Chapter 3: Sequential Logic

Transcription

1 Elements of Computg Systems, Nisan & Schocken, MIT Press, Chapter 3: Sequential Logic Usage and Copyright Notice: Copyright 2005 Noam Nisan and Shimon Schocken This presentation contas lecture materials that accompany the textbook The Elements of Computg Systems by Noam Nisan & Shimon Schocken, MIT Press, The book web site, features 13 such presentations, one for each book chapter. Each presentation is designed to support ab 3 hours of classroom or self-study struction. You are welcome to use or edit this presentation for structional and non-commercial purposes. If you use our materials, we will appreciate it if you will clude them a reference to the book s web site. And, if you have any comments, you can reach us at tecs.ta@gmail.com Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 1

2 Sequential VS combational logic Combational devices: operate on data only; provide calculation services (e.g. Nand ALU) Sequential devices: conta state and (optionally) operate on data; provide storage / synchronization services (e.g. flip-flop RAM) Sequential devices are clock-based; the clock cycles determe when the states are committed The low-level behavior of clocked / sequential gates is tricky The good news: the complex clock-dependency details can be encapsulated at a very elementary level the computer s logic design. Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 2

3 Lecture plan Clock A hierarchy of memory chips: Flip-flop gates Bary cells Registers RAM Counters Perspective. Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 3

4 The Clock HW simulator demo tock tock tock tock clock signal tick tick tick tick cycle cycle cycle cycle In our jargon, a clock cycle = tick-phase (low), followed by a tock-phase (high) In real hardware, the clock is implemented by an oscillator In our hardware simulator, clock cycles can be simulated either Manually, by the user, or Automatically, by a test script. Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 4

5 Flip-flop HW simulator demo DFF (t) = (t-1) A fundamental state-keepg device For now, let us not worry ab the DFF implementation Memory devices are made from numerous flip-flops All regulated by the same master clock signal Notational convention: sequential chip = (notation) sequential chip clock signal Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 5

6 1-bit register () Objective: build a storage unit that can: (a) Change its state to a given put (b) Mata its state over time (until changed) DFF load if load(t-1) then (t)=(t-1) else (t)=(t-1) (t) = (t-1) DFF load Basic buildg block (t) = (t-1)? (t) = (t-1)? MUX DFF Won t work if load(t-1) then (t)=(t-1) else (t)=(t-1) OK Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 6

7 1-bit register (cont.) HW simulator demo Interface load Implementation load MUX DFF if load(t-1) then (t)=(t-1) else (t)=(t-1) Load bit Read logic Write logic Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 7

8 Multi-bit registers HW simulator demo load load w... w if load(t-1) then (t)=(t-1) else (t)=(t-1) 1-bit register if load(t-1) then (t)=(t-1) else (t)=(t-1) w-bit register Register s width: a trivial parameter Read logic Write logic Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 8

9 Aside: Hardware Simulation HW simulator tutorial: HW simulator demo Built- chips Clocked chips GUI-empowered chips. Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 9

10 Random Access Memory (RAM) load HW simulator demo (word) register 0 register 1 register 2. register n-1 (word) address (0 to n-1) RAM n Direct Access Logic Read logic Write logic. Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 10

11 RAM terface load 16 bits address RAMn 16 bits log 2 n bits Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 11

12 RAM anatomy RAM 64 RAM8 RAM 8. 8 Register register. register 8 RAM8... register... Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 12

13 Historical aside: One of Intel s first RAM chips (c. 1972) Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 13

14 Counter Needed: a storage device that can: (a) set its state to some base value (b) crement the state every clock cycle (c) mata its state (stop crementg) over clock cycles (d) reset its state c load reset w bits PC (counter) w bits If reset(t-1) then (t)=0 else if load(t-1) then (t)=(t-1) else if c(t-1) then (t)=(t-1)+1 else (t)=(t-1) Typical function: program counter Implementation: register chip + some combational logic. Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 14

15 Sequential VS combational logic (revisited) Combational chip Sequential chip (optional) time delay (optional) comb. logic comb. logic DFF gate(s) comb. logic = some function of () (t) = some function of ((t-1), (t-1)) Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 15

16 Time matters tock tock tock tock clock signal tick tick tick tick cycle cycle cycle cycle Durg a tick-tock, the ternal states of all the clocked chips are allowed to change, but their puts are latched At the begng of the next tick, the puts of all the clocked chips the architecture commit to the new values. a Reg1 Implications: Challenge: propagation delays Solution: clock synchronization + Cycle length and processg speed. b Reg2 Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 16

17 Perspective All the memory units described this lecture are standard Typical memory hierarchy (listed creasg access time and decreasg cost): SRAM ( static ), typically used for the cache DRAM ( dynamic ), typically used for ma memory Disk (Elaborate cachg / pagg algorithms) A Flip-flop can be built from Nand gates But... real memory units are highly optimized, usg a great variety of storage technologies. Elements of Computg Systems, Nisan & Schocken, MIT Press, 2005, Chapter 3: Sequential Logic slide 17