672 Part 3 ½
Computer Classes
Section 3½
Minicomputers
Most of the fields of the microword supply signals for conditioning and clocking the data paths. Many of the fields act directly or with a small amount of decoding, supplying their signals to multiplexers and registers to select routings for data and to enable registers to shift, increment, or load on the master clock. Other fields are decoded according to the state of the data paths. An instance of this is the use of auxiliary ALU control logic to generate function-select signals for the ALU as a function of the instruction contained in the IR. Performance as determined by microcycle count is in large measure established by the connectivity of the data paths and the degree to which their functionality can be evoked by the data-path control fields of the microprogram word.
The complexity of the clock logic varies with each implementation. Typically the clock is fixed at a single period and duty cycle; however, processors such as the 11/34 and 11/40 can select from two or three different clock periods for a given cycle depending upon a field in the microword register. This can significantly improve performance in machines where the longer cycles are necessary only infrequently.
The clock logic must provide some means for synchronizing processor and Unibus operation, since the two operate asynchronously with respect to one another. Two alternate approaches are employed in midrange implementations. Interlocked operation, the simpler approach, shuts off the processor clock when a Unibus operation is initiated and turns it back on when the operation is complete. This effectively keeps microprogram flow and Unibus operation in lockstep with no overlap. Overlapped operation is a somewhat more involved approach which continues processor clocking after a DATI or DATIP is initiated. The microinstruction requiring the result of the operation has a function bit set which turns off the processor clock until the result is available. This approach makes it possible for the processor to continue running for several microcycles while a data transfer is being performed, improving performance.
The sequence of states through which the control unit passes would be fixed if it were not for the branch-on-microtest (BUT) logic. This logic generates a modifier based upon the current state of the data paths and Unibus interface (contents of the instruction register, current bus requests, etc.) and a BUT field in the microword currently being accessed from the control store, which selects the condition on which the branch is to be based. The modifier (which will be zero in the case that no branch is selected or that the condition is false) is ORed in with the next microinstruction address so that the next control-unit state is not only a function of the current state but also a function of the state of the data paths. Instruction decoding and addressing mode decoding are two prime examples of the application of BUTs. Certain code points in the BUT field do not select branch conditions, but rather provide control signals to the data paths, Unibus interface, or the control unit itself. These are known as active or working BUTs.
The JAM logic is a part of the microprogram flow-altering mechanism. This logic forces the microaddress register to a known state in the event of an exceptional condition such as a memory access error (bus timeout, stack overflow, parity error, etc.) or power-up by ORing all is into the next microaddress through the BUT logic. A microroutine beginning at the address of all is handles these trapped conditions. The old microaddress is not saved (an exception to this occurs in the case of the PDP-11/60); consequently, the interrupted microprogram sequence is lost and the microtrap ends by restarting the instruction interpretation cycle with the fetch phase.
The structure of the microprogram is determined largely by the BUTs available to implement it and by the degree to which special cases in the instruction set are exploited by these BUTs. This may have a measurable influence on performance as in the case of instruction decoding. The fetch phase of the instruction cycle is concluded by a BUT that branches to the appropriate point in the microcode based upon the contents of the instruction register. This branch can be quite complex, since it is based upon source mode for double-operand instructions, destination mode for single-operand instructions, and op code for all other types of instructions. Some processors can perform the execute phase of certain instructions (such as set/clear condition code) during the last cycle of the fetch phase; this means that the fetch or service phase for the next instruction might also be entered from BUT IRDECODE. Complicating the situation is the large number of possibilities for each phase. For instance, there are not only eight different destination addressing modes, but also subcases for each that vary for byte and word and for memory-modifying, memory-nonmodifying, MOV, and JMP/JSR instructions.
Some PDP-11 implementations such as the 11/10 make as much use of common microcode as possible to reduce the number of control states. This allows much of the IR decoding to be deferred until some later time into a microroutine which might handle a number of different cases; for instance, byte- and word-operand addressing is done by the same microroutine in a number of PDP-11s. Since the cost of control states has been dropping with the cost of control-store ROM, there has been a trend toward providing separate microroutines optimized for each special case, as in the 11/60. Thus more special cases must be broken out at the BUT IRDECODE, and so the logic to implement this BUT becomes increasingly involved. There is a payoff, though, because there are a smaller number of control states for IR decoding and fewer BUTs. Performance is boosted as well, since frequently occurring special cases such as MOV register to destination can be optimized.