The translation array comes into play on the following occasions:
• The opcode is used to determine the start of a microroutine.
• At appropriate times, address mode bits of the microinstruction are tested to
perform appropriate addressing.
• Interrupt conditions are periodically tested.
• Conditional branch microinstructions are evaluated.
LSI-11 MICROINSTRUCTION FORMAT The LSI-11 uses an extremely vertical microinstruction
format, which is only 22 bits wide.The microinstruction set strongly resembles
the PDP-11 machine instruction set that it implements. This design was
intended to optimize the performance of the control unit within the constraint of a vertical,
easily programmed design. Table 16.5 lists some of the LSI-11 microinstructions.
Figure 16.15 shows the 22-bit LSI-11 microinstruction format. The high-order
4 bits control special functions on the processor board.The translate bit enables the
translation array to check for pending interrupts.The load return register bit is used
at the end of a microroutine to cause the next microinstruction address to be loaded
from the return register.