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I type instructions (revisited)

• The I is for immediate.
• These instructions have an immediate third operand, i.e., the third operand is contained in the instruction itself.
• This means the operand itself, and not just its address or register number, is contained in the instruction.
• Two registers and one immediate operand.
• Compare I and R types: Since there is no shamt and no funct, the immediate field can be larger than the field for a register.
• Recall that lw and sw were I type. They had an immediate operand, the offset added to the register to specify the memory address.

addi (add immediate)

Example: addi $1,$2,100

• $1 = $2 + 100
• Why is there no subi?
  Ans: Make the immediate operand negative.

slti (set less-than immediate)

Example slti $1,$2,50

• Set $1 to 1 if $2 less than 50; set $1 to 0 otherwise.

lui (load upper immediate)

Example: lui $4,123

• Loads 123 into the upper 16 bits of register 4 and clears the lower 16 bits of the register.
• What is the use of this instruction?
• How can we get a 32-bit constant into a register since we can't have a 32 bit immediate?
  1. Load the word
     • Have the constant placed in the program text (via some assembler directive).
     • Issue lw to load the register.
     • But memory accesses are slow and this uses a cache entry.
  2. Load shift add
     1. Load immediate the high order 16 bits (into the low order of the register).
     2. Shift the register left 16 bits (filling low order with zero)
     3. Add immediate the low order 16 bits
     4. Three instructions, three words of memory
  3. load-upper add
     • Use lui to load immediate the desired 16-bit value into the high order 16 bits of the register and clear the low order bits.
     • Add immediate the desired low order 16 bits.
     • lui $4,123 -- puts 123 into top half of register 4.
       addi $4,$4,456 -- puts 456 into bottom half of register 4.
       

Homework: 3.1, 3.3, 3.4, and 3.5.

Chapter 4

Homework: Read 4.1-4.4

4.2: Signed and Unsigned Numbers

MIPS uses 2s complement (just like 8086)

To form the 2s complement (of 0000 1111 0000 1010 0000 0000 1111 1100)

• Take the 1s complement.
• That is, complement each bit (1111 0000 1111 0101 1111 1111 0000 0011)
• Then add 1 (1111 0000 1111 0101 1111 1111 0000 0100)

Need comparisons for signed and unsigned.

• For signed a leading 1 is smaller (negative) than a leading 0
• For unsigned a leading 1 is larger than a leading 0

sltu and sltiu

Like slt and slti but the comparison is unsigned.

Homework: 4.1-4.9