Configuration cycles on a PCI bus are TYPE-0 or TYPE-1 depending on how they are to be treated by a bridge. TYPE-0 configuration cycles are not forwarded by any bridge. The host uses TYPE-0 configuration cycles to access the configuration space of the i960RP and the bridge unit in order to configure them. The i960RP may generate TYPE-0 cycles to access any of the devices on its secondary bus, including (but not past) the DEC bridge.
The TYPE-1 cycles are used to send configuration requests through bridges to devices downstream. A TYPE-1 cycle is always sent to a bridge unit, but contains in it the bus number and device number needed to address the downstream device. The bridge uses this information to generate a TYPE-0 cycle. For example, the i960RP may generate a TYPE-1 cycle intended for the IMC device. It will send that cycle to the DEC bridge, which will translate it to a TYPE-0 cycle on the tertiary bus to satisfy the request.
TYPE-1 requests may also be forwarded further downstream as TYPE-1 requests, and this is how the host would send requests to PCI devices on the downstream side of the DEC bridge on the ISE board. A TYPE-1 cycle in this case gets translated by the i960RP to a TYPE-1 cycle for the DEC bridge, which in turn generates the correct TYPE-0 cycle on its bus.
Software on the i960 may itself generate TYPE-0 and TYPE-1 configuration cycles by using the ATU of the i960RP. The pertinant registers are listed in
Table 8: Registers for Configuration Cycles
Table 
.