Open Core Protocol Debug Interface Specification rev 1.0 - OCP-IP

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- Indirect Branches are caused by jump instructions with calculated target address (e.g.
1)
Return from subroutine ) or by exceptions (e.g. interrupts, traps). In these cases the
target address must be contained in the trace memory. FORGET on the base port is used to
indicate such a branch. Each FORGET received on any of the two ports invalidates the
current base address. The exact protocol definition is given in the tables below.
There is no need to serve both ports concurrently by the sender. The only requirement is that
the base is sent prior to (or latest concurrently with) the next discontinuity. If this is not
possible the sender may set both mode ports to FORGET. This is interpreted as .overrun. .
Note: This implies that the increments are accumulated starting from 0 after each
discontinuity. A valid instruction pointer however, e.g. for comparison, is only
available when the associated base was received.
A short examples may help to understand the deceptively simple interface protocol.
1)
It is not recommended to treat ”Return” instructions as direct branches: If the trace is turned
on after the subroutine was called there is no way for the decoder to know the return address.
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