An amount of interruptions accessible for assignment depends on devices connected to a system. For example, if the support of a mouse is disabled in BIOS (the parameter “Mouse support”) an interruption IRQ12 will be accessible.
To assign interruptions, you may also use the parameter “Slot n IRQ”, where n – a number of PCI-slot. This parameter enables to assign a particular interruption to a slot. The numbers of the appropriate hardware interruptions are used as the values of this parameter: IRQ3, IRQ4, IRQ5, IRQ7, IRQ9, IRQ10, IRQ11. Besides, you can use the value “Auto” that enables to set an interruption number automatically. It should be stated that this parameter cannot be used in Windows 2000/XP.
Another parameter for assignment of interruptions is “PIRQ_n Use IRQ”, where n – ranges from 0 to 3. This parameter also enables to assign interruptions individually to each device on PCI and ISA-buses. In contrast to the parameter “Slot n IRQ”, this parameter has a wide range of functional possibilities. Its values indicate free or accessible IRQ-numbers. There is also the value “Auto” that enables to perform autoconfiguration of devices.
The last parameter for assignment of interruptions is “PCI Device Search Order” that enables to change the sequence of PCI-slots. This parameter may have two values: First-Last – from the first to the last (a standard bypass); Last-First – from the last to the first (a reverse bypass).
Similar to the assignment of a separate interruption to video card, it is possible to assign a separate interruption to USB-bus in BIOS. To do this, use the parameter “Assign IRQ for USB” which has got two values: Enabled and Disabled. This parameter should be turned on if you use two or more USB-devices simultaneously, for example mouse and scanner. Turn this parameter off if there are no USB-devices or you use only one device, for example only scanner or only digital camera.
The other opportunity to get the additional free interruption is to switch off a floppy-drive. The parameter “FDD IRQ Can Be Free” (Report No FDD For Win 95) enables to free up an interruption IRQ6. This parameter has got two values: Enabled and Disabled. But if a floppy drive is connected to a system, you must switch off its controller for freeing up an interruption. To do this, set a value “Disabled” for the parameter “Onboard FDC Controller” (FDC Function) in a section “Integrated Peripherals”.
Use the parameter “IRQ n Assigned to” (IRQn) for manual assignment of the rest resources the hardware interruptions, where n – a number of interruption. This parameter may have values describing the type of device for which this interruption can be assigned.
- Legacy ISA (ISA/EISA) – an interruption for ISA-devices that do not support Plug and Play technology. The interruptions for such devices are assigned in accordance with Technical documentation;
- PCI/ISA PnP (PCI/PnP) – an interruption for PCI or ISA-devices that support Plug and Play technology.
Manual assignment of resources is needed if there are some conflicts not related to the device capacity after automatic allocation. The simplest example is “hanging” of one device when switching on another one. A system often begins functioning normally after reassignment of interruptions and/or changing the location of a device.
Some problems often occur within the operation of video card. This is due to the fact that your video card has got a collective interruption with other devices, and until it operates in an office mode, your system handles the allocation of device requests. But as soon as you begin playing, the processing of large volumes of information exchanged by a central processor, system memory and video processor begins. In this case the shared use of interruption leads to reducing in productivity of all devices.
Use the parameter “Assign IRQ For VGA” (IRQ to PCI VGA, Allocate IRQ to PCI VGA) to solve this problem. This parameter has got two values: Enabled (Yes) and Disabled (No). If this parameter is enabled, a system assigns a separate interruption to video card that has positive implication for your computer’s productivity during games or while operating with programs that process video streams. Remember that if you assign a separate interruption to video card, you decrease the amount of interruptions for the rest PCI-devices and reduce their interaction with a system. All of these highlight the need to define priorities while using your computer: if an office operation of your computer is primary, it is inappropriate to assign a separate interruption for your video card.
If you operate with outdated PCI-video cards, you can also use the assignment of a separate hardware interruption using the parameter “Slot n IRQ for VGA” where n – an amount of accessible PCI-slots in your computer. A slot with the video card in it and that requires a separate interruption should have a value “Enabled” while the rest slots should have values “Disabled”.
The working capacity of your computer can significantly rely on the correct assignment of interruptions. That’s why, while installing the expansion cards in slots, remember the following:
- the same interruption number is assigned to AGP-slot and the first PCI-slot;
- if there are five PCI-slots, one interruption number is split between the fourth and the fifth slots;
- while installing a sophisticated device that requires two interruptions, the next slot , wherever possible, should be free.
In modern systems there are schemes enabling to connect the inner peripherals via USB-ports. Such schemes are rather convenient as USB-bus takes one interruption and enables to connect all peripherals (mouse, keyboard, scanner, video camera and other) without any further expenditure of resources. Shifting to USB-devices enables to decongest system interruptions in order to facilitate an access to them for other devices.
All the rest interruptions are assigned to the expansion cards that support Plug and Play technology. In case if the amount of expansion cards is greater than the amount of free interruptions, several devices may be on the same interruption. In some operating systems, for example Windows 2000/NT, all interruptions are again assigned after the system booting and it may happen that several devices, for example the sound card, video card and modem, are simultaneously located on the same interruption without any reasons. As a result, if such devices would operate in parallel, there will be rather tangible delays in a system, for example audio may “hang” during a telephone call by modem. And if the manual configuration and reallocation of interruptions are allowed in Windows 98, you cannot do these procedures by default in Windows 2000/XP – they would be available only if you switch off a manager of ACPI power saving mode.
All parameters enabling to specify the method for allocation the hardware interruptions can be found in a section “PNP/PCI Configuration BIOS”.
IRQ-interruption (Interrupt Request) is a device request for data processing by a processor. If an interruption is received, the processor stops its operations, saves the current state and transfers its control to a special program (interruption handler) which contains the commands for processing the condition that caused this interruption. A system assigns a particular interruption to each device, both peripheral and internal. While using this interruption, a device informs a processor about the need to process the request for this device.
Some interruptions are strictly assigned to the particular devices while the other interruptions can be assigned according to the current requirements. There are 16 interruptions in a system: from IRQ0 to IRQ15. The interruptions IRQ0, IRQ1, IRQ8 and IRQ13 are reserved by a system (for a system timer, keyboard, clocks and mathematical co-processor) and do not take part in assigning between the rest devices and expansion cards.
How to assign interruptions? The assigning of interruptions to the system devices on a motherboard are carried out after switching on the computer and its testing, then ISA- and PCI-devices are assigned. The interruptions for ISA-devices are assigned first because not all such devices support Plug and Play technology and may need manual (for example, using some switches on a motherboard) indication of interruption number. After the assignment, the configuration of interruptions is saved in the non-volatile CMOS-memory. During the next loadings this configuration is automatically called from memory, compared with the current system state and loaded in case of no changes. But if there are some changes in a system, for example replaced or deleted expansion cards, the procedure of interruption assignment is performed again.
With the standard frequency of PCI-bus equal to 33 MHz, the value “PCICLK/4” ensures the standard frequency of ISA-bus. But as most ISA-devices are able to operate with much higher frequency, the value “OCICLK/3” can be used for this parameter. This value sets the frequency 11 MHz for ISA-bus. Such transition like any other increase of frequency causes the enhanced heat transition by the appropriate devices In case of poor heat exchange, such procedure may cause overheated elements of ISA-device and its failure. It is not recommended to increase frequency while using the disk controllers as ISA-devices. It is clear that if the frequency of PCI-bus is changed, for example during processor overclocking, the frequency of ISA-bus is also changed.
The parameter “ISA Bus Clock” or similar to it may have the following values: PCI/x, PCICLK/x, CLK/x where x – a number ranging from 2 to 12 (2, 3, 4, 5, 6, 8, 10 and 12). The fixed values, for example 7.159 MHz, or the value “Auto” (indicates the automatic selection of frequency by a system) can also be used.
An access to ISA-bus is carried out in the following way: if there is a master-device, it is the dominant one while the rest devices wait for the bus release and then all received requests are allocated by using the bus controller. Use the parameter “Slave Wait States” (ISA Wait States) to indicate a delay while waiting an access to a bus. This parameter may have values “4WS” (4ISACLKs) and “5WS” (5ISACLKs) that indicate four or five waiting states in the bus frequencies accordingly.
By default the frequency of ISA-bus is equal to 8,33 MHz. But this value can be changed using the parameter “ISA Bus Clock” (ISA Clock, ISA Bus Clock Frequency, ISA Clock Divisor, AT Bus Clock Selection, AT Bus Clock Source). The frequency of this bus is set via the southern bridge of a chipset and depends on the frequency of PCI-bus which, in turn, depends on the frequency of FSB-bus (processor-chipset). So, this parameter has got the values that are multiple of the frequency of PCI-bus, for example PCICLK/4 or PCICLK/3.
ISA-devices are hardly used in modern computers, that’s why most manufacturers do not equip their motherboards with these slots. Nevertheless, some time ago ISA-devices were very popular.
In contrast to PCI-bus with several master-devices on it, ISA-bus may have only one master-device. Use the parameter “Bus Mastering” to permit the use of this device in the mode “bus master”. This parameter may have two values: Enabled and Disabled. If the value “Disabled” is set, no one device connected to ISA-bus could operate as dial with it.
Most parameters in BIOS, related to ISA-bus setting, enable to control various delays resulting from the slowness of ISA-bus against the rest components, for example PCI-bus.
Thus, the parameter “16 Bit ISA I/O Command WS” (ISA 16 Bit I/O Wait States) enables to compensate the difference between the operating speed of system devices and computer peripherals. Such compensation is compulsory, for example, the additional waiting time or respond from a device were not identified. If there is no preset delay, a signal about device capacity may fail and your system will automatically assume it as a nonfunctional device. This parameter may have two values: Enabled and Disabled. If the usage of delay is enabled, the system sets it automatically. Some types of BIOS offer the direct identification of a number of wait states. The best speed is achieved without any delays (the value “Disabled” or 0) but this may cause failures in operation of ISA-devices. If there are no expansion cards of ISA in your computer, this parameter should be turned off.
PCI-bus allows the parallel operation of several devices. In such operating mode the additional buffering of reading-writing cycles is turned on in a chipset. This enables to increase the system speed but not all PCI-devices support this mode correctly. That’s why BIOS has got the parameter “Peer Concurrency” (PCI Concurrency) that points to the possibility of concurrent operation of several devices. This parameter may have two values: Enabled –the concurrent operation is allowed; Disabled – the concurrent operation is prohibited. The parallel operation of PCI and ISA buses is a special case. Use the parameter “Passive Release” to control the opportunity of such work. This parameter may have two values: Enabled and Disabled.
When you transmit data from PCI to ISA bus, there is a delay related to the significant difference in the operating speed of these buses. To speed up the system operation, use the parameter “PCI to ISA Write Buffer” that enables to write temporarily the data in a special buffer without any interruption in processor operation. This parameter has got two values: Enabled and Disabled. If this mode is switched off, PCI-bus will wait for finishing the transmission of data via ISA-bus.