2.2.4. Outstanding transaction regulation

The regulator enables you to program, at run-time, values for the number of AW and AR requests that it issues. See Chapter 3 Programmers Model.

At design time, you can configure the ASIB, or internal IBs, using maximum values for the number of AW and AR requests that the master can issue at any one time. You normally set these limits to match the characteristics of the attached master, in the case of an ASIB, but in either case, you must always observe these limits because they size the downstream components. If you program the QoS regulator with larger values than the configuration limits, it has no effect.

Fractional outstanding transactions

You can characterize a sequence of transactions, with periods when there are no outstanding transactions, by using a fractional outstanding transaction number. For example, if requests occur every 100 cycles, but it only takes 50 cycles for the last response to arrive, then this would correspond to an average of 0.5 outstanding transactions. This generalizes to give an average value for any sequence of transactions where the number of outstanding transactions varies.

Combined AW and AR outstanding transaction regulation

You can program a maximum outstanding transaction capability for the combined AW and AR channels. This is in addition to the individual channel maxima, so that it is possible to set a combined limit that is lower than the sum of the individual channel limits. A combined value of zero, or greater than or equal to the sum of the individual configuration limits, has no effect.

For example, if the AW and AR configuration issuing capabilities are four and four, the default issuing capability is eight. Setting a combined value of six lowers the combined issuing capability, but leaves the individual channel configuration limits unaltered. Therefore, if there are four outstanding AW requests, then only two AR requests are permitted. Alternatively, if there are only two outstanding AW requests, then four AR requests can be made.

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