engineering the pending interrupt sources from the current
state of the controller. For channel A we can always read the
interrupt pending register (RR3). For channel B we can read
the interrupt vector register (RR2) because it contains the
modified vector and thus includes the interrupt source.
Since we currently need puc(4) for the Z8530, we know that
the interrupt handler for both channels will be called and
thus that RR3 will always be read at least once, even if ch A
has no pending interrupt.
NOTE: The modified interrupt vector has no value that represent
a lack of pending interrupt for channel B. That is, the
value read when no interrupts are pending is the same as the
value for the special receive condition. Fortunately, we don't
actually have to depend on that interrupt source. This does
mean that we need to properly handle the overflow condition,
when we read received character from the chip.
o The DSR signal is represented by the SYNC bit in the external
status register (RR0). We now properly track DSR.
o It's save to enable the external/status interrupt source. We
now get interrupts when line signals (DSR, DCD or CTS) change.
Problems fixes:
o interrupt storms.
o blocked open(2).
o lack of (hardware) flow control.
o unable to report DSR.
MFC after: 5 days
3aef9c1b41