The Processor Data Path & Control Chapter 5 Part 3 - Microprogrammed Control Unit N. Guydosh 3/1/04+

Tradeoffs Tradeoffs between hardware vs microprogrammed controller –Advantages Of Microprogrammed: Usually Easier To Document And Understand More Flexible: Easier, Faster, And Cheaper To Fix Design bugs Disadvantages Of Microprogrammed –May Be Slower Than Direct Hardware Implementation –May Involve More Support People Team Of "Microcoders" & More Managers Needed

Microprogrammed controller Control lines directly generated (asserted) by “microinstructions” as they get executed by a data path Data path for microinstructions is distinct from the data path of the main architecture (MIPS) instructions An embedded computer within a computer The microprogrammed controller “computer” is generally special purpose “micro ‑ data path ”: Typically has only an instruction memory implemented as rom or flash memory rather than a ram has minimal computation capability Microinstructions: Main goal in life is to generate control signals or Execute (interpret) architecture instructions

Microprogramms Two basic categories of microprogramming horizontal vs vertical (see p. C ‑ 31 ) Vertical –Shorter word –More decoding needed to interpret fields (maximal encoding) – Words are more functional –Micro data hardware path more complicated –Ex: older IBM main frames (s360/s3670)used it Horizontal (used here): –Longer word –Direct relationship between fields/word bits and control signals –Minimal encoding of fields ‑ may have a bit significant relationship between word bits and control signals. –Micro data hardware path simpler –Ex:PowerPC’s employed horizontal code

Microinstructions Relationship to FSM –A state in the FSM usually corresponds to the execution of a microinstruction Sequencing of microinstructions –Next microinstruction to execute may be sequential, a conditional branch, or an unconditional branch Flow pattern follows the flow of the state diagram –sequential within a ”routine” –conditional branch based on opcode (and status) to various "subroutines" –unconditional branch usually at the end of a cycle transferring control to the “fetch “ routine –default is sequential –instruction carries a “next address” field.... Compare to assembly language instructions

Defining The Microinstruction Microinstruction is made up of a series of fields corresponding each corresponding to related groups of control signals and a sequencing field Criteria for choosing fields: –must encompass all control signals –Must be consistent don't make "overlapping fields require that a control signal be both asserted de-asserted. –use control signals defined on p. 384, fig for defining fields.... See also p. 383, fig for big picture … previous slides

Defining The Microinstruction Note: MDR “benignly” written by default Note: IRWrite added above, now 18 control lines., IRWrite

The Microprogram ==> 18 total Note “Read PC” includes IRWrite corresponding to fix in Fig. C21. Number of control lines:

Dispatch Tables The Microcode assembler uses the encoding of the sequence field in the microword the contents of the above dispatch tables the specification (field ==> control lines) of fig C21 microprogram itself To generate the microinstructions to execute If the sequence field is a dispatch specification selecting one of the tables above, then the opcode is used to access within one of the above tables.

Implementation  typo see below Note for those with earlier printing of book: BWrite doesn’t seem to get used anywhere. B register is unconditionally written. If BWrite is dropped, we have the correct number control lines (18). The control lines activated are the actual bits of the mico-word selected. … “horizontal microcode”