Instruction Register and Addressing Modes

Instruction Register

• Holds the program instruction currently being executed.
• Modern processors use more than the 16-bit representation shown in the diagram.
• Anatomy of a machine code instruction:
  • Operation Code (opcode): Represents operations like add, subtract, load, store.
  • Operand(s): Identifies the data to be operated upon. Could be a value, CPU register, or memory address.

Bit Allocation

• More opcode bits = larger instruction set, but fewer bits for operand = fewer memory locations accessible.
• Fewer opcode bits = fewer instruction types, more bits for memory addresses.

Addressing Modes

• Different modes help overcome memory addressing limitations and efficiently utilize registers and memory.
• Immediate Addressing: Operand is the actual value to be operated on (e.g., #5).
• Direct (Absolute) Addressing: Operand is a memory address containing the value.
• Indirect Addressing: Operand is a memory address that holds the effective memory address.
• Register Direct Mode: Operand is a processor register containing a value (very fast).
• Register Indirect Mode: Operand is a register containing a memory address.
• Relative Addressing: Effective memory address = Program counter base address + operand offset.
• Indexed Addressing: Operand = base memory address, effective memory address = base address + index register value.

Execution Examples

• Immediate Addressing Example:
  • Instruction: Load value #5 into accumulator.
  • Address in PC is copied to Memory Address Register.
  • Instruction copied to Current Instruction Register via Memory Data Register.
  • PC incremented after fetch.
  • Processor operates immediately on the fetched value.
• Direct Addressing Example:
  • Instruction: Load content of location 501 into accumulator.
  • Address in PC copied to Memory Address Register.
  • Instruction fetched in Current Instruction Register, PC incremented.
  • Processor retrieves value from memory address 501.
• Indirect Addressing Example:
  • Looks in memory location 501 for the address of the location with needed data.
  • Memory accessed twice during execution.
• Register Direct Mode Example:
  • Instruction: Load contents of register r1 into accumulator.
  • No memory access needed after fetch.
• Relative Addressing Example:
  • Unconditional jump instruction: Adds operand value 3 to program counter 101.
  • PC updated to 104, control passed to new instruction.
• Indexed Addressing Example:
  • Operand adds 3 to address 500 (fourth element of array).
  • Effective memory address calculated as base address + index register value.

Summary

• Immediate Addressing: Operand is the actual value.
• Direct Addressing: Operand is a memory address with the value.
• Indirect Addressing: Operand is a memory address with the effective memory address.
• Register Direct Mode: Operand is a processor register with a value.
• Register Indirect Mode: Operand is a processor register with a memory address.
• Relative Addressing: Effective address = PC base address + operand offset.
• Indexed Addressing: Effective address = base memory address + index register value.

Variations & Additional Modes

• Indexed addressing has variations:
  • Indexed Immediate
  • Indexed Indirect
• Displacement Mode: Uses a register (other than index register) containing a memory address.
• Some architectures include Auto increment, Auto decrement, and spec addressing modes.