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MIPS Logic & Shift

MIPS Logic & Shift. Bitwise Logic. Bitwise operations : logical operations applied to each bit Bitwise OR:. Immediate Value. Immediate Value Value hard coded into instruction. Immediate Value. Immediate Value Value hard coded into instruction 16 bits available to store immediate:.

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MIPS Logic & Shift

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  1. MIPS Logic & Shift

  2. Bitwise Logic • Bitwise operations : logical operations applied to each bit • Bitwise OR:

  3. Immediate Value • Immediate ValueValue hard coded into instruction

  4. Immediate Value • Immediate ValueValue hard coded into instruction • 16 bits available to store immediate:

  5. Register • Registers = 32 bits, Immediate 16 bits • Must extend immediate

  6. Max Immediate • Immediate value can't require 17+ bits • Unless you use Pseudo instructions • Must be UNCHECKED for now

  7. OR vs ORI • OR $6, $5, $4 • ORI $6, $0, 0xffff

  8. Logical Ops to Know • Immediate instructionsori, andi, xoriinstruction $dest, $source, immediate • Register-Register Instructionsor, and, xor, norinstruction $dest, $source1, $source2

  9. Quick Reminders • A or 0 = A • Anything or'd with 0 is that thing • A and 1 = A • Anything and'd with 1 is that thing • A and 0 = 0 • Anything and'd with 0 is 0 • A xor 1 = not(A) • Xoring with 1 flips bit

  10. Common Tricks • ORILoad a value to register by ori $0 and value

  11. Common Tricks • ORClear a register by oring $0 with self

  12. Common Tricks • ORCopy register by or with $0

  13. Common Tricks • ORCombine non-overlapping patterns:

  14. Common Tricks • ANDKeep only specified bits byanding with 1's in desireddigits

  15. Common Tricks • NORDo NOT by NOR with $0

  16. Logical Shift • Logical Shifts • Add 0's to fill empty space • sll, srl instructions:instruction$dest, $source, number of bits

  17. NOP • NOP : No operation • Done with sll $0, $0, $0 • Opcode for sll is 0 • 0x00000000 is nop!

  18. Loading Large Value • Loading 32bit immediate takes 3 steps:Load (ori) 16bits (in right part of register)Shift left 16 bits (move to left half of register)Load (ori) 16bits (in right part of register)

  19. Bit Isolation • Getting particular bits out of pattern • Strategy 1: • Shift to wipe out others • Want to clear left 8 bits:1010 1011 1111 0110 1010 1011 1111 0110 • Shift left 8 bits:1111 0110 1010 1011 1111 0110 0000 0000 • Shift back right 8 bits:0000 0000 1111 0110 1010 1011 1111 0110

  20. Bit Isolation • Getting particular bits out of pattern • Strategy 1: • Shift to wipe out others • Now want to isolate green five bits0000 0000 1111 0110 1010 1011 1111 0110 • Shift right 190000 0000 0000 0000 0000 0000 0001 1110

  21. Bit Isolation • Getting particular bits out of pattern • Strategy 2: • Masking : binary pattern showing bits to keep 0x00 : keep no bits 0x01 : keep bit 1 (0000 0001) 0x04 : keep bit 3 (0000 0100) 0x05 : keep bit 1 & 3 (0000 0101) 0xF0 : keep bit 5-8 (1111 0000)

  22. Using Masks • AND with a mask 0's out unmasked portions:

  23. Hex Mask • F (1111) mask a whole hex digit

  24. Bit Isolation • Getting particular bits out of pattern • Strategy 2: • Masking : binary pattern showing bits to keep • Want to keep just green bits1010 1011 1111 0110 1010 1011 1010 0110 • Create mask… 0x01F00000 0000 0000 0000 0000 0001 1111 0000 • AND0000 0000 0000 0000 0000 0001 1010 0000

  25. Bit Isolation • Want to keep just green bits1010 1011 1111 0110 1010 1011 1010 0110 • Create mask… 0x01F00000 0000 0000 0000 0000 0001 1111 0000 • AND0000 0000 0000 0000 0000 0001 1010 0000

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