Standard IRQ and I/O Address Assignments
IRQ	Assignment	I/O Address
0	System Timer	040-043
1	Keyboard	060-064
2	Link to IRQ9 for IRQs > IRQ7	0A0-0A1
3	COM2, COM4	2F8-2FF
4	COM1, COM3	3F8-3FF
5	Sound card	220-22F
6	Floppy disk	3F0-3F7
7	LPT1	378-37F
8	System clock	070-07F
9	Link to IRQ 2	N/A
10	Available	N/A
11	Laptop PC Cards	N/A
12	Bus mouse	238-23B
13	Math coprocessor	0F8-0FF
14	IDE hard disk	N/A
15	Secondary IDE drive	N/A

     DMA (direct memory access) channels allow a device to bypass some functions of the motherboard and write or read directly to or from RAM. Not every device gets to be a DMA device, but those that do should not be in conflict.

Standard DMA Channel Assignments
DMA Channel #	Standard Assignment
0	Available
1	Sound card
2	Floppy disk controller
3	16-bit sound card (uses 2 DMA channels)
4-7	Available (newer systems only)

     Bus mastering goes one step further than DMA and lets a device bypass not only the CPU, but RAM also. Bus-mastered devices are intelligent devices that have a microprocessor on board to manage the device, which allows the bus-mastered device to operate concurrent with and independant of the CPU, greatly improving its performance. In fact, bus-mastered devices can pass data and instructions directly to one another without assistance from or disturbing the CPU. Many EISA, MCA, and PCI adapter cards feature bus mastering.

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     IDE (Integrated Drive Electronics) allows a system to install up to two disk drives in a series. This interface protocol is also known as ATA (AT Attachment) interface. EIDE (Extended IDE) allows up to four drives to be installed, two each on two IDE cables. If you have just installed a second hard disk drive and the Hard Drive not Present error displays during the POST, you haven't set the master / slave jumpers.

     An IDE cable has a 40-pin ribbon cable that must be connected with the red or blue strip of the cable aligned to pin 1. Connecting the ribbon cable in backward on either the drive or the adapter, normally causes the system not to boot and the drive LED to light up and stay on.

     A SCSI chain can hold up to eight total devices, including the SCSI host adapter which is always designated as ID 7. If an SCSI disk will be the boot device, it must be ID 0. If both IDE and SCSI hard disk drives are on one system, the IDE device should be the boot device. An SCSI chain must be terminated with resistors at each end. The CMOS setup for a SCSI hard disk is either SCSI drive type or drive type 1.

     Device drivers are TSR (Terminate and Stay Resident) software that communicates between the operating system and the device itself. Devices displayed in the Windows 95 Device Manager with a yellow exclamation point symbol are missing a device driver, an IRQ, or another system resource.

Modem Control (AT) Commands
Command	Description
AT	Procedes all modem action commands.
ATDTnnnnnnn	Dial the nnnnnnn telephone number using Touch-Tone dialing.
ATDPnnnnnnn	Dial the nnnnnnn telephone number using Pulse dialing.
ATA	Answer an incoming call.
ATH0	Hang up immediately.
AT&F	Reset the modem to factory default settings.
ATZ	Reset the modem to its power-up settings, which may include modifications made by the user.

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Upgrading the Motherboard

     The motherboard, CPU, memory modules, SRAM, and virtually everything inside the case, except the case itself, are considered to be field replaceable units (FRUs).

     When adding memory to a system, it must be at the same speed as the existing memory unless all the memory is to be replaced. The easiest way to match the memory is to use the last part of the number on the chips. For example, on a 70 nanosecond SIMM, each chip will have a "-70" at the end of the chip number, or on an old 286 PC, the memory may have a "-12" for 120 nanosecond memory. You must fill the DIP memory banks, or SIMM banks, completely before you can begin adding memory to the next bank or banks.

     You will need to run the CMOS setup utility and update the CMOS settings to reflect the new memory. If the CMOS already reflects the new memory, save the settings and exit.

     You can add additional cache memory to help optimize the performance of the CPU and operating system. A cache controller anticipates the data or program module that the CPU is likely to request next. The data is copied from DRAM and stored in SRAM. The SRAM can transfer the data to the processor much faster than is possible with DRAM.

     Prior to the 486 processor, the cache controller was installed on the motherboard on a seperate integrated circuit chip. Since the 486, the cache controller is now integrated into the CPU along with an amount of SRAM, which is called internal, or Level 1 (L1) cache. Additional cache memory is called external, or level 2 (L2), cache. A point of diminishing returns exist to adding L2 cache memory more than 512K.

     CMOS data is usually organized into basic and advanced information. The basic data includes the following types of entries:

• System date and time - this is kept in the CMOS because it always has power and can keep the system up to date.
  
• Type of floppy disk drive.
  
• Type of each hard disk drive installed on the system.
  
• Amount and type of memory installed on the system.
  
• Type and number of ports along with their system resource assignments.
  
• Disk drive boot sequence.
  
• The video adapter card type or architecture.