Processor Organization and Architecture

Processor Organization and Architecture

• The CPU requirements include fetching instructions, interpreting/decoding them, fetching data, processing/executing data (arithmetic/logical operations), and writing data.
• The processor connects to the rest of the system via the system bus.
• The major components of the processor include:
• ALU (Arithmetic Logic Unit) for data processing.
• CU (Control Unit) for controlling the movement of data and the ALU.
• Registers as Internal Memory consists of storage locations

Internal Structure of CPU

• The CPU's internal structure includes data transfer paths between registers and the ALU, as well as logic control paths.
• An internal CPU bus facilitates data transfer between the ALU and Registers.

Register Organization

• Registers in the processor serve two main roles: User Visible Registers and Control and Status Registers.
• User Visible Registers minimize main memory references by optimizing register usage.
• Control and Status Registers are used by the control unit to manage processor operation.

User Visible Registers

• These are referenced using machine language and are categorized into:
• General Purpose
• Data
• Address
• Condition Codes

General Purpose Registers

• Registers contain operands.
• Can be used for addressing functions.

Data Registers

• Registers are used to hold data.

Address Registers

• Registers may be general purpose or devoted to an addressing mode.
• Segment Pointers hold the base address of a segment.
• Index Registers are for indexed addressing.
• Stack Pointer points to the top of the stack.

Condition Codes (Flags)

• Bits set by the processor hardware as a result of operations (e.g., Carry, Overflow, Sign).
• These codes can be tested as part of conditional branch operations.
• Condition code bits are collected into registers that the programmer cannot directly alter.

Design Issues with Registers

• Whether to use general purpose or specialized registers is a key design consideration.
• Specialized registers implicitly specify operands.
• Example: PUSH and POP in Stack.
• Specialization limits programmer flexibility.
• The number of registers to provide affects operand specifier bits and memory references.
• More registers require more operand specifier bits.
• Fewer registers lead to more memory references.

Register Length

• Register length is determined by the need to accommodate the largest address and most data types.

Control and Status Registers

• Control Registers essential for instruction execution include:
• Program Counter
• Instruction Register
• Memory Address Register
• Memory Buffer Register

Status Register

• A register or set of registers containing status information is called the Program Status Word (PSW).
• PSW contains condition codes and other status information.
• PSW includes:
• Sign: Sign bit of the last operation.
• Zero: Set when the result is 0.
• Carry: Set when a carry/borrow is generated/taken.
• Overflow: Indicates arithmetic overflow.
• Interrupt Enable/Disable: Used to enable/disable interrupts.
• Supervisor: Indicates supervisor/user mode.
• Other registers include interrupt vector register and system stack pointer.

Addressing Modes

• Addressing modes specify how operands are located.
• The different modes include:
• Immediate
• Direct
• Indirect
• Register
• Register Indirect
• Displacement (Indexed)
• Stack

Immediate Addressing

• The operand is part of the instruction.
• Example: ADD 5.
• Adds 5 to the contents of the accumulator.
• 5 is the operand.
• Advantage: No memory reference to fetch data; fast.
• Disadvantage: Limited range.

Direct Addressing

• The address field contains the effective address of the operand.
• Example: ADD A.
• Looks in memory at address A for the operand.
• Advantage: Single memory reference to access data; no additional calculations for the effective address.
• Disadvantage: Limited address space.

Indirect Addressing

• Memory cell pointed to by the address field contains the address of the operand.
• EA = (A).
• Looks in A, finds address (A), and looks there for the operand.
• Example: ADD (A).
• Adds the contents of the cell pointed to by the contents of A to the accumulator.
• Advantage: Large address space; 2n where n = word length; may be nested and multilevel.
• Disadvantage: Multiple memory accesses to find the operand, hence slower.

Register Addressing

• The address field refers to a register.
• EA = R.
• Advantage: Only a small address field is needed in the instruction; less access time.
• Disadvantage: Limited number of registers and limited address space.

Register Indirect Addressing

• Similar to indirect addressing.
• EA = (R).
• The operand is in the memory cell pointed to by the contents of register R.
• Requires one fewer memory access than indirect addressing.

Displacement Addressing

• Combines direct and register indirect addressing.
• EA = A + (R).
• The address field holds two values:
• A = base value.
• R = register that holds the displacement (or vice versa).

Stack Addressing

• The stack is a reserved block of locations.
• The stack pointer holds the address of the top of the stack.
• It implies addressing, requiring no memory reference, and operates on the top of the stack.