Specifications
Page B-2 Appendix B
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.
B.2READ AHEAD
The AM-137’s SCSI disk driver, SCZRR.DVR, is able to perform read-ahead directly
into AMOS disk cache. When any program attempts to read a physical block from a
disk, the SCZRR driver will also read up to an additional seven sequential blocks from
the disk drive and store these read-ahead blocks in the cache.
This read-ahead scheme works very well when jobs on the system are doing a large
number of sequential reads. For example, data base searches and programs like
REDALL execute much faster because the data they require is already in memory and
only has to be transferred from the cache into the user partition.
Programs that do significant random disk access (such as RNDRED) tend to slow down
with this read ahead scheme. Most of the slow down is caused by "thrashing" of the
cache, where cache entries that will be used again are removed from the cache due to
the allocation requirements of the read-ahead blocks (which typically are never used).
The actual data transfer overhead is very little, as most SCSI disk drives (especially fast
SCSI-2 drives) have a track cache built into the drive allowing both the target and read
ahead blocks to be transferred over the SCSI cable without delay.
B.2.1Controlling Read-Ahead
In order for read-ahead to occur on the AM-137, the AMOS disk cache, DCACHE.SYS,
must be installed as normal and additionally, the full SCSI dispatcher (SCZRR.SYS)
must also be installed. The number of read-ahead blocks to be transferred into cache on
every physical disk access is controlled by the FIXLOG program.
PDI-00137-50, Rev. A01