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CPU must:Fetch instructions

Interpret instructions

Fetch data

Process data

Write data

Organizational requirements that are derived from these

functions:

ALU

Control logic

Temporary storage

Means to move data and instructions in and around the CPU

Processor organization

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CPU With System Bus (External view of the CPU)

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Internal Structure Of CPU

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Register Organization

Registers form the highest level of the memoryhierarchy:

Small set of high speed storage locations

CPU must have some working space (temporarystorage for data and control information), which are

called registers

Number and function vary between processor designs

One of the major design decisions

Top level of memory hierarchy

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Two types of registers

User-visible May be referenced by assembly-level

instructions and are thus visible to the

user

Control and status registers

Used to control the operation of the CPU Most are not visible to the user

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User-visible RegistersGeneral categories based on function

General purpose Can be assigned a variety of functions

Ideally, they are defined orthogonally to the operations

within the instructions

Data

These registers only hold dataAddress

These registers only hold address information

Examples: general purpose address registers, segment

pointers, stack pointers, index registers

Condition codes Visible to the user but values set by the CPU as the result

of performing operations

Example code bits: zero, positive, overflow

Bit values are used as the basis for conditional jump

instructions

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Design trade off between general purpose and specialized registers

General purpose registers maximize flexibility in instructiondesign

Special purpose registers permit implicit register specification in

instructions reduces register field size in an instruction

No clear best design approachHow many registers are enough

More registers permit more operands to be held within the CPU --

reducing memory bandwidth requirements to some extent

More registers cause an increase in the field sizes needed to

specify registers in an instruction word Locality of reference may not support too many registers

Most machines use 8-32 registers

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How big (wide)

Address registers should be wide enough to hold the longest

address

Data registers should be wide enough to hold most data types

Would not want to use 64-bit registers if the vast majority

of data operations used 16 and 32-bit operands

Related to width of memory data bus Concatenate registers together to store longer formats

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Control and status registersThese registers are used during the fetching, decoding and

execution of instructions Many are not visible to the user/programmer

Some are visible but can not be (easily) modified

Typical registers

Program counter Points to the next instruction to be executed

Instruction register

Contains the instruction being executed

Memory address register

Memory data/buffer register

Program status word(s)

Superset of condition code register

Interrupt masks, supervisory modes, etc.

Status information

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CPU Register Classification:

1 User-visible registers

General Purpose

Data

Address

Condition Codes

2 Control and status registers