Releasing The Vmebus; Arbiter/Requester Configuration Register - DEC Digital Alpha VME 4/224 User Manual

Table 10–7 Arbiter/Requester Configuration Register
Field Name Description
<3:0> Fairness The fairness timeout field accepts the following values:
timeout
0
Non-zero
F
<4> Not used
<6:5> Request Sets the request level of each device to 0, 1, 2, or 3. The setting works
level with the value in <7> to specify the arbitration scheme according to the
following table:
Scheme
Priority
(PRI)
Round-robin
(RRS)
Single-level
(SGL)
<7> Works with a device's request level to specify the arbitration scheme.
When clear, the arbitration scheme is round-robin. When set, other
arbitration types are possible.

10.3.1.2 Releasing the VMEbus

Once a Digital Alpha VME 4 system has acquired ownership of the VMEbus, it is
important to control the manner in which it is relinquished. Four release modes
are supported: release-on-request (ROR), release-when-done (RWD), release-on-
clear (ROC), and bus capture and hold (BCAP). The release mode is configured in
the VIC release control register (VIC_RCR, offset D0), shown in Figure 10–12 and
Table 10–8.
Fair request is not enabled.
Number of 2-microsecond intervals that make up the
fairness timeout period.
Fairness timeout period is not enabled, that is, the
Digital Alpha VME 4 system can only request the
VMEbus if no other requests for the VMEbus are being
asserted.
Bit 7 Bus request level
<7>= 1 Level 3 has the highest priority; level 0
has the lowest priority.
<7>= 0 When a request is being handled on a
bus request level n, the next request to
be handled is on level n-1. If a request
is being handled on level 0, the next
request to be handled is on level 3.
<7>= 1 All bus requests are set to the same
level.
VME Interface 10–19