On-line Debug

This is a very powerful technique for debugging. This allows read and write control to some of the internal states of the processor while the system is still operating. Using several customized instructions, referred to as Shadow and Mask, this is accomplished. These instructions are part of the RAS (Reliability, Availability, Serviceability) features of the processor and are mainly used for system debug. The speed of operation is up to 10Mhz.

The system groups wanted some visibility into the processor while the system was operating, this included the Program Counter (PC) and some other important architectural registers. These registers were chosen as they would provide the best insight into the processor during system debug phase. Mask and Shadow chain techniques are used for implementing this. Mask chains are simple scan chains that are used to control the logging of data that is eventually accessible through the shadow chain. This is very useful as the mask chain can log the errors and they can be viewed later by the Shadow scan chain. Shadow/Mask combination can successfully detect bus protocol violations.