Today's Topics

• Memory-Mapped I/O
• Video I/O System
• Text-Mode Video

Memory-Mapped I/O

• Use system bus for perphial I/O

• Map hardware devices to memory addresses

Video I/O (Introduction)

• Video is the primary form of I/O between a human and a computer

• The Video Monitor (CRT) converts the analog RGB signal to an optical image.

• The Video Adapter converts digtial information from the CPU to analog signals for a monitor. The major components of video adapter are:

  

  • VRAM/DRAM Video/Dynamic Random Access Memory
    Stores screen content

  • Video BIOS
    Stores character mappings

  • Palette Registers
    Defines R/G/B color values

  • Graphic Accelerator
    Hardware implementing graphic routines

  • D/A (Digital to Analog) Converter
    Generates analog Red/Green/Blue (RGB) signals

Text-Mode Video

• VGA adapters support multiple modes of operation

• In text 80x25 color Text Mode, ASCII letters and attributes are written to the video card.

• The video buffer is memory mapped
  An array of consective bytes which are displayed on the screen.

  

• Each Text Page occupies just under 4KByte of memory
  80 (columns) * 25 (rows) * 2 (bytes/char) = 4000 Bytes < 4096 == 2^12

• There are Actually Four Pages of text
  • B8000: Page 0 (Using this for MP0)
  • B9000: Page 1
  • BA000: Page 2
  • BB000: Page 3

• To access Memory, Use Extra Segement
  • Set ES=VIDSEG=B800.
    
  • Use Segment override to write to memory

• Complete example of Writing a character to the screen

  MOV AX,VIDSEG ; VIDSEG = Defined Constant = B800 MOV ES,AX ; Load ES with B800 MOV DI,180 ; ( (1*80) + 10 )*2 = Row 1, Col 10 MOV AH, 00001010b ; Attribute: Intense Green on black MOV AL, 'A' ; Character: The ASCII Letter 'A' MOV [ES:DI],AX ; Write Attribute:Character to video

• Pages can be switched via BIOS calls
  (See lab manual for more information on INT 10h)

More Examples

• TMODE.EXE Demo Program (80x86 Executable)