System Resources

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Channels, addresses, and other signals hardware devices use to communicate on the bus.
SYSTEM RESOURCES (the order in which you would experience problems with)
  • Memory Addresses –most troublesome and most difficult to explain
    • With IRQ’s your system can respond to external events in a timely manner
    • Hardware interrupts also known as maskable interrupts can mask or turn off interrupts for a short time while the CPU is used for critical operations.
    • The highest priority hardware interrupts have the lowest numbers.
    • IRQ chips were wired to cascade through IRQ2 leaving 14 channels open.
    • High-priority interrupts takes precedence over low-priority interrupts (by interrupting them).
    • Out of Stack Resources –too many interrupts generated too quickly (error occurs and your system halts).
    • Two types of interrupts:
      • PCI interrupts (level sensitive/shareable) –INTA# to INTD#
      • ISA interrupts (non-shareable)
    • PCI IRQ Steering allows for several PCI devices to be mapped on a single IRQ.
    • You can have several PCI devices mapped to the same ISA IRQ only if:
      • No ISA devices are using the IRQ
      • The BIOS and OS support PCI IRQ Steering
      • PCI IRQ Steering is enabled
    • IRQ Steering is controlled by one of four routing tables Windows attempts to read (in sequence).
      • ACPI BIOS table
      • MS Specification table
      • Protected Mode PCIBIOS 2.1 table
      • Real Mode PCIBIOS 2.1 table
        • IRQ Holder indicates that an IRQ has been mapped to a PCI and is unavailable for ISA devices, even if no PCI devices are using the IRQ.
        • PCI buses enable two types of devices to exist:
          • Bus masters (initiators) are devices that can take control of the bus and initiate a transfer.
          • Slaves (targets) are the intended destination of the transfer.
        • Arbitrated bus –part of the PCI bus controller in the motherboard chipset that governs all bus transfers, giving fair and controlled access to all the devices on the bus.
        • APIC (advanced programmable interrupt controller) replacement controller for 8259 to support virtual PCI IRQ’s above 15 up to 24.
        • 8-bit IRQ priority (0-7)
        • 16-bit IRQ priority (0-2, 8-15, 3-7 [7 is the last; 2 is replaced by IRQ9])
        • It’s up to the system BIOS or programs to manage interrupts properly by assigning unique IRQ’s to PCI devices.
        • IRQ conflicts arise in most serial or integrated COM ports.


    • DMA (direct memory access) Channels –used by the obsolete ISA standards
      • DMA channels are used by communication devices that must send and receive information at high speeds.
      • 8-bit DMA channel range (0-3)
      • 16-bit DMA channel range (0-7)
      • 16-bit ISA cascaded channel 4 leaving 7 channels open; all with prioritized cards
      • DMA 2 is the only standard channel universally used in all systems by the floppy controller.


  • I/O port Addresses –used by all hardware devices on the bus (64KB of them)
    • I/O ports allow communication between devices and software in your system.
    • Virtually all devices use I/O ports
    • There are exactly 65,536 ports/addresses in a 16-bit design.
    • I/O ports are designated by hexadecimal addresses similar to memory addresses.
    • Windows Device Manager will show you current addresses used on your motherboard.
  • If your system is PnP-compatible, potential conflicts should be resolved automatically.
  • You know you have a problem when one of your devices stops working.
    • Inaccurate data transfer
    • Your system locks up
    • Sound card doesn’t sound right
    • Mouse doesn’t work
    • Cant format a floppy
  • Conflicts would be highlighted with red or yellow in the Device Manager.
  • Run a virus scan before acting on a resource conflict.
  • Disable unused Comx: ports to free up IRQS
  • Avoid using ISA devices in general.
  • The first thing you should check at start up of a new system is the BIOS setup to ensure PnP is enabled.
  • Minimize initial configuration at startup with graphics card, memory, and storage drives only.
  • Reboot between devices.
  • Add one device at a time (loading sequence for additional cards):
    1. Sound Card
    2. Internal or External modem
    3. Network Card
    4. Auxiliary Video Devices
    5. SCSI Adapter
    6. Anything else

It is the responsibility of the operating system to inform users of conflicts that can’t be resolved by the BIOS.
IRQs cause more problems than DMAs because they are much higher in demand.
Bus Mastering –bypasses processor [maybe system resources as well]
DMA & IRQ both use cascading and prioritized cards
PnP was not an entirely new concept, MCA and EISA preceded it by almost 10 years.
The USB bus only uses 1 IRQ.
Sound cards are the biggest resource hog.
COM1 (03F8; IRQ4)
COM2 (02F8; IRQ3)
COM3 (03E8; IRQ4)
COM4 (02EB; IRQ3)
Parallel Port (0378; IRQ7)
IDE Primary (01F0; IRQ14)
IDE Secondary (0170; IRQ15)
Primary Floppy (03F0; IRQ6)