User`s manual

Special Topics

5-16 Single Board Computers SCSI Software UserÕs Manual

Cache Coherency

The user is responsible for making sure all of the memory used by the

Firmware is cache coherent. This includes the Firmware data area (siopdata),

the command structure (siop_struct), and all buffers contained within the

command structure. Generally, the proper snoop mode setting (refer to Table

B-5) handles cache coherency for all onboard memory on MotorolaÕs 68040-

and 88110-based SBCs. On the MVME187, which is based on the 88100, cache

coherency can be accomplished by allocating the Firmware memory in cache-

inhibited areas.

Local Bus Usage by the NCR 53C710

Because the NCR 53C710 is a local bus master, it consumes local bus cycles.

The NCR 53C710 uses the local bus to fetch instructions and to transfer data.

The percentage of the local bus bandwidth consumed by SCSI data transfers is

determined by the SCSI data transfer rate (SDTR) and the NCR 53C710 local

bus transfer rate (LBDTR). The bandwidth percentage is calculated using the

equation:

BWP = (SDTR / LBDTR) * 100

The SCSI data transfer rate depends on the SCSI device which is connected to

the SBC and how that device is used. In a typical system, if we assume a disk

transfer is composed of an average seek (16 milliseconds), followed by an

average rotational latency (8 milliseconds), followed by an 8KB data transfer,

the average transfer rate would be about 300KB/sec.

The NCR 53C710 transfers SCSI data using the burst protocol defined by the

MC68040 bus. The NCR 53C710 transfers a maximum of four longwords (16

bytes) during each bus tenure. At 25 MHz, the NCR 53C710 local bus transfer

rate is 57MB/sec for writes and 44MB/sec for reads with parity off and

40MB/sec for reads with parity on. The typical percentage of local bus

bandwidth consumed by SCSI data transfers is summarized in the following

table:

Table 5-3. Typical NCR 53C710 Local Bus Usage for SCSI Data Transfers

SCSI Data Rate

(MB/sec)

Write Read

(With Parity Off)

Read

(With Parity On)

0.6 1.1% 1.5% 1.4%

5.0 9.0% 12% 13%