Motorola 700/800-Series User Manual

Disk I/O Support

3-15

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. 

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 

 command. 

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 

 command. 

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.