MIPS%20instructions

1
MIPS instructions
2
Outline

• MIPS Instructions continued
• Logical operations
• Instructions for making decisions

3
MIPS Instructions So Far

• Arithmetic instructions
• Each MIPS arithmetic instruction performs only
  one operation and has three operands
• All operands from registers
• Or one operand is an immediate (constant)
• Data transfer instructions
• Load from memory or store a register value to
  memory
• How to implement Ai using MIPS instructions?
• Instructions are encoded using 0s and 1s
• They are stored in memory along with data
• Stored-program concept

4
Logical Operations

• Often we need to operate on bit fields within a
  word
• For example, how to access a byte of word
• How to extract op code from an instruction?
• We need logical operations
• Which allow us to pack and unpack bits into words
  and perform logical operations such as logical
  and, logical or, and logical negation

5
Shifts

• Shift instructions move all the bits in a word to
  the left or to the right
• Shift left logical (sll) move all the bits to the
  left by the specified number of bits
• Shift right logical (srl) move all the bits to
  the right
• Filling the emptied bits with 0s

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Example

• Suppose register s0 (16) is 9ten
• What do we have in t2 (10) after

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1
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Example

• Suppose register s0 (16) is 9ten
• We have in t2 (10) after
• The value is 144ten 9ten ? 24
• In general, shifting left by i bits gives the
  same result as multiplying by 2i

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1 0 0 0 0
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Example

• Suppose register s0 (16) is 9ten
• We have in t2 (10) after
• The value is NOT 9ten ? 228
• Note that overflow happens this time

31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 1
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
1 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
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How to Extract Opcode and Other Fields

• We want to use the minimum number of instructions
• Extract the opcode?
• Shift the instruction to right by 26 bits
• How about rs?

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How to Extract Opcode and Other Fields

• We want to use the minimum number of instructions
• Extract the opcode?
• Shift the instruction to the right by 26 bits
• How about rs?
• Shift the instruction to the left by 6 bits
• Then shift the result to the right by 27 bits

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Shift Left Logical Instruction Encoding
31
26
25
21
20
16
15
11
10
6
5
0
opcode
rs
rt
rd
shamt
funct
rd
shamt
sll 10, 16, 4
rt
31
26
25
21
20
16
15
11
10
6
5
0
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
1
0
1
0
0
0
1
0
0
0
0
0
0
0
0
0
0
opcode
rs
rt
rd
shamt
funct
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
1
0
1
0
0
0
1
0
0
0
0
0
0
0
0
0
0
Encoding 0x00105100
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Bit-wise AND

• Apply AND bit by bit
• The resulting bit is 1 if both of the input bits
  are 1 and zero otherwise
• There is also a version of AND with an immediate
• Note that the immediate is treated as an unsigned
  16-bit number
• In other words, the immediate is zero-extended to
  a 32-bit number

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Bit-wise OR

• Apply OR bit by bit
• The resulting bit is 1 if at least one of the
  input bits is 1 and zero otherwise
• There are also two versions of OR
• As in andi, the immediate is treated as an
  unsigned 16-bit number

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NOT

• Since NOT takes one operand and results in one
  operand, it is not included in MIPS as an
  instruction
• Because in MIPS each arithmetic operation takes
  exactly three operands
• Instead, NOR is included
• The resulting bit is 0 if at least one of the
  input bits is 1
• How to implement NOT using NOR?
• Using zero as one of the input operands
• It is included in MIPS as a pseudoinstruction

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Instructions for Making Decisions

• A distinctive feature of programs is that they
  can make different decisions based on the input
  data

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Instruction beq (branch if equal)

• To support decision making, MIPS has two
  conditional branch instructions, similar to an
  if statement with a goto
• In C, it is equivalent to
• Note that L1 is a label and we are comparing
  values in register1 and register2

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Instruction bne

• Similarly, bne (branch not equal) means go to the
  statement labeled with L1 if the value in
  register1 does not equal to the value in regster2
• Equivalent to

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Instruction j (jump)

• MIPS has also an unconditional branch, equivalent
  to goto in C
• Jump to the instruction labeled with L1

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Compiling if-then-else

• Suppose variables f, g, h, i, and j are in
  registers s0 through s4, how to implement the
  following in MIPS?

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Compiling if-then-else

• Suppose variables f, g, h, i, and j are in
  registers s0 through s4, how to implement the
  following in MIPS?

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Compiling if-then-else

• Suppose variables f, g, h, i, and j are in
  registers s0 through s4, how to implement the
  following in MIPS?

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MIPS Assembly for if-then-else

• Now it is straightforward to translate the C
  program into MIPS assembly