After  a  short  review  of general  feat ures  of  microprogrammed  control  units  some  design 
problems  are  treated. especially  optimization of control store  by  means  of compacting  and 
coding

Möschwitzer, A.

Müller, J.

English

Periodica Polytechnica (Budapest University of Technology and Economics)

PERIODICA POL,'TECHNICA SER. EL. ENC. VOL. 34. NO. 1. PP. 5-11 (1990) SOME ASPECTS OF MICROPROGRAMMED CONTROL OF PROCESSORS J. MULLER and A. MOSCHWITZER Technical University Dresden Received January, 31. 1989. Abstract After a short review of general feat ures of microprogrammed control units some design problems are treated. especially optimization of control store by means of compacting and coding. KeYW07'ds: microprogralllming. control store optimization. Introduction It is well known that a processor can be divided into two parts (see Pig. 1). There is at first a so called data path with a data input and a data output. The tasks of such a data path are any transformations of the data input signals into the data output signals. Secondly there is a so-called control path. The control path will send control signals for each clock period to the date path in order to determine in which way the next data transformation has to be done. On the other hand, the data path also will send signals of internal states to the control path, for example flags for overflow, negative etc. The control path can receive also external signals, for example HOLD, POvVER-FAIL etc. The control tasks for a processor, for example the machine instruct.ions are implemented in the control pat.h. One important task for the designer of a digital system is to find an effective, flexible and low-cost realization for such a control path. One successful v .. -ay to do this is t.he design of a microprogrammed control unit. The first idea for this way was published by vVilkes in 1953 (\YILEEs, 1953). In Figure 2 the basic struct.ure of such a unit is repre-sented. The basic idea is t.o place the necessary control signals system-atically in a control store. Each column of the control store matches one elementary control signal. Some of such elementary control signals can produce a simple data path action, for example a register-transfer with an AL IT-operation. Such actions are named microoperations. For each data path there is a set of possible microoperations. 6 J. MULLER and A. MOSCHWITZER ! Data> input trol Con signa ls A • Data path Fla Control path , External control signals gs Data V> output Fig. 1. P.arting a digital system in a data path and control path Each line in the control store represents a microinstruction. Such a ml-croinstruction can have one or more microoperations. If two or more microoperations can be executed in parallel, they can be placed in one microinstruction. A sequence of microinstructions forms a microprogram. For example, for each machine instruction of a microprogrammed proces-sor such a microprogram exists. That means, the execution of a machine instruction is the execution of a sequence of microinstructions. Furthermore we can find in such a microprogrammed control unit a microinstruction address register, 3. microinstruction register and a subunit for realization of next-address-logic. Some of the next-address information can be part of the microinstruction. This structure is the simplest way to realize a microprogrammed control unit. The simplest micro cycle works in the scheme: SOME ASPECTS OF MICROPROGRAMMED CONTROL OF PROCESSORS 7 States Jump/branch -logic flags .. (next -address) ... , Microinstruction address register Clock __ li!iiilIiiDc:o:::Jli:!CiiI_GCIl;I:I_c;;::I;GIi!:/I+ I Next address M icroinstruction I inform. ~ I Con t r 0 I I s t ore I I I Microinstruction register J Clock -_c::::=:a_-~~c:z=:a_-cIa! , Control signal s Fig. 2. Structure of a simple microprogrammed control unit 1. generate the next valid microprogram address; 2. read the next microinstruction from the control store into the ml-croinstruction register; 3. execute microinstruction. Such a control unit has some advantages. The control path is highly flexi-ble, because another control task can be implemented by simply changing the content of the control store. Further advantages are low design costs or low costs in silicon area. Both together are difficult to realize, because a small silicon area (that means primarily a small control store) needs an excellent design with good and expensive design tools. The major problem 8 J .. \[ULLER and A. MOSCHWITZER of using a microprogrammed control unit is the speed of the unit. In order to get a new microinstruction in the microinstruction register a memory access is needed, and so the memory access time will be a major part of the microcycle time. Another problem is the bit-level design. Bit-level design is very difficult for the designer and needs sophisticated design tools in order to get a useful design. Some design problems In Fig. 3 is represented an overview of a possible way to design a micropro-grammed control unit. There are two kinds of information in order to start the design process. First we need a description of the data path of the processor. The best may be a register-transfer description in a hardware description language (H .. 'dlTE:\STEI:\. 1987). Secondly we need a functional specification of the processor. ·With the first information a designer is able to define the set of possible microoperations of the data path, and from the second information we get detailed task specifications of the microprograms. After this the de-signer can create a primary design of the necessary microprograms. There are many tools for this design step (so microprogramming languages, micro-programming compiler and assembler etc.). Generally such microprograms are sequences of single microoperations. The correctness of these micro-programs should be verified by simulation. Now the compacting of the microprograms can be done. In this step the single microoperations will be arranged to microinstructions with several microoperations in order to get a maximum parallelism. By doing so we obtain a reduction of the length of the control store. This red uces the necessary silicon area, the number of microinstructions of a microprogram and the run time (see also Fig. 4). The compacting process needs an analysis of the micro programs in or-der to detect clata dependencies and resource conflicts. Only two microop-erations without data dependencies and resource conflicts can be executed in parallel (\Ii··J.l.I:H. 1985). The remaining steps belong to the bit-level design of microprogram-med control 1lnits. That means that the designer works with single bits and no more with mnemonics or similar higher level constructions. An important step is to minimize the v;idth of the microinstructions. Originally there is eXclctly one elementary control signal mapped to one column. This is the horizontal· format. \Vith n bits in a format we can represent 2" different microinstrllctions. The major part of this is not suggestive for the data path, another part is not 1ls(:d. And so k hits, with k less than n, an: nu:d(:d in ()ffler to n:pff:S('nt t he necessary rnicroinst ructions. For this SO.IIE ASPECTS OF .\[JCROPROGRAMMED CONTROL OF PROCESSORS Register transfer description of 0 data path '" Microopero tions definition of the data path '" ~ Primary design of the Functional specification of the system + Task specification microprogrammes ! microprogrammes (sequences of single microoperations) n y Si m u lot i o n ;=a-=c::::alla:lle=l;:l:l==, ! I ;, H Compact ion of the microprogrammes I I (arrange microoperations to micro-I instructions with several microoperotions) ~~~~----=-=~{ « Minimize the width of the microin struct ion s I (encoding) i "'----=======1 I I Design of the jump/branch logic ~==========-i, I Address encoding I f4'IC11.============~ I Minimize the control store area by I topological transformalions ~ -----------i 9 of reason we can flnd a new code for each microinstruction, with a number of bits between J.: and Il. \Vith that we reduce the necessary silicon area. But 10 1 .. HOLLER and A. .\IOSCHWITZER Compoction c:=====~> I' 1'<1 Fig .. i. Prillciple, or lllicr()pn)~ralll cOlllpactillg we must have a decoder in order to generate the original format needed by the data path. The decoder leads to additional silicon area and run time. A maximal encoding with J.: bits is therefore not practical because of the expensin decoder. In practice we fine! so called 'quasi-horizontal' formats (see also Fig. 5). Here the original format is divided in suggestive groups and these groups will be encoded separately. This results in a good compromise between bit-width-reduction and decoder expense. Other steps in the bit-level design are for example the address encoding and also topological transformations, for example folding. Conclusions EspeciallY for so called CrSe-processors microprogrammed cont rol units are an important way to realize the very complex control path. There must be an excellent CAD-process for these microprogrammed control units in order to get low cost and practicable designs. Control store optimization is one of the most important steps in this design process. SO.\fE ASPECTS OF ,HICROI'ROGRAMMED CONTROL OF PROCESSORS Horizontal format • • • Elementary control signal K< n • • 0 Decoder ••• 2 • • • Decoder m Quasi horizontal format Vi!!,.'i, Thp (jllasi-Iiorizontal forlllal of a microinstructioll References 11 IL\ In l'::;:::n': le; , H,\\", (l!hi): lIardware Description Languages, In: Advances in CAD for \'LSr. "O!. 'I. \(,rl I1 Holland. FIi'7, \U'I.I.U{' .l, (Ul".'i): Lin B('irrag zur r('chnprge,;tiitzlcL EIltwllrf von \Iikroprogramm-,1"'I('!'\\'('rl-;<'II, Di,,,ertatioll .. \. Tt" Drf';;dell. S('klion Inforrnatioflstechnik. 108.5. \\'Ii.I-:l>. \1.\', SII',j:;(;I::fL .J.B, (10.Y3): \licroprogralIlllling and the Design of the ('()nlf'()1 ('ircllit, ill all Elcctrollic Digital ('omputer, ill: Bell. C.G .. \ewell. A.: ("l!I'i"lt('r SI l'llcl Ill'(',,: i\pading.o and Examples, \!cGra,,;-,IIill. 1071. _-lddT'f.ss: ProL Dr.-Ing. habil A, :\10SClI\\'lTZEH, Dr.-Ing . .J. :\lCLLEH Technische tniversitat Dresden Se kt ion Informationstechnik Bereich Bauelemente und Systeme ,\lommsenstr. 13 SO:?, Dresden DDR 

SOME ASPECTS OF MICROPROGRAMMED CONTROL OF PROCESSORS

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SOME ASPECTS OF MICROPROGRAMMED CONTROL OF PROCESSORS

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