Chapter 5: Processor Design Advanced Topics. Microprogramming: Basic Idea

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1 5-1 Chapter 5 Processor Desig Advaced Topics Chapter 5: Processor Desig Advaced Topics Topics 5.3 Microprogrammig Cotrol store ad microbrachig Horizotal ad vertical microprogrammig 5- Chapter 5 Processor Desig Advaced Topics Microprogrammig: Basic Idea Recall cotrol sequece for 1-bus SRC Step Cocrete RTN Cotrol Sequece T0 MA PC: C PC + 4; PC out, MA i, INC4, C i, Read T1 MD M[MA]: PC C; C out, PC i, Wait T IR MD; MD out, IR i T3 A R[rb]; Grb, R out, A i T4 C A + R[rc]; Grc, R out, ADD, C i T5 R[ra] C; C out, Gra, R i, Ed Cotrol uit job is to geerate the sequece of cotrol sigals How about buildig a computer to do this? Page 1

2 5-3 Chapter 5 Processor Desig Advaced Topics The Microcode Egie A computer to geerate cotrol sigals is much simpler tha a ordiary computer At the simplest, it just reads the cotrol sigals i order from a read-oly memory The memory is called the cotrol store A cotrol store word, or microistructio, cotais a bit patter tellig which cotrol sigals are true i a specific step The major issue is determiig the order i which microistructios are read 5-4 Chapter 5 Processor Desig Advaced Topics Fig 5.16 Block Diagram of Microcoded Cotrol Uit Ck CCs Other Sequecer k IR Opcode PLA (computes start addr) Icremet 4 1Mux µpc Cotrol store Exteral source Microistructio has brach cotrol, brach, ad cotrol sigal fields Microprogram couter ca be set from several sources to do the required sequecig m µbrach cotrol µir Cotrol sigals PC out,etc. Brach Page

3 5-5 Chapter 5 Processor Desig Advaced Topics Parts of the Microprogrammed Cotrol Uit Sice the cotrol sigals are just read from memory, the mai fuctio is sequecig This is reflected i the several ways the µpc ca be loaded Output of icremeter µpc + 1 PLA output start for a macroistructio Brach from µistructio Exteral source say for exceptio or reset Micro coditioal braches ca deped o coditio codes, data path state, exteral sigals, etc. 5-6 Chapter 5 Processor Desig Advaced Topics Cotets of a Microistructio Microistructio format Brach cotrol Cotrol sigals Brach PC out MA i PC i C out A i Ed Mai compoet is list of 1/0 cotrol sigal values There is a brach i the cotrol store There are brach cotrol bits to determie whe to use the brach ad whe to use µpc + 1 Page 3

4 . 5-7 Chapter 5 Processor Desig Advaced Topics Fig 5.17 The Cotrol Store 0 µcode for istructio fetch a1 Micro a a3 µcode for add µcode for br µcode for shr Commo istructio fetch sequece Separate sequeces for each (macro) istructio Wide words -1 m bits wide k µbrach cotrol bits c cotrol sigals brach addr. bits 5-8 Chapter 5 Processor Desig Advaced Topics Tbl 5. Cotrol Sigals for the add Istructio Addresses are the istructio fetch Addresses 00 0 do the add Chage of µcotrol from 103 to 00 uses a kid of µbrach Page 4

5 5-9 Chapter 5 Processor Desig Advaced Topics Uses for µbrachig i the Microprogrammed Cotrol Uit (1) Brach to start of µcode for a specific ist. () Coditioal cotrol sigals, e.g. CON PC i (3) Loopig o coditios, e.g Goto6 Coditios will cotrol µbraches istead of beig ANDed with cotrol sigals Microbraches are frequet ad cotrol store es are short, so it is reasoable to have a µbrach field i every µ istructio 5-10 Chapter 5 Processor Desig Advaced Topics Illustratio of µbrachig Cotrol Logic We illustrate a µbrachig cotrol scheme by a machie havig coditio code bits N ad Z Brach cotrol has parts: (1) selectig the iput applied to the µpc ad () specifyig whether this iput or µpc + 1 is used We allow 4 possible iputs to µpc The icremeted value µpc + 1 The PLA lookup table for the start of a macroistructio A exterally supplied The brach field i the µistructio word Page 5

6 . BrNotZ BrNotN BrN 5-11 Chapter 5 Processor Desig Advaced Topics Fig 5.18 Brachig Cotrols i the Microcoded Cotrol Uit ZN Sequecer Mux cotrol BrU BrNotZ BrZ BrNotN BrN Icr PLA 4 1 Mux µpc Cotrol store Exteral Cotrol sigals Mux Ctl Brach Select Icremet µpc PLA Exteral Brach 5 brach coditios NotN N NotZ Z Ucoditioal To 1 of 4 places Next µistructio PLA Exteral Brach 5-1 Chapter 5 Processor Desig Advaced Topics Some Possible µbraches Usig the Illustrated Logic (Refer to Tbl 5.3) Cot rol Sigals Brach Address Brachig act io XXX XXX XXX Noe ext ist ruct io Brach to output of PLA Br if Z t o Ext er. Addr. Br if N t o 300 (else ext ) Br if N t o 06 (else ext ) Br to 04 If the cotrol sigals are all zero, the µistructio oly does a test Otherwise test is combied with data path activity Page 6

7 5-13 Chapter 5 Processor Desig Advaced Topics Horizotal versus Vertical Microcode Schemes I horizotal microcode, each cotrol sigal is represeted by a bit i the µistructio I vertical microcode, a set of true cotrol sigals is represeted by a shorter code The ame horizotal implies fewer cotrol store words of more bits per word Vertical µcode oly allows RTs i a step for which there is a vertical µistructio code Thus vertical µcode may take more cotrol store words of fewer bits 5-14 Chapter 5 Processor Desig Advaced Topics Fig 5.0 Completely Horizotal ad Vertical Microcodig µpc µpc Horizotal cotrol store Vertical cotrol store C i Ic4 MA i PC out todecoder Data path PC out MA i Ic4 C i Page 7

8 5-15 Chapter 5 Processor Desig Advaced Topics Savig Cotrol Store Bits with Horizotal Microcode Some cotrol sigals caot possibly be true at the same time Oe ad oly oe ALU fuctio ca be selected Oly oe register out gate ca be true with a sigle bus Memory read ad write caot be true at the same step A set of m such sigals ca be ecoded usig log m bits (log (m + 1) to allow for o sigal true) The raw cotrol sigals ca the be geerated by a k to k decoder, where k m (or k m + 1) This is a compromise betwee horizotal ad vertical ecodig 5-16 Chapter 5 Processor Desig Advaced Topics Fig 5.19 A Somewhat Vertical Ecodig µir ALU ops field F5 Register-out field F decoder 3 8 decoder 16 ALU 7 Reg out cotrol cotrol sigals sigals Scheme would save (16 + 7) - (4 + 3) = 16 bits/word i the case illustrated Page 8

9 Chapter 5 Processor Desig Advaced Topics A Microprogrammed Cotrol Uit for the 1-Bus SRC Usig the 1-bus SRC data path desig gives a specific set of cotrol sigals There are o coditio codes, but data path sigals CON ad = 0 will eed to be tested We will use µbraches BrCON, Br = 0, ad Br 0 We adopt the clockig logic of Fig Logic for exceptio ad reset sigals is added to the microcode sequecer logic Exceptio ad reset are assumed to have bee sychroized to the clock 5-18 Chapter 5 Processor Desig Advaced Topics Tbl 5.4 The add Istructio Addr Ot her Br Cot rol Addr. Sigals Actios XXX MA PC: C PC+ 4 ; XXX MD M[ MA] : PC C; XXX IR MD; µpc PLA; XXX A R[rb] ; XXX C A+R[rc]; R[ra] C: µpc 100; Br(!=0) Ed MA i Microbrachig to the output of the PLA is show at 10 Microbrach to 100 at 0 starts ext fetch Page 9

10 5-19 Chapter 5 Processor Desig Advaced Topics Fig 5.16 Block Diagram of Microcoded Cotrol Uit Ck CCs Other Sequecer k IR Opcode PLA (computes start addr) Icremet 4 1Mux µpc Cotrol store m Exteral source Microistructio has brach cotrol, brach, ad cotrol sigal fields Microprogram couter ca be set from several sources to do the required sequecig µbrach cotrol µir Cotrol sigals PC out,etc. Brach 5-0 Chapter 5 Processor Desig Advaced Topics Fig 5.1 SRC Microcode Sequecer CON =0 Exceptio Reset 400 Sequecer 10 1Mux Icremet 000 PLA Brach Exteral 4 1 Mux µpc BrCON BrN 0 BrN = 0 Ed Mux cotrol BrU Mux Ctl Select Icremet µpc PLA Exteral Brach Page 10

11 5-1 Chapter 5 Processor Desig Advaced Topics Tbl 5.6 Somewhat Vertical Ecodig of the SRC Microistructio F1 F F3 F4 F5 F6 F7 F8 F9 Mux Ct l Brach cotrol 000 BrU 001 BrCON 010 BrCON 011Br =0 100 Br Noe Ed Out sigals 0 Cot. 000 PC out 1 Ed 001 C out 010 MD out 011 R out 100 BA out 101 c1 out 110 c out 111 Noe I sigals Misc. Gat e regs. 000 MA i 000 Read 00 Gra 001 PC 001 i Write 01 Grb IR Wait 10 Grc i 011 Decr 11 Noe 011 A i 100 CON i 100 R i 101 C i 101 MD i 110 Stop 110 Ld 111 Noe 111 Noe Brach ALU 0000 ADD 0001 C=B 10 bits 0010 SHR 0011 Ic NOT bits 3 bits 1 bit 3 bits 3 bits 3 bits bits 4 bits 10 bits 5- Chapter 5 Processor Desig Advaced Topics Other Microprogrammig Issues A hardware push-dow stack for the µpc ca tur repeated µsequeces ito µsubrouties Vertical µcode ca be implemeted usig a horizotal µegie, sometimes called aocode Page 11

12 5-3 Chapter 5 Processor Desig Advaced Topics Microprogrammig Summary Microprogrammig is a desig method with a target of easig the desig task ad allowig for easy desig chage or multiple compatible implemetatios of the same istructio set Page 1