I/O Address, DMA Channel, Common IRQ and DMA Assignments

I/O Address

• An I/O address (also known as a port address) allows two devices in a computer to send information to each other.
• When a device wants to send information to another device, it addresses the data to the receiving I/O port number and sends it out on the system bus.
• Any device set with the receiving address can receive the information. I/O addresses are often confused with memory-mapped I/O (a scheme to map devices into system memory along with RAM and ROM).
• Each device in a computer must have its own I/O address.

DMA Channel

• A DMA channel allows a device to directly access RAM without going through the CPU first. 
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  • Devices such as hard drives, CD-ROM drives, tape drives, and sound cards use DMA channels to increase the speed of data transfers.
  • Devices must have a unique DMA channel.
  • If devices share a DMA channel, memory errors and system lockups will occur.
  • Ultra DMA supports burst mode data transfer rates. UDMA provides for direct transfers to and from RAM at twice the speed of standard DMA performance. Configure software support in Device Manager.
  • In Windows, the DMA checkbox only appears if an ATAPI driver is controlling the drive.
  ·         Common IRQ Assignments

  IRQ Channel	Assignment
  0	System clock (timing pulse)
  1	Keyboard
  2	Gateway (cascade) to IRQs 8-15
  3	COM 2 and COM 4
  4	COM 1 and COM 3
  5	LPT 2 (but often available and used for a sound card)
  6	Floppy drive
  7	LPT 1
  8	Real time clock
  9	Redirected as IRQ 2. Also, used by USB.
  10	Available
  11	Available (often used for network cards)
  12	Mouse (other than serial mouse)
  13	Math coprocessor
  14	Primary IDE interface
  15	Secondary IDE interface

  
  
   
  Common DMA Assignments

  DMA Channel	Assignment
  0	Available
  1	Sound card
  2	Floppy drive
  3	Available
  4	Second DMA controller
  5	Available
  6	Available
  7	Available

  
  
  
  
  
  
  
  Common I/O Address Assignments

  Device	I/O Address Range
  COM 1	3F8 to 3FF
  COM 2	2F8 to 2FF
  COM 3	3E8 to 3EF
  COM 4	2E8 to 2EF
  LPT 1	378 to 37F
  LPT 2	278 to 27F
  Floppy Drive	3F0 to 3F7
  Sound Card	Often set to base I/O 220
  Musical Instrument Digital Interface (MIDI) Port	Often set to base I/O 330
  Network Interface Card	Often set to base I/O 300