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Execution Team

Execution Team. Phase 1 Presentation 2/14/2011. Wisdom of the Day. 学 而不思则罔，思而不学则 殆 Learning without thought is useless; thought without learning is perilous . Today we will encourage thoughts while you learn about the execution stage.

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Execution Team

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  1. Execution Team Phase 1 Presentation 2/14/2011

  2. Wisdom of the Day • 学而不思则罔，思而不学则殆 • Learning without thought is useless; thought without learning is perilous. • Today we will encourage thoughts while you learn about the execution stage. Confucius (551Bc-479BC) was a Chinese thinker and social philosopher whose teachings and philosophy have deeply influenced Chinese, Korean, Japanese, and Vietnamese thought and life.

  3. Execution Engine Overview • Responsible for producing the result of almost every instruction • Three design points: • The ALU • PC Manipulation • Memory Accesses

  4. Block Diagram

  5. Execute Stage Block Diagram

  6. THE ALUPowerhouse of the CPU

  7. ALU Overview • The ALU is the main math unit of the CPU • It takes two inputs and then returns the results of various operations on them

  8. ALU Datapath

  9. ALU Operations • Add (ADDU) • Subtract (SUBU) • Or (OR) • And (AND) • Nor (NOR) • Set Less Than (SLT) • Set Less Than Unsigned (SLTU) • Shift Logical Left (SLL) • Shift Right Logical (SRL) • Load Upper Immediate (LUI)

  10. Addition • Uses a ripple carry adder to produce the sum • Other options include a carry look ahead adder • We believe the ripple carry adder is sufficiently fast for our implementation. C Code int ADDU(int a, int b){ return a+b; }

  11. Subtraction • Modified existing adder to handle both Add and Subtract • Subtraction is the addition of one number and the 2s compliment of a second number • 2s compliment: invert bits then add 1 • A-B = A + (B! + 1) C Code int SUBU(int a, int b){ return a-b; }

  12. Subtraction • For our adder to handle both Add and Subtract, we place a mux in front of the adder • Choice 1: signal unmodified • Choice 2: inverted part of B • Then to add one we set the carry in bit of the adder to high C Code SUBU(int a, int b){ return a-b; }

  13. Set Less Than • On a set less than instruction, the ALU determines if the first input is less than the second. • To implement, we subtract the two operands and then determine if the output is negative. • The sign becomes the result. • 1 means it is less than. • 0 indcates that it is not. C Code boolean SLT(int a, int b){ return a<b; }

  14. Set Less Than Unsigned • First, let's try using the exact same logic as SLT: • 12<3? • 12-3<0? • 1100-0011 = 1100+1101 = 1001 = -7 • We have determined this won't work. • Ideas? C Code boolean SLTU(uint a, uint b){ return a<b; }

  15. Set Less Than Unsigned • Let’s add an extra zero to make them positive • 12<3? • 12-3<0? • 01100-00011 = 01100+11101 = 01001 = 9 • This does work. • Sign bit is only the XOR of 0, 1 and the last carry out bit. • If you're savvy, you'll notice this is simply the NOT of the carry-out bit. C Code boolean SLTU(uint a, uint b){ return a<b; }

  16. Shifting • The output bits choose their bit based on the input shamt • Implemented using MUXes. • We made it faster by converting shamt to one-hot encoding. • No cascading of MUXes • Uses a bunch of and gates in parallel • Any ”leftover” bits are 0's. C Code uint SLL(uint a, intshamt){ return a<<shamt; }

  17. Logic Operations • AND • OR • NOR • In order to complete these operations we use bitwise, AND, OR, and NOR. • Uses 32 gates for each operation. C Code int AND(int a, int b){ return a&b; }

  18. Load Upper Immediate • Returns the value of the immediate shifted left 16 times. • Reusing SLL complicates the logic of the ALU circuit. • Uses a dedicated 16-shift left module. C Code int LUI(int a){ return a<<16; }

  19. ALU Control • How is the ALU controlled? • How can we accomplish this? • Alternative options?

  20. ALU Control • Nearly every operation is computer in parallel • When ADDU is performed, SUBU, SLT and SLTU are not, and vice-versa • There is a MUX inside the ALU • The MUX chooses the output based on the requested operation

  21. ALU Control • How was this accomplished? • One hot encoding • SIG0 is for indicating the adder must do subtraction.

  22. ALU Control • Alternative Options • Binary Encoding • One Cold Encoding • We use one hot to make the MUX less complex • Just as we did for the SLL and SRL

  23. Immediates • Adding special instructions for immediates would be painstaking and wasteful • Determine second input from outside the ALU

  24. Immediate Datapath

  25. PC ManipulationMoving About the Code

  26. PC Manipulation Overview • Dynamic alteration of the PC allows for programs to be non-linear • Greatly increases the capability of computers

  27. Branching • Branch instructions modify the program counter to skip over sections of code or to go back to repeat previous code. • Our branches allow for conditional movement to an offset.

  28. Conditional Branch • BEQ: Branches when the values in the two registers are equal • BNE: Branches when the values in the two registers aren’t equal • Two things must be calculated: • New address • Comparison C Code if(a == b) goto c; /* BEQ */ if(a != b) goto c; /* BNE */

  29. Conditional Branch • Solution: dedicated adder to calculate new address • The ALU can then do the comparison • Extra output to determine if subtraction results in a 0 C Code if(a == b) goto c; /* BEQ */ if(a != b) goto c; /* BNE */

  30. Branching Datapath

  31. Jumping • Jumps unconditionally change the PC • Their addresses are absolute rather than offsets of the current PC C Code goto c;

  32. Jumps • Two unlinked jumps • Jump(J): jumps directly to the instruction in the immediate field • Jump Register (JR): jumps to the instruction whose location is the value of the given register C Code goto 0xDEADBEEF; /* J */ goto c; /* JR */

  33. Jumping Datapath

  34. Linked Jumps • Two linked jumps • Jump and Link(JAL): jumps to the instruction in the immediate field and saves the return address in $ra • Jump and Link Register (JALR): jumps to the instruction in the immediate field and saves the return address in the specified register C Code $ra = PC+8; goto 0xDEADBEEF;

  35. Linked Jumps • Linked jumps record the address of PC+8 • This is the instruction after the delay slot instruction • More MUXes on the ALU inputs to choose when return the link address C Code $ra = PC+8; goto 0xDEADBEEF;

  36. Linked Jumping Datapath

  37. PC Change Determination • PC will change on a successful branch or a jump command • Use and a combination of AND gates and an OR gate • Use JUMP? bit to choose the new address C Code if(BEQ && !Not Zero || BNE && Not Zero || Jump){ gotoNewAddress; }

  38. MEMORY INTERFACINGLoading and Storing

  39. Memory Interfacing Overview • There are a limited number of registers in the CPU • To maintain and obtain more data, we need to be able to access a larger pool of data

  40. Load Word • Loads some word into a register from memory • The address is determined by adding an offset to the first operand • Easily implemented in the ALU C Code b = mem[a+0x000016]

  41. Store Word • Stores a register to memory • We use the same method as load word to calculate the address C Code mem[a+0x000016] = b

  42. Memory Datapath

  43. Conclusion • Three design points: • ALU • PC Manipulation • Memory Accesses • Take advantage of repeated logic • If all else fails, more hardware

  44. Questions

  45. Works Cited • Confucius • http://www.all-famous-quotes.com/images/uploads/confucius1.jpg • Exeggutor • http://www.pokecommunity.com/signaturepics/sigpic136542_1.gif • Block Diagram • http://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/MIPS_Architecture_%28Pipelined%29.svg/800px-MIPS_Architecture_%28Pipelined%29.svg.png • Doctor of Math • http://brownsharpie.courtneygibbons.org/?p=452 • Flowchart • http://xkcd.com/210/ • Bank Branch • http://media.glassdoor.com/m/7f/db/98/f9/a-typical-suntrust-bank-branch-this-one-located-at-4235-university-drive.jpg • Navy Seal • http://upload.wikimedia.org/wikipedia/commons/c/cf/NavySeal.png • One Tree Hill • http://gmovietrends.com/wp-content/uploads/2010/11/One-Tree-Hill.jpg • Van Halen • http://flavio.castelli.name/wp-content/uploads/2010/08/van-halen-jump.jpeg • Paratroopers • http://upload.wikimedia.org/wikipedia/commons/1/13/720th_Special_Tactics_Group_airmen_jump_20071003.jpg • Jumper • http://www.themovieblog.com/wp-content/uploads/2007/12/jumper-poster-2.jpg • Jumping Spider • http://fc03.deviantart.net/fs23/f/2007/356/1/a/green_jumping_spider_closeup_by_troypiggo.jpg • RAM Upgrade • http://farm1.static.flickr.com/145/329883185_6b80bde503.jpg • Battering Ram • http://4.bp.blogspot.com/_86fa3woQOHU/SwPiRaktsdI/AAAAAAAAA4c/pn35L4Ei7B8/s1600/BatteringRam.jpg • Tron Ram • http://obsoletegamer.com/wp-content/uploads/2010/03/RamMyBuddy300.jpg • Animal Ram • http://www.bigfoto.com/themes/nature/animals/ram-animal-i4t.jpg • Fry • http://scalp.plaxmol.com/wp-content/uploads/2009/02/fry-panique-questions.jpg

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