Vax 613 0S Instruction Manual download pdf (Page 151)

9.4 TURBOchannel Usage (System-Speciﬁc)

The following sections discuss TURBOchannel usage on 300 models and on

400/500/600/700/800/900 models:

• DMA size (Section 9.4.1)

• DMA arbitration (Section 9.4.2)

• I/O timeout (Section 9.4.3)

9.4.1 DMA Size

The maximum size of DMA transaction bursts on the TURBOchannel varies

according to the model:

Model

Minimum

Burst Size Maximum Burst Size

300 1 longword 64 longwords

400/500/600/700/800/900 1 longword 128 longwords

If a hardware request exceeds the allowed maximum, the following occurs:

1. An error is signaled to the device.

2. The system aborts the transaction.

3. An Uncorrectible Error interrupt is generated back to the CPU.

As a result, data in the ﬁrst 128 longwords of the DMA burst request may not

have been transferred correctly.

9.4.2 DMA Arbitration

DMA arbitration is performed differently on the 300 models and the 400/500/600

/700/800/900 models.

• Section 9.4.2.1 discusses DMA arbitration in 300 models.

• Section 9.4.2.2 discusses DMA arbitration in 400/500/600/700/800/900 models.

9.4.2.1 DMA Arbitration (300 Models)

DMA Arbitration requires acknowledgement of DMA requests according to their

priority. Possible requestors are:

• TURBOchannel dual SCSI ASIC (TCDS)

• TURBOchannel slot 0 (TC0)

• TURBOchannel slot 1 (TC1)

• IOCTL ASIC

Requests are arbitrated according to the following priorities:

• TCDS (or on-board SCSI) has the highest priority.

• If the arbitrator has already selected a request, no further arbitration takes

place, although the request may not yet have been acknowledged on the

TURBOchannel.

• DMA requests from the TC0, TC1, and IOCTL follow a round-robin priority

scheme; last serviced requestor goes to the bottom of the queue.

I/O Programming 9–5

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Vax 613 0S Instruction Manual Page 151