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CS61C – Discussion 0b100 MIPS Instructions

CS61C – Discussion 0b100 MIPS Instructions. Worksheet Time. Get started with the first and last problem on the front Work with a (new!) partner, please. Pseudo -instructions. Something in C translate awkwardly in MIPS a = b;  add $s0, $s1, $0

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CS61C – Discussion 0b100 MIPS Instructions

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  1. CS61C – Discussion 0b100MIPS Instructions

  2. Worksheet Time • Get started with the first and last problem on the front • Work with a (new!) partner, please

  3. Pseudo-instructions • Something in C translate awkwardly in MIPS • a = b;  add $s0, $s1, $0 • Pseudo-instructions make assembly programming easier, but get translated later • move $s0 $s1 • Some common pseudo-instructions: • move • li (load immediate) • la (load address)

  4. MAL vs TAL • MAL: MIPS Assembly Language • Includes pseudoinstructions • TAL: True Assembly Language

  5. Machine Code • Instructions are numbers too! • 32 bits, in fact • PC – program counter • Register of the currently running instruction

  6. Instruction Types • 3 Types of instructions: • R • I • J • Different type means different fields • Most important field?

  7. Instruction Fields

  8. Instruction Types • I Type instructions • Immediates • lw/sw • beq/bne • J type instructions • j and jal, but not jr • R type instructions • Everything else, including shifts (sll, srl)

  9. Encoding MIPS Machine Code • Get opcode for instruction type • Convert arguments into fields • Order the fields correctly • Convert to binary, hex, or decimal

  10. Example • addi $s5, $s6, -50 • addi $21, $22, -50 • addi is I-type • opcode= 8 • rs (source) = 22 • rt (target) = 21 • immediate = -50

  11. Decoding MIPS Machine Code • Read the opcodeof the instruction • Using green sheet, determine instruction type • Decode the fields • Convert to assembly!

  12. Announcements • Proj 1-1 due 09/24 23:59 • Proj 1-2 due 10/01 23:59 • HW3: Floating Point and Caches • Due 10/05 23:59 • Midterm coming up soonish • Lots of old MT on HKN site!

  13. Worksheet Time • Work on 4 and 5 in the back

  14. BEQ and BNE • Offset is relative to PC + 4 • We interpret immediate as two’s complement offset to PC • 16 bits • But we know next instruction will end in 00 so we don’t include it • So it’s really like 18 bits • Signed means +- 217 bytes around PC • Formula: (PC+4) + (immediate * 4)

  15. JUMP AROUND • Jump instructions use absolute addresses • Again, next address will always end in 00 • We take the top 4 bits from PC + 4 • So formula is:{PC+4, 26 bits, 00}

  16. Don’t stress – it’s on the sheet

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