Several of the arithmetic/logical instructions also have an immediate form. They are I-type and one I-type ALU instruction is
addi, add immediate:
addi $10, $8, 9 # $10 = $8 + 9 # rt = rs + imm16 |
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The instruction format includes the following features:
- The
opfield, also called the opcode, is always contained in bits 31:26. - The two registers
rsandrtare at the positions bits 25:21 and 20:16. - The 16-bit offset is in positions bits 15:0.
| The figure shows a datapath for I-type ALU instructions. Other I-type instructions such as load and store will be discussed later. |
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The datapath has the following features:
- The second ALU input comes from the extended immediate.
RWnow comes fromrt, instead ofrd.RBandBusBare not used.
ALUCtrlis derived from theopfield.RegWriteis used to enable the writing of theALU result.ExtOpis used to control the extension of the 16-bit immediate.