Page 287 374 Overview of the Hard Disk Driver in MINIX 3 The hard disk driver

Page 287 374 overview of the hard disk driver in

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[Page 287] 3.7.4. Overview of the Hard Disk Driver in MINIX 3
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The hard disk driver is the first part of MINIX 3 we have looked at that has to deal with a range of different types of hardware. Before we discuss the driver, we will briefly consider some of the problems hardware differences can cause. The "PC" is really a family of different computers. Not only are different processors used in different members of the family, there are also some major differences in the basic hardware. MINIX 3 has been developed on and for newer systems with Pentium-class CPUs, but even among these there are differences. For instance, the oldest Pentium systems use the 16-bit AT bus originally designed for the 80286 processor. A feature of the AT bus is that it was cleverly designed so older 8-bit peripherals could still be used. Later systems added a 32-bit PCI bus for peripherals, while still providing AT bus slots. The newest designs have dropped AT-bus support, providing only a PCI bus. But it is reasonable to expect that users with computers of a certain age may want to be able to use MINIX 3 with a mix of 8-bit, 16-bit, and 32-bit peripherals. For every bus there is a different family of I/O adapters. On older systems these are separate circuit boards which plug into the system parentboard. On newer systems many standard adapters, especially disk controllers, are integrated parts of the parentboard chipset. In itself this is not a problem for the programmer, as integrated adapters usually have a software interface identical to that of removable devices. Also, integrated controllers can usually be disabled. This allows use of a more advanced add-on device, such as a SCSI controller, in place of a built-in device. To take advantage of this flexibility the operating system should not be restricted to using just one kind of adapter. In the IBM PC family, as in most other computer systems, each bus design also comes with firmware in the Basic I/O System Read-Only Memory (the BIOS ROM) which is designed to bridge the gap between the operating system and the peculiarities of the hardware. Some peripheral devices may even provide extensions to the BIOS in ROM chips on the peripheral cards themselves. The difficulty faced by an operating system implementer is that the BIOS in IBM-type computers (certainly the early ones) was designed for an operating system, MSDOS, that does not support multiprogramming and that runs in 16-bit real mode, the lowest common denominator of the various modes of operation available from the 80x86 family of CPUs. The implementer of a new operating system for the IBM PC is thus faced with several choices. One is whether to use the driver support for peripherals in the BIOS or to write new drivers from scratch. This was not a hard choice in the design of early versions of MINIX, since the BIOS was in many ways not suitable to its needs. Of course, to start MINIX 3 the boot monitor uses the BIOS to do the initial loading of the system, whether
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