Hardwired control unit

Hardwired control unit
hardwired control unit is very difficult to design and it is very costly. Now CAD tools is used for logic design. Therefore almost all processors of today use hardwired control unit.


Starting with a behavioral description of the control unit, the state diagram of micro-operations is constructed.  Most states are simply driven by clock and only transition to the next state.  Some states branch to different states depend on conditions such as testing conditional codes or decoding the instruction.  


  The control circuit is implemented using Programmable Logic Array (PLA).  In general, any sequential circuit (which can implement any state machine) can be constructed from combinational circuits with feedback.  The feedback information is the states.  If the feedback path uses no clock, the circuit is called asynchronous.  If the feedback path uses a latch with clock, the circuit is called synchronous.  Synchronous circuits are used almost exclusively for sequential circuits today as they are easier to design and can be implemented reliably.  Most of the CAD tools handle synchronous circuits.


Asynchronous circuit has been used for the reason of speed as in many early computer designs, for example, ILLIAC and many computers in the class called supercomputer.  But it is difficult to implement reliably and it is still much more difficult to do systematic design of a complex machine using asynchronous circuits. The combinational part of the control circuit can be regarded as a memory where its content is the map of the inputs to the outputs (states are considered to be a part of the outputs). This view of combination circuit as a memory is called Random Access Memory model (RAM) of computation machines.




Microprogrammed control unit
 a control unit can be implemented as a memory which contains patterns of the control bits and part of the flow control for sequencing those patterns.  Microprogram control unit is actually like a miniature computer which can be "programmed" to sequence the patterns of control bits.  Its "program" is called "microprogram" to distinguish it from an ordinary computer program. Using microprogram, a control unit can be implemented for a complex instruction set which is impossible to do by hardwired.


Microprogram approach for control unit has several advantages:
1.     One computer model can be microprogrammed to "emulate" other model.
2.     One instruction set can be used throughout different models of hardware.
3.     One hardware can realised many instruction sets. Therefore it is possible to choose the set that is most suitable for an application.


How microprogram work
Like the RAM model, a microprogrammed control unit consists of microPC, micromemory, output buffer and a sequencing unit. A micromemory (sometimes called microstore) contains bit patterns that are used to control the datapath.  Each word of the micromemory is called "microword".  Each word of the micromemory is separated into several fields used for internal control, external control, conditional and specifies the next address.  Internal control bits are the signals that control the datapath.  External control bits are the signals that control external units such as memory (read, write), interrupt acknowledge etc.  Conditionals are the bits that are used to determine the flow of microprogram; loop, branching, next instruction etc. Its input comes from the datapath (usually from the conditional code register).  Next address determines the next microword to be executed.


A microprogram is executed as follow
1.     a word from microprogram at the location specified by the microPC is read out, control bits are latched at the output buffer which is connected to the datapath.
2.     if conditional field is specified and the test for conditional is true, the next address of microprogram will come from the next address field otherwise the microPC will be incremented (execute the next microword).


What that has been described is called horizontal microprogram in which there is a one-to-one relationship between internal/external control bits and the actual control signal of the datapath (hence it is wide or "horizontal"). The microword can have other formats.  There are several possibilities :
1.     single format - one address, as just described above.
2.     single format - two addresses, each microword contains two next addresses field, one for result of test true, the other for result of test false.
3.     multiple format, such as, one format for the control bits without the next address field and another format for "jump on condition" with the address field.  The advantage is that the microword can be shorter than the single format.  The disadvantage is that to "jump" will take one extra clock.


Horizontal microprogram allows each control bit to be independent from other therefore enables maximum simultaneous events and also offers great flexibility. It is also waste a lot bit.


For each field of microword, there may be a group of bits that are not activated at the same time therefore they can be "encoded" to use a fewer bit.  A decoder is required to  "decode" these bits and to connect them to the datapath.  This approach is called vertical microprogram. There are many possibilities to compact the micromemory to be as small as possible, sometime trading off speed for space, for example, two-level microprogram.  The first level is "vertical" i.e. maximally encoded, the microword of the level one is pointed to the "horizontal word" of the second level.  This is rather like the first level is composed entirely from "subroutine call" and the second level is the subroutine.


Microprogram becomes obsolete mainly because the present design emphasizes the performance and microprogram is slower than hardwired.  The change in instruction set design toward a minimum number of clock per instruction simplifies the instruction set to the point that microprogram is not really required.  Also the design of hardwired control unit can be mostly automated as opposed to microprogram which must be written and debug.  Hence, for the current instruction set architecture, hardwired control unit offers a lower engineering cost.


Equivalence of hardware and software
The definition of microprogramming is due to Wilkes, who in 1953 suggested a method for designing the control unit of a processor, based on the use of sequence of microwords - a microprogram - held in a read only memory (ROM).  In this context, microprogramming is generally understood as the technique of producing interpreters for high-level language.


At that time random access memory (RAM) that was available was much slower than the processor, leads to CISC (Complex Instruction Set Computer) to achieve high speed  the microprogram of CISC are organised horizontally; the need to control a complex processing unit requires each microword to consist of a large number of bits, often over 100.


Firmware, specification of a microprogram, is not an interpretation algorithm but a logic system.  The concept of vertically organised microprogram follows that each microword is of fewer bits than in horizontally organised microprogram.  The resulting simplicity enables a true optimization of the software to be achieved.  Firmware is the transformation and equivalence between hardware (logic systems) and software (microprogram). This hardware-software equivalence is a particular case of the equivalence between space and time




Micro programmed control


Micro programmed control is a control mechanism to generate control signals by using a memory called control storage (CS), which contains the control signals.  Although micro programmed control seems to be advantageous to CISC machines, since CISC requires systematic development of sophisticated control signals, there is no intrinsic difference between these 2 control mechanisms.


Hard-wired control


Hardwired control is a control mechanism to generate control signals by using appropriate finite state machine (FSM). The pair of "microinstruction-register" and "control storage address register" can be regarded as a "state register" for the hardwired control. Note that the control storage can be regarded as a kind of combinational logic circuit. We can assign any 0, 1 values to each output corresponding to each address, which can be regarded as the input for a combinational logic circuit.