6_2_Addressing_mode

Computer Composition Chapter VI: Instruction System Principles

6.1 Basic Concepts

  • Introduction to the fundamental concepts related to computer instruction systems.

6.2 Addressing Methods

  • Different techniques through which instructions locate operands or next instruction addresses.

6.3 RISC vs. CISC

  • Overview of the differences between RISC (Reduced Instruction Set Computer) and CISC (Complex Instruction Set Computer) architectures.

Addressing Methods

Instruction Addressing

  • Refers to the method for determining the address for the next instruction in a sequence.

  • The address of the next instruction is typically given by the Program Counter (PC).

Key Concepts in Instruction Addressing

  • Opcode (OP): The part of the instruction that specifies the operation to be performed.

  • Address Code: May involve specifying multiple addresses depending on the instruction structure.

  • Instruction Structure: An instruction usually consists of an opcode and its corresponding address codes.

  • Addressing Mode: The method used to locate an operand or an instruction's address in memory.

Sequential Addressing

  • Instructions are usually stored sequentially in memory.

  • If the address of the first instruction is known, the next instruction's address can be calculated by adding the instruction length to the current address.

Branching and Jump Instructions

  • Branch Instructions: Change the execution order based on certain conditions.

  • Jump Instructions: Change the execution order unconditionally.

Program Counter (PC)

  • Holds the address of the next instruction to be executed.

  • The value in PC increments by one (or by the length of the instruction) after each instruction fetch.

Instruction Formats

Fixed-Length Instruction Word Structure

  • Memory is accessed word by word, typically with an instruction length of 16 bits (2 bytes).

  • Example sequence with opcodes and address codes (simplified):

    • LDA 1000: Load Accumulator with data from memory address 1000.

    • ADD 1001: Add data from memory address 1001 to the Accumulator.

    • JMP 7: Jump to instruction at address 7.

Overview of Addressing Types

Immediate Addressing

  • The operand itself is represented in the instruction (denoted by #).

  • Advantages include fast execution as no additional memory access is required.

Direct Addressing

  • The address field in the instruction directly points to the operand’s location in memory.

  • This method offers simplicity but may have limited addressing range based on the bits allocated.

Indirect Addressing

  • The address field points to another memory location that contains the effective address of the operand.

  • It allows accessing a broader range of memory but requires multiple memory accesses.

Register Addressing

  • Operand address is directly given as a register identifier.

  • Benefits include a fast execution phase since it does not access main memory.

Stack Addressing

  • Uses the stack pointer (SP) to implicitly reference operands stored on a stack.

  • Operands are accessed in a Last In, First Out (LIFO) manner.

Implicit Addressing

  • The operand’s address is implied by the instruction itself (e.g., accumulator-based instructions).

Relative Addressing

  • The effective address is computed using the program counter (PC) and an offset specified in the instruction.

  • Facilitates code relocation and supports branching instructions.

RISC vs. CISC

RISC (Reduced Instruction Set Computer)

  • Characterized by a small, highly optimized instruction set with fixed instruction formats.

  • Focuses on executing instructions in fewer clock cycles, often one cycle per instruction.

CISC (Complex Instruction Set Computer)

  • Contains a large variety of instructions, many addressing modes, and instruction formats leading to more complicated instruction decoding.

  • Typically takes multiple clock cycles to execute most instructions.

Comparison of RISC and CISC

  • RISC simplifies command sets and shifts processing to register, while CISC tends toward complex instructions that appeal to high-level language support.

  • Compilation and optimization strategies vary, with RISC being more efficient in runtime execution due to its streamlined instructions.