| IRQ 10 | Sometimes COM 4 — not considered fixed |
| IRQ 11 | Sometimes COM 3 — not considered fixed |
| IRQ 12 | PC/2-style mouse |
| IRQ 13 | Coprocessor |
| IRQ 14 | Primary Integrated Device Electronics (IDE) controller |
| IRQ 15 | Secondary IDE controller |
Table B.3 Legacy ISA hardware DMA considered fixed (PC 99)
| Hardware DMA | System function (default) |
|---|---|
| DMA 0 | ISA expansion |
| DMA 1 | |
| DMA 2 | FDC |
| DMA 3 | extended capabilities port (ECP) parallel port on LPT 1 |
| DMA 4 | DMA controller cascading |
| DMA 5 | |
| DMA 6 | |
| DMA 7 |
Over the years, various hardware developments have seen the light of day, such as MCA, EISA, PCI, and SCSI buses, with Plug and Play capability. Not all these developments have won market support.
PC 99 still supports all these 'legacy' peripherals, if they are present in the system.
PCs have a Basic I/O System (BIOS) in ROM that runs when the computer is first switched on. This ROM finds the relevant operating system and runs its boot loader to start DOS, Windows 98, Windows 2000, or whatever.
Changing World
Most programs that are used nowadays run using Win32 (or at least Win16) functions. These routines do not allow direct access to I/O ports, etc. For example, when keyboard input is required, W98 or W2000 carries out the necessary operation. In the IBM-compatible environment, it interacts with the keyboard controller using techniques dating from time immemorial.
Less DOS applications are being run. In W98, NT, and W2000, these do not get direct access to the hardware. Instead, a Virtual Device Driver arbitrates access to these peripherals, allowing multiple DOS and Windows applications to run at the same time.
These two developments allow the Windows operating systems to move away from the original IBM-compatible specification. As long as the right keystroke messages get through to user programs, it does not matter where in hardware they come from. For old DOS programs that do try to access the actual hardware, virtual device drivers on faster new machines can emulate the required functionality.
The new wisdom is that the USB and IEEE 1394 (originally called FireWire) devices represent an easier way for users to connect assorted peripherals to the system, including standard system components such as a keyboard and a mouse. The idea is that any devices can be plugged in easily (even when the computer is on) and — within limits — daisy chained together. USB and IEEE 1394 devices cope with being plugged in anywhere. The Windows Plug and Play system sorts out any resource allocations on the fly.
Whether users will take to the new doctrine remains to be seen. Plugging a new USB device in will certainly be easier than dismantling a case, fiddling with jumpers, and inserting a card. The new Device Bay specification will allow easy insertion of cards within the chassis of the PC.
USB's 12Mbs is designed for slower-speed devices, such as keyboards, mice, joysticks, printers, modems, telephones, and monitors. IEEE 1394 runs faster, at 100 Mbs or more, and is more useful for devices such as scanners, storage units, and audio/visual kit, such as digital cameras.
ACPI, OnNow, and Plug and Play
New PCs aim to use less power and switch on more quickly, as well as supporting Plug and Play. Windows and new BIOS chips use the Advanced Configuration and Power Interface (ACPI) specification to help with these tasks.
ACPI-based hardware knows how to turn off or reduce power to some devices. Windows drivers can now help in this process by using the Power Management IRPs and routines, and the power management routines that are relevant for its device class. The aim of the OnNow initiative is that a PC may appear to be off, when in fact it is just 'sleeping', waiting for a soft power on button to be
