Hardware devices are categorized as block devices, character devices, mmapped devices, or networked devices. A block device is a mass storage device (such as a disk) that can accept data, store it, and return data to the processor in fixed-length transfers. A block device driver uses the integrated page cache for all data transfers. Device drivers that support the block interface are complex and are not covered in this manual.
A character device (such as a terminal, network interface, or plotter) is a device that deals with arbitrary streams of data that typically have no particular structure. In addition, many character devices impose alignment restrictions (such as quad-aligned) and often require that you transfer data in multiples of the device's fundamental size. In particular, most IRIX devices doing DMA require the starting address at least to be aligned on a 32-bit boundary (the lowest two bits of the address are zeros). Unlike block devices, character devices do not use the integrated page cache.
Mmapped device drivers are those in which the hardware is memory mapped into a user's address space. No interrupt or DMA service routine is available to the user process.
Networked device drivers are covered in Chapter 9, "Writing Network Device Drivers".
It is possible, however, for some devices to fit both systems. Disk drivers often allow blocked, cached access as well as character, uncached access. Generally speaking, though, custom device drivers are most often written for character devices.
In some cases, a controller board may have more than one device connected to it. A SCSI-bus controller board, for example, normally has up to seven devices attached to it, and there may be multiple boards.