Alpha Microsystems Eagle 100 Specifications download pdf (Page 71)

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. 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 Roadrunner 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 Roadrunner’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) with much 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 about 4 times faster).

The data transfer path is much faster with the RISC SCSI-2 controller. It is able to

read from or write to system memory 32 bits at a time, taking 150ns per read or

write. The AM-520 transfers data 16 bits at a time, taking 210ns per 16 bit transfer.

PDI-00172-60, Rev. A02

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Alpha Microsystems Eagle 100 Specifications Page 71