 17 ( )
2: LOAD 30 ( )
3: STOR 18 ( )
4: LOAD 16 ( )
5: ADD 18 ( )
6: STOR 16 ( )
7: LOAD 16 ( )
8: SUB 19 ( )
9: STOR 16 ( )
10: SUB 17 ( )
11: JPOS 4 ( )
12: JZER 4 ( )
13: LOAD 16 ( )
14: STOR 31 ( )
15: HALT ( )
16: 0 ( )
17: 0 ( )
18: 0 ( )
19: 1 ( )

43. Von Neumann Machine
Will see how hardwired operations are accomplished later one
Comparison and addition
Build up other operations/tasks using those basic/hardwired operations ones
Programmed operations

44. Fetch-Execute Cycle Programs are made up of instructions
Fetch-Decode-Execute
CU supervises the fetch-execute cycle
Fetches one instruction from memory
Decodes instruction
Increments PC
Executes instruction

45. Fetch-Execute Cycle
To supervise the fetch-execute cycle, the CU has two registers
PC (program counter)
Contains the address of the next instruction in memory to fetch
Initially points to first instruction of a program
After fetching an instruction, it is incremented by one unless there is a jump to some other instruction
IR (instruction register)
Holds the current instruction
Also has a timing clock
Clock chip that generates a steady stream of electric pulses
At every pulse, the CPU proceeds by one step
Fetch, send data to ALU, etc …
Higher frequency clocks result in faster computers
MHz (~3.0 GHz)

46. … ... … ... Cycle PC IR AC - 1 – F 1 – D 1 – I 1 1 – E 2 – F 5 2 – D-
1 – F
(LOAD 30)
1 – D
1 – I 1
1 – E
5 (user input for N1)
2 – F
(STOR 17) 5
2 – D
2 – I 2
2 – E …
...
Cycle PC IR AC
5 – F 5
(ADD 18)
0 (sum)
5 – D
5 – I 6
5 – E
7 (0+ N2 which is 7) …
...
11 – F 11
(JPOS 4)
1 (N2-N2)
11 – D 1
11 – I 12
11 – E 4