Specifications

APPENDIX B
READ-AHEAD AND WRITE
BUFFERING
B.1INTRODUCTION
In the past, AMOS systems achieved high levels of performance by using a "Herbie"
style disk controller (such as the AM-520) to offload a large portion of the overhead
associated with disk access. One additional benefit of this offloading, is that extra cycles
are available on the Herbie controller to perform functions such as read-ahead and write
buffering. Both of these schemes are used by the current AM-520 firmware, but cannot
be used on other non-intelligent interfaces such as the Alpha Micro SASI interface
because the main processor running AMOS has to handle control of the SASI interface,
stealing CPU cycles away from other resources, such as the terminal service system
and user jobs.
In the case of the AM-137 board, a "hybrid" Herbie style controller has been
implemented. A programmable RISC controller is used for SCSI bus communications
and for data transfer to and from the AM-137’s SCSI bus. The 68030 CPU is only
involved with setup before and cleanup after a SCSI command is sent to a device—the
rest of the command, including data transfer, is handled by the RISC processor.
Having the RISC processor take care of these details, allows us to implement both
read-ahead and write buffering without the need for a separate Herbie controller. Also,
higher levels of performance will be seen when using a fast SCSI-2 disk drive than with
an AM-520 using ESDI drives for the following reasons:
Physically, SCSI-2 drives are faster than most ESDI drives. They spin the
platters twice as fast (reducing latency) and have significantly faster seek
times.
Data transfer rates are higher with fast SCSI-2 drives. ESDI drives have a
maximum transfer rate of 18Mbits/s, whereas fast SCSI-2 drives transfer
data at 80Mbits/s (or around 4 times faster).
PDI-00137-50, Rev. A01