Today's Topics

• Table-lookup funtions
• Jump Tables
• Interrupt Vectors
  • Hardware/Software Interrupts
  • DOS/BIOS/vBIOS calls
• Hash functions

Table-lookup functions

• Consider movement in an array by one element
  • Let Array = Matrix of ROWSIZE x COLSIZE elements
  • Let Pos = Position in array = Row * COLUMNSIZE + Column
  • Let Dir = Direction (UP=0, RIGHT=1, DOWN=2, LEFT=3)

• Slow and Tedious Original Code:

  Compare each possible direction value

  MBegin: CMP [DIR],UP JE MoveUP CMP [DIR],RT JE MoveRT CMP [DIR],DN JE MoveDOWN CMP [DIR],LT JE MoveDown MoveUP: SUB Pos,COLSIZE JMP MDone MoveRT: ADD Pos,1 JMP MDone MoveDN: ADD Pos,COLSIZE JMP MDone MoveLT: SUB Pos,1 JMP MDone MDone: Finished!

• Fast and Compact Table-lookup Code

  Eliminate conditional jumps by defining a table of movements indexed by the Direction. This method runs in constant O(1) time.

  Movement dw -COLSIZE ; Movement[0] == Go UP dw +1 ; Movement[1] == Go RIGHT dw +COLSIZE ; Movement[2] == Go DOWN dw -1 ; Movement[3] == Go LEFT MBegin: MOV BX, [DIR] ; Direction = {0..3} SHL BX,1 ; Index words, not bytes MOV AX, [Movement+BX] ADD Pos,AX ; Position += Movement MDone: Finished!

• In General: Table lookups provide an efficient method of mapping, altering, or updating values when the input has a well-defined sequential range of values.

Jump Tables

• Suppose table entries are pointers to functions

• Suppose BX holds a command in the range of 0..N-1.

• Let cmd index a function the code

  JFUNCTION: ; Input BX = Function number to execute (0,1,..,n-1) SHL BX,1 ; Words, not bytes JMP [Jtable+BX] ; Jump to function ; Note: Be Very careful NOT to put the contents ; of the table in-line with executable code. ; Note that the JMP statement above will prevent ; the Instruction pointer from attempting to ; execute type bytes in the Jtable variable Jtable dw offset Funct0 dw offset Funct1 .. dw offset FunctN-1 Funct0: do something .. RET Funct1: do something else .. RET .. FunctN-1: do something else .. RET

Interrupt Vectors

• Jump Tables are the basis for interrupt vectors

• On 80x86, low 1024 bytes of memory hold vector table
  • There are 256 pointers in the table
  • Each entry is a far (4 byte segment:offset) pointer

    Memory Address (hex)	Interrupt Function Pointer
    003FC	INT 255
    4 * x	INT x
    00008	INT 2
    00004	INT 1
    00000	INT 0

• Interrupts can be triggered by hardware or software
  • CPU (divide by zero)
  • Perphials (Keyboard input)
  • Software Opcode (INT n)

• A Few Interrupt Functions (see Class-resources or Art of Assembly for a more complete listing)
  • SYSTEM BIOS Functions

    All PCs come with a BIOS ROM (or EPROM). The BIOS contains standard 80x86 opcodes to provide basic functions such as bootstraping and primitive I/O.

    • INT 19h: Bootstrap
    • INT 11h: Equipment checker
    • INT 16h: Keyboard I/O
    • More..

  • DOS Functions

    With software interrupts, we can also define a sub-function by placing a value in a register. For example, to execute of the many DOS functions, you can specify a sub-function by loading a value into AH just before calling INT 21

    • INT 21h, Subfunction:
      • AH=3: CS306's KBDOUT (char in DL)
      • AH=7: CS306's KBDIN (result in AL)
      • AH=3Dh: Open File
      • AH=3Fh: Read File
      • AH=3Eh: Close File
      • AH=13h: Delete File (!)
      • AH=2Ah: Get system date
      • AH=2Ch: Get system time
      • AH=2Ch: Read DOS Version
      • AH=47h: Get Current Directory
      • AH=48h: Allocate Memory block (specified in paragraphs==16 bytes)
      • AH=49h: Free Memory block
      • AH=4Bh: Load and execute program
      • AH=4Ch: Terminate program (and free resources)

  • VIDEO BIOS Functions

    Video cards come with 80x86 commands burned in a ROM. Video BIOS is located at C0000-C7FFF and holds routines for handling basic functions. To execute a function in video BIOS ROM, call INT 10 with AX=sub-function. Video BIOS calls can also be used to place characters or pixels on the screen; but it MUCH SLOWER to use this method as compared to writing directly to video memory.

    • INT 10h, Subfunction:
      • AH=0,AL=2h: 80x25 Text
      • AH=0,AL=13h: 320x200, 256-color graphic

  • MOUSE Functions : INT 33h

    Routines to read position and status of mouse

    • AX=0000h: Reset
    • AX=0001h: Show Mouse
    • AX=0003h: Get Position & Status
      • BX = Buttons Pressed: bit[0]=left, bit[2]=right
      • CX = Column Position (*8)
      • DX = Row Position (*8)

  • HARDWARE Functions
    • IRQ 0-7 FUNCTIONS : INT 8-Fh
      • System timer (IRQ 0 == INT 8h)
      • Keyboard (IRQ 1 == INT 9h)
      • Bus/DMA (IRQ 2 == INT Ah)
      • Serial Port (IRQ 3 == INT Bh)
      • SoundBlaster (IRQ 5 == INT Dh)

        INT = 8 + IRQ

    • IRQ 8-15 FUNCTIONS : INT 70h-77h
      • SCSI Request (IRQ 10d == INT 72h)

        INT = 68h + IRQ

• The function executed by an interrupt can be changed by updating the vector table.
  We will do this in later MPs

Hash Tables

• Consider using a table lookup when the input (I) can span a large range of values but is sparsely populated.
  • Example: Count incoming packets from each of 100 hosts scattered throughout the internet using the 32-bit IP source address.

• A Table-Lookup would require 4 billion table entries, most of which would be empty.

• A Hash Function, H(x), can be used to re-map The four-byte (W.X.Y.Z) IP address to smaller (8-bit) value.
  • Example: H(W.X.Y.Z)=W xor X xor Y xor Z

    H(128.174.5.58) = 17
    H(224.2.4.88) = 190
    H(141.142.44.33) = 14

• A good hash function evenly distributes the input vales to outputs

• Collisions, however, can occur when two inputs map to the same output.

• Collision Resolution: Linear Probing
  • Use next-available location in the hash table
  • Recall that Unhashed values are stored in the table to differentiate between two entries that hash to the same location.