CSC103 How Computers Work

Notes 9

Programming Languages

1. History of Programming Languages
2. Programming Language components
3. Syntax: grammar for parsing
• Must be unambiguous (unlike natural languages)
4. Semantics: the meaning/function/action of each "statement" (sentence)
5. Examples:
   • LISP
   • C
   • C++
   • Perl
   • Python
6. Compiling/Interpretting: source code → byte code → machine language
7. byte code: generic machine-like language
8. Example:

   // Bytecode stream: 03 3b 84 00 01 1a 05 68 3b a7 ff f9
// Disassembly:
iconst_0 // 03
istore_0 // 3b
iinc 0, 1 // 84 00 01
iload_0 // 1a
iconst_2 // 05
imul // 68
istore_0 // 3b
goto -7 // a7 ff f9

9. Machine code, in machine language (CPU specific)
10. Example:

    op | rs | rt | rd |shamt| funct]
0 1 2 6 0 32 decimal
000000 00001 00010 00110 00000 100000 binary

11. Logo: Turtle graphics
12. Scratch

13. Free download, easy install, works on all platforms
14. Interface: Blocks Palette, Scripts area, Stage
15. Stage: 480 x 360, origin (0,0) in middle
• So: -240,+240, -180,+180
16. Program the action of sprites
17. We'll just use one, and draw pictures with it.
18. Right-click on any block to get explanation
19. Rounded blocks can fit in rounded positions, to, e.g., replace a number
20. Comments added by right-click in script area
21. Blocks: Motion, Pen, Control, Variables
22. Motion
• moving the sprite
23. Pen
• controlling color and thickness
24. Control
• repeat an action
25. Directions
• based on clock: 0=12noon, 90=3pm, 180=6pm, -90=270=9pm

26. Draw a square
27. Draw a spiral