The BSD UNIX kernel supports various VAX hardware by having a CPU-independent support subsystem for each bus and all peripherals that can be attached to it, and having a module for each supported CPU that identifies and attaches all buses found on this CPU and performs supporting functions for the CPU itself, such as controlling the cache and the clock and handling machine checks.
During the course of VAX hardware development, DEC has created two major subsystems that are found on a variety of different CPUs and buses. They are specific to the VAX hardware and architecture, but not to any particular CPU or bus, making them a new class of code in the BSD UNIX kernel, belonging to the machine-dependent hardware support code, but not to the CPU or bus support modules. One of them is MSCP and cluster interfaces, the other is SCSI.
Finally, hardware support expansion has significant implications for booting and initial installation.
The VAX hardware support code in the BSD UNIX kernel thus consists of 4 major components:
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