CPU

Register File:

• While Von Neumann has only an accumulator, MIPS has 32 registers
• Prefixes:
  • R#: Register
  • M#: Memory

Control Unit: All computer operations are controlled by the CU.

• Timing signals that govern the IO transfers and generated by the CU
• Usually distributed throughout the machine, not just a single unit

ALU: Most computer operations are executed in the processor’s ALU

Example: Data flow in and out of the ALU

1. Load operands into memory
2. Bring operands from memory into the processor’s registers
3. Perform operation in ALU
4. Store result in the processor or move back into memory

Memory Unit

Memory Unit: Stores programs and data

• Main Memory: Volatile
  • Fast.
  • Processed in words (e.g., 4 bits at a time)
• Secondary Storage: Non-volatile
  • Larger and cheaper.

Basic Operational Concepts

To perform tasks, we use programs (lists of instructions and data), which are stored in memory, which are moved into the processor’s registers and/or cache as needed.

Example: A Typical Instruction

ADD R0, M1

• What it does: R0 <- R0 + M1
  • M1’s contents are preserved.
  • R0 is overwritten.
• Note: The leftmost operand is the destination

Note: In the real world, the BIU is accessible only to the control unit, and the IR and PC are only indirectly accessible through instructions.

Example: Typical Operating Steps: \text{Program Execution: } \\ \text{Secondary Storage $\to$ Main Memory $\to$ CPU Registers $\to$ Execution}

1. Program loaded from secondary storage into main memory.

• Done by the loader

2. PC set to point to the first instruction.
3. Content of PC is transferred to the MAR
4. Read signal is asserted to main memory
5. Instruction retrieved from memory and loaded into MDR
6. Contents of MDR transferred to IR

• (This is necessary for the decode cycle to start.)

7. Instruction get decoded by CU.
8. Get operands for the ALU

• Operands can be in the general-purpose registers or in memory.
• Note: Some instructions don’t use ALU
  • e.g., Loading & storing memory and interrupt instructions don’t use ALU

9. Perform operations using ALU
10. Store result

• Can be stored in a general-purpose register or memory.

11. PC incremented to next step

• (Hardware-wise, this is conditional (example: doesn’t happen on jump instructions))