CPU components (cont.)

Example: Steps to executing an sw (store word) instruction:

1. Instruction goes to instruction memory
   • Instruction memory goes to RAM at address given by PC
   • Instruction gets stored in "instruction register" within instruction memory
2. Instruction is disassembled into parts which go to different components of the CPU
   • e.g., register #1 goes to register file - 5 bits
3. Register in register file goes to ALU to be added with immediate value, so that the store word instruction gets the address with the correct offset

Additional components:

• "sign-extend" component - 16 bits in, 32 bits out. Takes leftmost bit and extends to 16 more bits for a total of 32

Note: Not expected to memorize circuitry for exam, but expected to understand picture. Should be able to, e.g.:

• Remove part of picture, ask to fill in missing part (infer from context)
• What is the meaning of the load word instruction's ALU output? (answer: address (register value) + offset (immediate value) gives address in RAM to load from)
• What is the bit width of a given channel? (infer from context)

Recall the machine cycle steps:

• Fetch
• Decode
• Execute
• Write back

Each time the "fetch" instruction happens, the PC is immediately updated to the new value.

Argument registers:

• Registers that need to be read
• Always first two registers in the instruction (e.g., rs, rt)
• Need to be first two registers because those bits of the instruction are wired up to the "read register" inputs of the register file

Example: BEQ instruction steps

1. PC passes instruction address to instruction memory
2. Instruction register arguments passed to register file
3. Argument registers go to ALU - compare values by subtracting
4. Check difference:

• If difference was 0, produce a value of 1, to send along the "zero" indicator (meaning that the values were equal)
  • This goes to an AND gate with the Branch control line from the opcode, to update the Program Counter to reflect the fact that the branch has occurred
  • Branch amount:
    • take 16-bit immediate value (recall that BEQ is an I-type instruction)
    • sign-extend using the sign extend component
    • shift left two bits (due to addressing mode)
    • add to program counter (not necessary to use full ALU for this - can just use adder, because the immediate value could be negative)
• If the difference was nonzero, send 0 along the "zero" indicator line, so the AND gate is zeroed out and the jump does not occur, and the PC is left as is

Control units

ALU control unit:

• Gets a channel of width 4 bits, which are set based on the opcode
• Uses the below table (from the textbook; will be provided with it if necessary on the final exam)
• Controls which operation is performed by the ALU (e.g., AND, OR, NOT, ADD, etc.)

(big) Control Unit:

• Takes opcode as input (6 leftmost bits of instruction)
• Generates corresponding output, which sends bits through control lines as needed

Example use of control units: determine the values of each of the control lines for the below instruction:

add $s0, $t5, $t6

Opcode: 000000 because this is an R-type instruction.

Control line values:

• RegDst: 1
• Branch: 0
• MemRead: 0
• MemtoReg: 0
• ALUOp: 10
• MemWrite: 0
• ALUSrc: 0
• RegWrite: 1