CPU Structure

• CPU must:

—   Fetch instructions

—   Interpret instructions

—   Fetch data

—   Process data

—   Write data

CPU With Systems Bus

CPU Internal Structure

Registers

• CPU must have some working space (temporary storage)


• Called registers


• Number and function vary between processor designs


• One of the major design decisions


• Top level of memory hierarchy

User Visible Registers

• General Purpose


• Data


• Address


• Condition Codes

General Purpose Registers

• May be true general purpose


• May be restricted


• May be used for data or addressing


• Data

—   Accumulator

• Addressing

—   Segment

Condition Code Registers

• Sets of individual bits

—   e.g. result of last operation was zero

• Can be read (implicitly) by programs

—   e.g. Jump if zero

• Can not (usually) be set by programs

Control & Status Registers

• Program Counter


• Instruction Decoding Register


• Memory Address Register


• Memory Buffer Register

Program Status Word

• A set of bits


• Includes Condition Codes


• Sign of last result


• Zero


• Carry


• Equal


• Overflow


• Interrupt enable/disable


• Supervisor

Supervisor Mode

• Intel ring zero


• Kernel mode


• Allows privileged instructions to execute


• Used by operating system


• Not available to user programs

Example Register Organizations