Memory Addressing Techniques in Computer Science

Indirect Addressing

• The binary equivalent of the address part is 300; it points to where the operand's address is stored.
• Operand's address is 1350, where the actual operand is located.
• Two memory references are required to fetch an operand: the first for the address of the operand, the second for the operand itself.
• Indirect addressing acts as a pointer to an array of data found in memory.

Computer Registers

• Program Counter (PC): Holds the address of the current instruction.
• Address Register (AR): Holds the address for memory operations.
• Instruction Register (IR): Holds the instruction code currently being executed.
• Temporary Register (TR): Stores temporary data during operations.
• Data Register (DR): Holds the memory operand for processing.
• Accumulator (AC): Primary processor register for arithmetic operations.
• Output Register (OUTR) and Input Register (INPR): Store output and input characters respectively.

Memory Specifications

• Memory consists of 4096 words, with each word being 16 bits in size.
• A common bus system connects various components of the computer for data transfer.

Instruction Types

• Memory Reference Instructions: Carry an opcode and an address.
  • Examples include operations like AND, ADD, LDA (load to AC), STA (store from AC), BUN (branch unconditionally), BSA (branch and save return address), ISZ (increment and skip if zero).
• Register Reference Instructions: Target specific registers to perform operations.
  • Examples include CLA (clear AC), CLE (clear E), CMA (complement AC), and INC (increment AC).
• Input-Output Instructions: Manage data transfer between the computer and peripheral devices.

Instruction Format

• Instruction format comprises an opcode and an address section.
• For a memory with 4096 words, 12 bits are necessary for addressing, supporting 16 possible operations with 4 bits for opcodes.

Direct vs. Indirect Addressing

• Direct Addressing: The instruction includes the operand's address directly.
• Indirect Addressing: The instruction specifies a location holding the address of the operand.
• The indirect address can be identified by a mode bit; a value of 0 indicates direct addressing, while 1 indicates indirect.

Control Flow

• Instructions that require operand addresses can be fetched based on the type of addressing specified.
• In direct addressing, the control retrieves and processes data from the specified address directly.
• In indirect addressing, it first retrieves the address from a specified location, then accesses the operand.

Interrupt Cycle Operations

• If both input and output flags are zero, the control continues to execute the next instruction.
• When a flag is set while the Interrupt Enable (IEN) is active, the control shifts to an interrupt cycle.
• The control assesses the flip-flop R at the end of the execute phase; if it's set, the next step is an interrupt cycle instead of a regular instruction cycle.

Register Transfer Statements

• The interrupt cycle involves specific register transfer steps, including:
  • Setting AR to zero and transferring PC content to TR.
  • Storing the return address in memory and clearing PC.
  • Incrementing PC, clearing IEN and R, and preparing to resume instruction execution.

Execution of Operations

• The instruction cycle that fetches and executes takes place after the reset of the control during an interrupt cycle, bridging back into fetch operations for subsequent instructions.

This quiz explores the concept of indirect addressing in memory management. It covers how binary equivalents are used and the process involved in fetching operands from memory addresses. Test your understanding of memory addressing techniques and their implications in computing.