The memory (or I-type) instruction
lw, load word, operates in five steps:
lw $t1, offset($t2) # $t1=mem[[$t2]+offset] # rt=mem[ [rs]+offset] |
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- An instruction is fetched and the PC is incremented.
- A register (
$t2orrs) value is read from the register file. - The ALU computes the sum of the value read from the register file and the sign extended, lower 16 bits of the instruction (
offset). - The sum from the ALU is used as the address for the data memory.
- The data from the memory unit is written into the register file; the register destination is given by bits 20:16 of the instruction (
$t1orrt).
ExtOp=“sign”sign-extends the 16-bit immediate to 32 bits.ALUSrc=‘1’selects the extended immediate as the second ALU input.ALUCtrl=“ADD”calculates data memory address as[rs]+sign-extended(imm16)MemRead=‘1’reads data memory.RegDst=‘0’selectsrtas the destination register.MemtoReg=‘1’places the data read from memory onBusW.RegWrite=‘1’writes the memory data onBusWto registerrt.