Blocks Versus Sectors; Device Probe Function - Motorola MVME167 Series Installation Manual

Debugger General Information

Blocks Versus Sectors

4

Device Probe Function

4-14
The logical block defines the unit of information for disk devices. A
disk is viewed by 167Bug 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 IOT 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 IOT command.
When a disk transfer is requested, the start and size of the transfer
is specified in blocks. 167Bug 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.
A device probe with entry into the device descriptor table is done
whenever a specified device is accessed; i.e., when system calls
.DSKRD, .DSKWR, .DSKCFIG, .DSKFMT, and .DSKCTRL, and
debugger commands BH, BO, IOC, IOP, IOT, MAR, and MAW
are used.
The device probe mechanism utilizes the SCSI commands "Inquiry"
and "Mode Sense". If the specified controller is non-SCSI, the probe
simply returns a status of "device present and unknown". The
device probe makes an entry into the device descriptor table with
the pertinent data. After an entry has been made, the next time a
probe is done it simply returns with "device present" status (pointer
to the device descriptor).