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CS 161Computer Architecture Chapter 5 Lecture 12

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1 CS 161Computer Architecture Chapter 5 Lecture 12
Instructor: L.N. Bhuyan Give qualifications of instructors: DAP teaching computer architecture at Berkeley since 1977 Co-athor of textbook used in class Best known for being one of pioneers of RISC and RAID Member of NAE

2 Datapath + Control Points
MemRead IRWrite RegWrite PCWrite IorD MemWrite RegDst ALUSrcA PCSrc P C PCWrite- Cond M u x M u x Address Read Reg1 25:21 Mem Read data1 z A A L U Read Reg2 ALU- Out M u x 20:16 Read Data Read data2 Write Reg B 15:0 M u x Write Data 15:11 4 IR 0 1M 2 u 3 x Regs Write Data 3 M D R M u x ALU Control Sgn Ext- end << 2 2 2 (funct) 5:0 ALUSrcB MemtoReg ALUOp

3 Multicycle Instruction Execution
All instructions execute in 3-5 cycles 3 cycles: beq 4 cycles: R-type, sw 5 cycles: lw 1: fetch instruction, PC=PC+4 2: decode, fetch registers, brnch target 3: execute/compute address/branch 4: access memory/complete R-type 5: (lw) store memory

4 Summary:

5 Implementing a Finite State Machine
internal storage (current state register) two combinational circuits: next state function; output function N e x t s t a t e Next-state Function current state reg C l o c k I n p u t s Output Function O u t p u t s

6 FSM diagram for Multicycle Machine
MemRead ALUSrcA = 0 IorD = 0 IRWrite ALUSrcB = 1 ALUOp = 0 PCWrite PCSrc = 0 start new instruction ALUSrcA = 0 ALUSrcB = 3 ALUOp = 0 1 state 0 lw/sw R-format beq cycle 3 8 6 2 ALUSrcA = 1 ALUSrcB = 0 ALUOp =1 PCWriteCond PCSrc = 1 ALUSrcA = 1 ALUSrcB = 2 ALUOp = 0 ALUSrcA = 1 ALUSrcB = 0 ALUOp =2 Branch Completion Memory Access R-format execution

7 FSM controller: execution cycles 3-5
from state 2 from state 6 sw lw to state 0 3 5 7 cycle 4 RegDst = 1 RegWrite MemtoReg = 0 MemRead IorD = 1 MemWrite IorD = 1 memory access (step 4) memory access (step 4) R-format completion (step 4) 4 cycle 5 RegDst = 0 RegWrite MemtoReg = 1 write-back (step 5)

8 Add Jump Note: How many state bits will we need?
don’t care if not mentioned asserted if name only otherwise exact value How many state bits will we need?

9 Simple Questions How many cycles will it take to execute this code? lw $t2, 0($t3) lw $t3, 4($t3) beq $t2, $t3, Label #assume not add $t5, $t2, $t3 sw $t5, 8($t3) Label: What is going on during the 8th cycle of execution? In what cycle does the actual addition of $t2 and $t3 takes place?

10 Implementing the FSM controller
W r i t e P C W r i t e C o n d PLA or ROM implementation of both next-state and output functions I o r D M e m R e a d M e m W r i t e Datapath Control Points I R W r i t e M e m t o R e g P C Src A L U O p O u t p u t s A L U S r c B A L U S r c A R e g W r i t e R e g D s t N S 3 } N S 2 Next-state N S 1 I n p u t s N S 5 4 3 2 1 p p p p p p 3 2 1 O O O O O O S S S S Instruction register opcode field state register

11 PLA Implementation

12 ROM Implementation ROM = "Read Only Memory"
values of memory locations are fixed ahead of time A ROM can be used to implement a truth table if the address is m-bits, we can address 2m entries in the ROM. our outputs are the bits of data that the address points to. m is the "height", and n is the "width“ equal to number of outputs. m n

13 ROM Implementation How many inputs are there? 6 bits for opcode, 4 bits for state = 10 address lines (i.e., 210 = 1024 different addresses) How many outputs are there? 16 datapath-control outputs, 4 state bits = 20 outputs ROM is 210 x 20 = 20K bits (and a rather unusual size, so go for next size chip) Rather wasteful due to lots of don’t care situations => the outputs only depend on states, not opcodes.

14 ROM vs PLA Break up the table into two parts — 4 state bits tell you the 16 outputs, x 16 bits of ROM — 10 bits tell you the 4 next state bits, 210 x 4 bits of ROM — Total: 4.3K bits of ROM => Lots of savings. PLA is much smaller — can share product terms — only need entries that produce an active output — can take into account don't cares Size is (#inputs ´ #product-terms) + (#outputs ´ #product- terms) For this example = (10x17)+(20x17) = 510 PLA cells PLA cells usually about the size of a ROM cell (slightly bigger)

15 Alternative to FSM for Multi-cycle?
MIPS-lite has (about) 7 instructions, 10 FSM states Real machines have 100 or more instructions; real controllers have hundreds, or even thousands of states! Problem: FSM Bubble-diagram too large

16 Observation about real machines
Machine Language: next instruction to be executed is usually implied PC register determines instruction next instruction always at PC+4 (unless branch or jump) FSM Controller: often only one exit arc from current state to next state Suppose borrow idea from Machine Language, represent each control step as some kind of “instruction”? Leads to Microprogrammed Control n n+1 n+2 n+3

17 Micro-programmed Control
In microprogrammed control, FSM states become microinstructions of a microprogram (“microcode”) one FSM state=one microinstruction usually represent each micro-instruction textually, like an assembly instruction FSM current state register becomes the microprogram counter (micro-PC) normal sequencing: add 1 to micro-PC to get next micro-instruction microprogram branch: separate logic determines next microinstruction

18 Microprogramming Vs Hardwired Control
Microprogramming offers flexibility for design and architectural changes. The control memory (ROM) can be reprogrammed or replaced. Hardwired control is difficult to design for complex set architecture. Once it is designed, no further change is possible Microprogramming is slow because the control memory is accessed in every cycle. Memory access is slow. Hardwired control is fast because the cycle time depends on the combinational logic delay of the control unit, which is much less than memory access time.

19 Microprogramming What are the “microinstructions” ?

20 Microprogramming A specification methodology
appropriate if hundreds of opcodes, modes, cycles, etc. signals specified symbolically using microinstructions Will two implementations of the same architecture have the same microcode? What would a microassembler do?

21 Microinstruction format

22 Maximally vs. Minimally Encoded
No encoding: 1 bit for each datapath operation faster, requires more memory (logic) used for Vax 780 — an astonishing 400K of memory! Lots of encoding: send the microinstructions through logic to get control signals uses less memory, slower Historical context of CISC: Too much logic to put on a single chip with everything else Use a ROM (or even RAM) to hold the microcode It’s easy to add new instructions

23 Microcode: Trade-offs
Distinction between specification and implementation is sometimes blurred Specification Advantages: Easy to design and write Design architecture and microcode in parallel Implementation (off-chip ROM) Advantages Easy to change since values are in memory Can emulate other architectures Can make use of internal registers Implementation Disadvantages, SLOWER now that: Control is implemented on same chip as processor ROM is no longer faster than RAM No need to go back and make changes

24 Historical Perspective
In the ‘60s and ‘70s microprogramming was very important for implementing machines This led to more sophisticated ISAs and the VAX In the ‘80s RISC processors based on pipelining became popular Pipelining the microinstructions is also possible! Implementations of IA-32 architecture processors since 486 use: “hardwired control” for simpler instructions (few cycles, FSM control implemented using PLA or random logic) “microcoded control” for more complex instructions (large numbers of cycles, central control store) The IA-64 architecture uses a RISC-style ISA and can be implemented without a large central control store

25 Pentium 4 Somewhere in all that “control we must handle complex instructions Processor executes simple microinstructions, 70 bits wide (hardwired) 120 control lines for integer datapath (400 for floating point) If an instruction requires more than 4 microinstructions to implement, control from microcode ROM (8000 microinstructions) Its complicated!

26 Chapter 5 Summary If we understand the instructions… We can build a simple processor! If instructions take different amounts of time, multi-cycle is better Datapath implemented using: Combinational logic for arithmetic State holding elements to remember bits Control implemented using: Combinational logic for single-cycle implementation Finite state machine for multi-cycle implementation

27 Techniques illustrated in chapter 5 are at the heart of every computer
Pipelining (Chap 6) Techniques illustrated in chapter 5 are at the heart of every computer All recent computers, however, go beyond techniques of chapter 5, and use pipelining to improve performance By overlapping execution of multiple instructions, pipelining can achieve: throughput close to 1 instruction per clock cycle (like single-cycle machine) with a clock cycle time determined by the delay of individual datapath components (like multi-cycle machine)

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