System Software

1
System Software

• Making von Neumann Friendly

2
The Naked Machine

• Von Neumann hard to work with
• Binary numbers everywhere
• Have to tell the computer where to start
• User Interface
• Hide messy details of HW from user
• Present info in form not requiring CS degree
• Easy user access to resources
• Prevent/repair accidental damage to HW, programs
  data

3
Virtual Machine

• Describing SW not HW
• No System Software circuit build from AND, OR
  NOT gates
• Automobile is similar
• HW Engine, power train, etc
• For most, HW a pain. Just want to drive
• Interface (SW) dashboard steering wheel

4
System Software

• Operating System
• Communicates with user
• Determines what user wants
• Activates required SW

5
Types of System Software
Operating System
Language Translators
Information Managers
Utilities
Editor, Graphics Etc.
Assemblers Compilers
File System Data Bases
Memory Managers
Scheduler
Linkers Loaders
6
Writing a Program

• Use editor to create source for program P
• Use file system to store source on hard disk
• Use compiler to translate to machine code
• Use loader to allocate enough memory to hold
  machine code put into memory
• Use scheduler to run the program
• Use file system from program to store output in
  data file
• If program failed, use debugging utility to
  locate problem.

7
Programming the Computer

• Whats wrong with machine code?
• Uses binary
• No English-like words (or even math)
• Only numeric addresses (hard to keep track of)
• Difficult to change (entering binary is a pain!)
• Difficult to create data (you have to convert
  everything yourself.

8
Assembler

• First language translator
• Early 1950s
• Second generation computers
• Not as frequently used because of newer high
  level languages like BASIC, Fortran, COBOL, C,
  JAVA and C
• More later in the course

9
Programming a Computer
Source
Editor
Assembler
Errors
Machine Code
Loader
In RAM
Results
Hardware
10
The Assembler

• Each line in an Assembler program has the same
  format

Label Opcode Address --Comment
Used if you want To JUMP here
Symbolic Address (Label)
Like ADD, COMPARE, JUMPGT, etc.
11
The Pseudo-Ops

• Used to generate data values instead of having to
  convert the values to binary ourselves

X .data 7
Value being stored in X
Label or Symbolic Address
Pseudo-Op to Create a Data Value
12
The Pseudo-Ops

• Where the program begins and ends
• .begin First line of assembler program
• .end Last line of assembler program

13
An Example

• Example1.mac

.begin LOAD FIRST ADD SECOND STORE
ANSWER HALT FIRST .data 2 SECOND .data 4 ANSWE
R .data 0 .end
14
Lets try a few

• Suppose we want to enter three numbers and add
  them up. Well call our three numbers A, B, C.

15
First Problem
.begin IN A -- Enter three numbers IN B IN C
LOAD A -- Get first number ADD B -- Add
second number ADD C -- Add third number STORE
ANS -- Store Answer OUT ANS -- Output
Result HALT A .data 0 B .data 0 C .data
0 ANS .data 0 .end
16
Your Turn

• You try it with the calculation
• RESULT (A B) (C D)

17
My Solution
.begin IN A -- Enter four numbers IN B IN C IN
D LOAD A -- Get first number ADD B -- Add
second number STORE TMP -- Store AB LOAD C --
get third number ADD D -- Add fourth
number STORE TMP2 -- Store CD LOAD TMP --
Get AB SUB TMP2 -- Subtract CD STORE ANS --
Store Answer OUT ANS -- Output Result HALT
A .data 0 B .data 0 C .data 0 D .data
0 ANS .data 0 TMP .data 0 TMP2 .data 0 .end
18
How about an If-Then-Else

• If A gt B Then
• Output A
• Else
• Output B

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A Solution
.begin IN A -- Enter two numbers IN B LOAD B
-- Compare A and B COMP A JUMPGT ABIG -- A gt
B OUT B JUMP DONE ABIG OUT A DONE HALT A .da
ta 0 B .data 0 .end
20
Your Turn

• If First gt Second Then
• Diff First Second
• Else
• Diff Second First
• Output Diff

21
A Solution
.begin IN FIRST -- Enter two numbers IN SECOND
LOAD SECOND -- Compare numbers COMP
FIRST JUMPGT BIG1 -- First gt
Second LOAD SECOND -- Diff 2 nd-1
st SUB FIRST STORE DIFF JUMP DONE BIG1 LOAD F
IRST -- Diff 1 st - 2 nd SUB SECOND STORE DI
FF DONE OUT DIFF -- Output difference HALT FIR
ST .data 0 SECOND .data 0 DIFF .data 0 .end

22
And a While Loop

• Twos 1
• While Twos lt 16000 Do
• Output Twos
• Twos Twos Twos

23
A Solution
.begin WHILE LOAD TWOS -- Compare TWOS
32000 COMPARE END JUMPLT DONE -- TWOS gt 16000
Done! OUT TWOS -- Output twos
power LOAD TWOS -- Double the value in
Twos ADD TWOS STORE TWOS JUMP WHILE -- Go
back for another pass DONE HALT TWOS .data
1 -- Starting Twos value END .data 16000 --
Biggest Value .end
24
Your Turn

• Sum 0
• Input a value
• While the value ? 0 Do
• Add the value to the Sum
• Input a value
• Output Sum

25
My Solution
.begin IN VALUE -- Get a value WHILE
LOAD VALUE -- Compare Value 0 COMPARE
ZERO JUMPGT DONE -- Value lt 0
Done! LOAD SUM -- Sum Sum
Value ADD VALUE STORE SUM IN VALUE -- Get A
Value JUMP WHILE -- Go back for another
pass DONE OUT SUM -- Output the
sum HALT SUM .data 0 -- Starting Sum
value ZERO .data 0 -- Zero value VALUE .data 0
-- Input value .end