Debugger General Information

Disk I/O Support

197Bug can initiate disk input/output by communicating with intelligent disk controller modules over the VMEbus. Disk support facilities built into 197Bug

3consist of command-level disk operations, disk I/O system calls (only via one of the TRAP #496 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 197Bug. Default values for these parameters are assigned at power-up 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 197Bug 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. 197Bug 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.

Device Probe Function

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

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Installation Guide

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Motorola MVME197LE manual Disk I/O Support, Blocks Versus Sectors, Device Probe Function