Today's Topics

1. Pixel-based Graphics Overview
2. Programming with VGA Mode 13h
3. Higher SVGA Resolutions with/VESA
4. String operations for fast graphics

Pixel-based Graphics Overview

• Review video card hardware

  

• Pixels represented by bytes in memory

• Typically Memory-Mapped (like text-mode)

• Multiple Color Depths (D)

  Mode	Bits/ Pixel	Number of colors	Color Mapping
  256 Color	8	256	Palette
  High Color	16	65,536	Palette
  True Color	24	16,777,216	Red/Green/Blue

• Multiple Resolutions (R)
  • 320x200 = 64k pixels
  • 640x480 = 307k pixels
  • 800x600 = 480k pixels
  • 1024x768 = 768k pixels
  • 1280x1024 = 1.28M pixels

• Amount of video memory required
  
  • Color Depth * Resolution
    M (bytes) = D (bytes/pixel) * R (pixels)

• Palette Registers
  • Diagram on board
  • Maps pixel values to colors
  • Table-Lookup provides many color shades (R/G/B values)
  • Color Depth limits # of different colors on screen
  • Artist Analogy

Programming with VGA Mode 13h

• Low-Resolution Graphics
  • 320x200 Mode w/256 Colors

  • One screen fits into one 64k segment

  • Mode used for ECE291 MP4

  • vBIOS call to set video mode
    • AH=0h (Set Mode)
    • AL=13h (320x200 @ 256 colors)
    • INT 10h (vBIOS)

• Palette Registers Define Color
  • Table-lookup maps 8-bit byte to 18-bit word

  • Palette entries store 6 bits/pixel (Red,Green,Blue)

  • Default (power-on) Palette color values
    • Entries 0-15: Basic Colors: [0,0,0,0,I,R,G,B]
    • Entries 16-31: Grayscale: [0,0,0,1,g3,g2,g1,g0]
    • Entries 32-255: User-Defined colors

  • Setting Palette Entries on VGA card
    • Method 1: Writing to VGA registers directly:
      • Write 0 to Port 3C8h
      • Write red, green, blue bytes to Port 3C9h for all 256 colors

    • Method 2: Using vBIOS call
      • BX = Palette Number (0..255)
      • CH = Green (0..63)
      • CL = Blue (0..63)
      • DH = Red (0..63)
      • AX = 1010h : Set VGA Palette Registers
      • INT 10h (vBIOS)

• Base Address = VidGrSEG = 0A000h

• Writing a (single) pixel

  MOV AX,VidGrSEG ; VidGrSEG = 0A000h MOV ES,AX ; Use Extra Segment MOV BX,Location ; Location = R*320+C MOV ES:[BX],Color ; Palette Entry Number

SVGA Graphics

• Higher resolution graphics require more than one segment of data
  • 640x480 * 256 colors
  • 800x600 * 256 colors

• VESA SVGA cards use bank switching to plot pixels

• Example Code
• More Info

String operations for fast graphics

• Repeat Prefix Opcode
  • REP/REPE/REPZ:
    CX--; Repeat until CX=0 while ZF=ZR=1

  • REPNE/REPNZ: CX--; Repeat until CX=0 while ZF=NZ=0

• Direction Flag
  • CLD: CLear Direction flag
    DF=0: Auto-increment
  • STD: SeT Direction flag
    DF=1: Auto-decrement

• String Instructions
  • MOVS: Move String
    
    • MOVSB (byte) ; MOVSW (word) ; MOVSD (double)
    • ES:[DI] = DS:[SI]
    • SI and DI incremented or decremented

  • CMPS: Compare String
    
    • CMPSB (byte) ; CMPSW (word) ; CMPSD (double)
    • Set ZF if ES:[DI] = DS:[SI]
    • SI and DI incremented or decremented

  • SCAS: Scan String
    
    • SCASB (byte) ; SCASW (word)
    • Set ZF if AL/AX=ES:[DI]
    • DI incremented or decremented

  • LODS: Read AL/AX/EAX from string
    • LODSB : Read AL from DS:[SI]
    • LODSW : Read AX from DS:[SI]
    • LODSD : Read EAX from DS:[SI]
    • SI incremented or decremented

  • STOS: Write AL/AX to string
    • STOSB : Write AL to ES:[DI]
    • STOSW : Write AX to ES:[DI]
    • STOSD : Write EAX to ES:[DI]
    • DI incremented or decremented

  • Fast Graphics
    • REP MOVSB : Transfer image to video
      • CLD : Auto Increment
      • CX=count
      • DS:SI=source
      • ES:DI=destination

    • REP STOSB : Draw Solid Horizontal Line
      • CLD : Auto Increment
      • CX=count
      • AL=source
      • ES:DI=destination

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