Rhapsody Developer Release Copyright 1997 by Apple Computer, Inc. All Rights Reserved.

Rhapsody Developer Release Notes:
SCSI Driver

The SCSI Driver is a software module that directly drives the SCSI Hardware Bus Adapter. Rhapsody provides two SCSI drivers, one for the internal bus driver controlled by a MESH chip, and another for the external bus, controlled by the Curio chip. The MESH chip is an Apple design, also used for CHRP systems, while the Curio chip is based on the industry standard NCR 53C96 design. Both drivers provide a common interface to operating system software, such as disk drivers.

Notes Specific to Developer Release

New Features and Caveats

The SCSI drivers permit hard-disk booting in place of network booting on PowerSurge class machines. The drivers are of alpha quality; some features planned for subsequent releases have been disabled (see below) to minimize the risk of bugs that cause crashes or data corruption arising from timing dependencies.

Disabled Features

The following features have been disabled or have not been implemented for the Developer Release. While the lack of these features might affect performance, it do not impede development or use of the software.

• The disconnect and reselect options are disabled. As a result, once an I/O request has begun, it continues to completion without releasing the SCSI bus.
  
  
• Tagged queuing is disabled. Tagged queuing permits a disk drive to operate on several I/O requests simultaneously. This feature can improve performance for virtual-memory paging. Because tagged queuing requires the disconnect and reselect options (which are disabled), it has been disabled.
  
  
• Synchronous data transfer is disabled. This feature is supported on the internal (MESH) SCSI bus, but the Curio chip does not support synchronous data transfer. Synchronous transfers can significantly improve transfer throughput of SCSI bus data.
  
  
• The initiator bus ID is hardwired to ID 7. This is only sigificant for systems where two Rhapsody computers share a common SCSI bus.
  
  
• The alpha-release drivers are coded for reliability and functionality. A number of coding improvements have been deferred to a subsequent release. These improvements include inline compilation of critical-path modules and improvements to inter-task queuing.