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Disk I/O Support
162Bug can initiate disk input/output by communicating with
intelligent disk controller modules over the VMEbus. Disk support
facilities built into 162Bug consist of command-level disk
operations, disk I/O system calls (only via one of the TRAP #15
instructions) for use by user programs, and defined data structures
for disk parameters.
Parameters such as the address where the module is mapped and
the type and number of devices attached to the controller module
are kept in tables by 162Bug. Default values for these parameters
are assigned at powerup and cold-start reset, but may be altered as
described in the section on default parameters, later in this chapter.
Appendix B contains a list of the controllers presently supported, as
well as a list of the default configurations for each controller.
Blocks Versus Sectors
The logical block defines the unit of information for disk devices. A
disk is viewed by 162Bug as a storage area divided into logical
blocks. By default, the logical block size is set to 256 bytes for every
block device in the system. The block size can be changed on a per
device basis with the
The sector defines the unit of information for the media itself, as
viewed by the controller. The sector size varies for different
controllers, and the value for a specific device can be displayed and
changed with the
When a disk transfer is requested, the start and size of the transfer
is specified in blocks. 162Bug translates this into an equivalent
sector specification, which is then passed on to the controller to
initiate the transfer. If the conversion from blocks to sectors yields a
fractional sector count, an error is returned and no data is
transferred.
command.
IOT
command.
IOT
Disk I/O Support
3-15
3
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