1. High-level Languages

2. Assembly language benefits
Hides details of hardware

3. Assembly language benefits
Hides details of hardware
Easier to remember instruction names

4. Assembly language benefits
Hides details of hardware
Easier to remember instruction names
A tiny step closer to how humans think

5. Assembly language benefits
Hides details of hardware
Easier to remember instruction names
A tiny step closer to how humans think
Assembly language is a useful level of abstraction from the bare hardware

6. Assembly language benefits
Hides details of hardware
Easier to remember instruction names
A tiny step closer to how humans think
Assembly language is a useful level of abstraction from the bare hardware
Assembly language programs must be "assembled" into machine language executables.

7. Assembly Language issues
Still tied directly to the machine language instruction set

8. Assembly Language issues
Still tied directly to the machine language instruction set
Most machine language instructions do only a tiny amount of work

9. Assembly Language issues
Still tied directly to the machine language instruction set
Most machine language instructions do only a tiny amount of work
Humans solve problems at a higher level, using much larger steps

10. Example For each value I read in from a file
If the value is negative, then
add it to the list of expenditures
Otherwise
add it to the list of income
(This is pseudo-code, like the way we write textual storyboards)

11. High-level languages
Def: a programming language that provides statements that are a closer match to the way humans solve problems than assembly language.
Why can't we just write programs in English (or any other spoken language)?

12. Examples COBOL – business application language
FORTRAN – scientific application language
BASIC, Pascal – teaching languages
Ada – designed and used for US military software
Smalltalk – early object-oriented language
C, C++ - systems/general purpose languages

13. Can a program in a high-level language be executed on a computer?

14. High-level language translation
compiler – a program that translates a program written in a particular high level language into another form.
What form should a compiler output?

15. Compiled languages
High level languages that are translated into a machine language executable are called "compiled languages."
Executables might be called "native code."

16. HLL correspondence to AL
Benefit of HLL is being able to work in "bigger steps."
One HLL statement might translate to many AL/ML instructions

17. HLL correspondence to AL
Benefit of HLL is being able to work in "bigger steps."
One HLL statement might translate to many AL/ML instructions
Java statement
sum = sum + value;

18. HLL correspondence to AL
Benefit of HLL is being able to work in "bigger steps."
One HLL statement might translate to many AL/ML instructions
Woody equivalent
CopyFrom sum
Add value
CopyTo sum
Java statement
sum = sum + value;

19. Hierarchy of abstraction
High-level lang.
program
Language closest to "how humans think."
Input for compiler
Compiler
Assembly language
program
Assembler
Machine language
program

20. Hierarchy of abstraction
High-level lang.
program
Translates programs from a specific HLL
into a particular architecture's assembly
language.
Compiler
Assembly language
program
Assembler
Machine language
program

21. Hierarchy of abstraction
High-level lang.
program
Compiler
Assembly language
program
"Human readable" form of a particular
architecture's machine language.
Input for assembler.
Assembler
Machine language
program

22. Hierarchy of abstraction
High-level lang.
program
Compiler
Assembly language
program
Translates a particular architecture's
assembly language into that archi-
tecture's machine language.
Assembler
Machine language
program

23. Hierarchy of abstraction
High-level lang.
program
Compiler
Assembly language
program
Assembler
Machine language
program
Binary form executable version
of program.

24. Interpreted Languages
Interpreter – a program that can execute high-level language programs "directly," without first being translated to machine language.
Code in the interpreter emulates the circuitry that understands how to carry out each machine language instruction.
Some are "scripting" languages, used to stitch together existing executables to perform higher level tasks.

25. Examples TCL/TK Python Windows Batch files/Windows Scripting Host Perl
PHP – web scripting language

26. Compiled vs. interpreted
Compiled execution
Hardware follows instruction cycle for each machine language instruction in the executable
Interpreted execution
Interpreter translates HLL statement, then imitates hardware to execute it.

27. Interpreted pros and cons

28. "Hybrid" languages
Goal: High Level Language with platform-independent executable format, performance close to compiled languages.
How?

29. Examples
Java – originally intended for set-top TV/Video boxes, widely used on the web
C# - Supported by MS .Net platform, their answer to Java

30. How it works
Source program is compiled to an intermediate form – byte code
Byte code is the assembly language for an imaginary architecture
For each supported platform, write a "virtual machine" emulator that reads byte code and emulates its execution.

31. Java source
program
Java compiler
Java byte code
Java
Virtual
Machine
Java
Virtual
Machine
Java
Virtual
Machine
Java
Virtual
Machine
Wintel
Hardware
Sun
Hardware
IBM
Hardware
Mac
Hardware

32. web request web request web request Web server Java Virtual Java
Machine
Applet
Java
Virtual
Machine
Applet
Java
Virtual
Machine
Applet
web
request
web
request
web
request
HTML
HTML
Java
Applet
Java
Applet
Java
Applet
HTML
Web server

33. "Just In Time" compilation
JVMs actually compile each bytecode instruction to native code the first time it is used.
Subsequent executions of that instruction are faster.
Why would the same instruction be used again?

34. Computer Systems
A computer system is the combination of hardware and operating system
An operating system is the software that controls the overall operation of the computer, managing the processor, memory, files and attached devices

35. Computer System examples
PC – MS Windows OS running on Intel CPU
Mac – Mac OS running on IBM or Intel CPU
Linux – Linux OS running on Intel CPU
Unix – Unix OS running on various hardware platforms
IBM – OS/400 running on IBM servers