Xerox Alto I Hardware Manual page 64

Alto Hardware Manual
Section 7: Ethernet 55
*
These functions have a peculiar timing restriction associated with them. The microinstruction that
executes one of them must stop the clock for one cycle. On Alto I, the microprogrammer must do
this using memory timing (i.e., by referencing
MD
in the same microinstruction, during the third or
fourth cycle of a memory reference). On Alto II, the hardware automatically stops the clock for
one cycle when necessary; however, due to a design error, the instruction following the one
specifying
EIDFCf
or
EILFCT
is occasionally stopped instead. Consequently, the programmer must
not permit a task switch to occur between these two microinstructions, nor start a memory
reference in the following microinstruction.