from hard disk CD ROM or floppy disk there is no practical alternative to doing

From hard disk cd rom or floppy disk there is no

This preview shows page 92 - 95 out of 140 pages.

from hard disk, CD-ROM, or floppy disk there is no practical alternative to doing it this way. Once we have loaded the system, including our own I/O drivers, we can do better than the BIOS. [Page 288]
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The second choice then must be faced: without the BIOS support how are we going to make our drivers adapt to the varied kinds of hardware on different systems? To make the discussion concrete, consider that there are two fundamentally different types of hard disk controller usable on the modern 32-bit Pentium systems for which MINIX 3 has been designed: the integrated IDE controller and add-on SCSI controllers for the PCI bus. If you would like to take advantage of older hardware and adapt MINIX 3 to work on the hardware targeted by earlier versions of MINIX, there are four hard disk controller types to consider: the original 8-bit XT-type controller, the 16-bit AT-type controller, and two different controllers for two different types of IBM PS/2 series computers. There are several possible ways to deal with all these alternatives: 1. Recompile a unique version of the operating system for each type of hard disk controller we need to accommodate. 2. Compile several different hard disk drivers into the boot image and have the system automatically determine at startup time which one to use. 3. Compile several different hard disk drivers into the boot image and provide a way for the user to determine which one to use. As we shall see, these are not mutually exclusive. The first way is really the best way in the long run. For use on a particular installation there is no need to use up disk and memory space with code for alternative drivers that will never be used. However, it is a nightmare for the distributor of the software. Supplying four different startup disks and advising users on how to use them is expensive and difficult. Thus, another method is advisable, at least for the initial installation. The second method is to have the operating system probe the peripherals, by reading the ROM on each card or writing and reading I/O ports to identify each card. This is possible (and works better on newer IBM-type systems than on older ones), but it does not accommodate nonstandard I/O devices. Also, probing I/O ports to identify one device sometimes can activate another device which seizes control and disables the system. This method complicates the startup code for each device, and yet still does not work very well. Operating systems that do use this method generally have to provide some kind of override, typically a mechanism such as we use with MINIX 3. The third method, used in MINIX 3, is to allow inclusion of several drivers in the boot image. The MINIX 3 boot monitor allows various boot parameters to be read at startup time. These can be entered by hand, or stored permanently on the disk. At startup time, if a boot parameter of the form label = AT
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is found, this forces the IDE disk controller (at_wini) to be used when MINIX 3 is started. This depends upon the at_wini driver being assigned this label. Labels are
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