Back in the late 1990s I worked on firmware for a dual PRI ISDN card that used an i960, and was quite impressed with its performance. Register windows made interrupts surprisingly performant. The hardest part of that project was fixing a stability issue caused by buggy logic in one of the CPLDs for bus arbitration between the i960, HDLC controllers and the PCI bridge. After fixing every firmware bug I could find, the issue simply had to be hardware. Once I had a few stress tests with the same firmware running on the older ISA version of the board and a borrowed logic probe, it was easy to prove the issue and come up with a fix. It was an OEM board that we were thankfully able to do the product stabilization work ourselves once the vendor realized they weren't going to get it done themselves any time soon.