The Processor Data Path Control Chapter 5 Part

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 (s 360/s 3670)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: Power. PC’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. 5. 34 for defining fields. . See also p. 383, fig. 5. 33 for big picture … previous slides

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

The Microprogram Number of control lines: 2 1 2 3 4 4 Note “Read PC” includes IRWrite corresponding to fix in Fig. C 21. 2 ==> 18 total

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 C 21 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 1 1 1 typo see below The control lines activated are the actual bits of the mico-word selected. … “horizontal microcode” 1 2 2 2 1 1 1 2 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).