Peter,

Any instruction that references External Program memory generates either the wr or rd strobes.

A "movx a,@r0" or a "movx a,@dptr" both generate a rd strobe as you are "reading" (fetching) from External Memory. This is the "X" in the MOVX instruction coding.

A "movx @r0,a" or a "movx @dptr,a" both generate a wr strobe as you are "writing" (storing) to External Memory.

Notice that Program memory (a read only mechanism) generates a PSEN signal automatically as it fetches instructions from an external program memory (prom) and in response to instructions such as "movc a,@a+dtpr".

regards,
p