996 B
Known differences between w11a and KB11-C (11/70)
CLR and SXT do a write
In most instructions the destination is modified, only in MOV(B), CLR(B),
and SXT the destination value is overwritten regardless of its previous
content.
The 11/70 microcode implements MOV(B) such that it ends with a DATO.
However, CLR(B) and SXT use a specifier flow like other updating
instructions and perform a DATIP, without using the value, and DATO.
The J11 does only a DATO in all three cases. See point 36 in the
PDP-11 differences table.
The w11a uses the dstw flow for all three instructions, and in this
case behaves like a J11 and rather than an 11/70.
This subtle difference might cause observable effects when reading a
device register changes the state of a device.
But for those registers it is unlikely to use MOV or CLR.
Also, drivers are usually written to run on 11/70 and J11 systems.
Therefore, this is considered an acceptable implementation difference.