Computer Organization and Architecture

Control Unit Operation & Microprogrammed Control

Outline

• Control Unit Operation

  • Micro-Operations

  • Control of the Processor

  • Hardwired Implementation

• Microprogrammed Control

  • Basic Concepts
  • Microinstruction Sequencing
  • Microinstruction Execution

Control Unit Operation

The function of a processor

• Instruction Fetch and Execute

  • Execute program

• Interrupts

  • Handling performance differences between CPU and other components

• I/O Function

  • Inter-working with peripherals

Composition of instructions

• Instructions include opcodes and operands

• Opcodes

  • Determines what type of operation the instruction does

• Operands

  • Determine the object of instruction operation

  • Operands may be in registers, memory, I/O, or immediate

  • Finding operands through addressing mode

Micro-operations

• A computer executes a program to complete user specified functions

  • Program contains many instructions

  • Instruction execution includes several cycles, such as fetch cycle, execution cycle, indirect cycle, etc

• Each cycle has a number of steps

  • Called micro-operations

  • Each step does very little

  • Atomic operation of CPU

• 程序的执行是由若干个指令执行组成的

• 每个指令的执行就是一个指令周期

• 指令周期由若干个子周期组成，比如取指周期、执行周期等

• 每个子周期包含若干个更小的操作，这些操作称为微操作

Fetch

• Program Counter (PC)

  • Holds address of next instruction to be fetched

• Memory Address Register (MAR)

  • Connected to address bus

  • Specifies address for read or write op

• Memory Buffer Register (MBR)

  • Connected to data bus

  • Holds data to write or last data read

• Instruction Register (IR)

  • Holds last instruction fetched

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Step1

• 下一个指令的地址是放在PC里

• MAR是与地址总线连接的唯一寄存器

• 取指的第一步是PC把下一个指令的地址给MAR

• 经过第一步之后，PC寄存器的内容复制到MAR寄存器中

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Step2

• MAR收到地址后，把地址放到地址总线上

• 控制单元发一个读命令到控制总线上

• 存储器收到读命令后，根据地址读出指令内容，并放到数据总线

• 数据总线上的数据读到MBR

• PC中还需要递增一个指令长度，以得到下一个指令的地址

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Step3

• MBR的内容传送给IR

• 传送完成后，MBR就可以释放，用于下一步操作

• IR中保存的就是下一个需要执行的指令

• 通过这几个微操作，完成了取指

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Fetch sequence

• 取指周期实际上由三步、四个微操作组成。每个微操作都涉及到数据在寄存器之间的流动

• 如果数据的流动是独立的，这些操作可以在同一个时钟周期内完成

• PC递增微操作，既可以在t2完成，也可以在t3完成

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Rules for Micro-operations grouping

• Proper sequence must be followed

  • MAR <- (PC) must precede MBR <- (memory)

• Conflicts must be avoided

  • Must not read & write same register at same time

  • MBR <- (memory) & IR <- (MBR) must not be in same cycle

• Also: PC <- (PC) +1 involves addition

  • Use ALU

  • May need additional micro-operations

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Indirect cycle

• In indirect addressing, it is necessary to get the address of the operand from the memory first. This process is called indirect cycle

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Interrupt cycle

• After instruction processing is completed, check whether there is an interrupt

• Interrupt cycle needs to complete these operations

  • The instruction address before the interrupt needs to be saved

  • Get the instruction address of the start of the interrupt handler to the PC

• After the PC gets the interrupt processing address, it enters the index retrieval cycle

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Execute cycle

• Fetching, indirect and interruption cycle can be determined in advance

  • Each cycle contains a fixed sequence of micro operations

• For execution cycle

  • Different operation codes correspond to different operations

  • Each operation has a specific micro operation sequence

Flowchart for instruction cycle

• 取指周期后，看是否是间接寻址。如果是间接寻址，进入01间接周期，如果不是，进入执行周期。

• 间接周期后，读取地址，然后进入10执行周期

• 执行周期，根据操作码来确定做什么操作。然后判断是否有中断。如果有中断，进入11中断周期。如果没有，进入00取指周期。

• 中断周期，设置中断，然后进入中断处理程序的取指周期，继续取指

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• Control of the Processor

  • Functional Requirements

  • Control Signals

  • A Control Signals Example

  • Internal Processor Organization

Functional requirement

• Micro-operations are the most basic function of the processor

• Three elements of the controller include

  • Basic elements of processor

  • Micro-operations that processor performs

  • How to control

  • ALU

  • Registers

  • Internal data paths

  • External data paths

  • Control Unit

Types of micro-operation

• Micro-operations can be divided into the following four categories

  • Transfer data between registers

  • Transfer data from register to external

  • Transfer data from external to register

  • Perform arithmetic or logical ops

Functions of control unit

• Operation object and operation type have been determined

• Function of the controller is to determine how to do it

  • Sequencing

  • Execution

  • This is done using Control Signals

Model of control unit

• 控制器的输入包括：时钟信号，各种标志，指令寄存器中的指令，来自控制总线的控制信号(中断)

• 控制器的输出包括：CPU内部的控制信号，到控制总线的控制信号

• 控制器内部就是如何根据输入，来生成控制信号

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Control signals

• Clock

  • Provide clock signal for timing of controller
  • One micro-instruction (or set of parallel micro-instructions) per clock cycle

• Instruction register

  • Op-code for current instruction

  • Addressing mode

  • Determines which micro-instructions are performed

• Flags

  • State of CPU

  • Results of previous operations

• Control signal from control bus

  • Interrupts

  • Acknowledgements

• output

  • Within CPU

    • Cause data movement

    • Activate specific functions

  • Via control bus

    • To memory，control read or write

    • To I/O modules

  • It generates three types of output control signals

    • Data paths: the signals control the internal flow of data

    • ALU: the signals control the operation of the ALU

    • System bus: the control unit sends these signals to the control lines of the system bus

Internal organization

• CPU adopts single bus structure

  • All components, including ALU and registers, are connected to the bus

  • A gate is set between each component and the bus to control the data transmission between the component and the bus

• Data transfer to and from external systems bus is controlled by the control signal

• Temporary registers needed for proper operation of ALU

• 各个寄存器都链接到内部总线上。寄存器和内部总线之间有个逻辑门，用于寄存器和内部总线的连接控制

• ALU没有内部存储部件。如果ALU的操作涉及2个操作数的话，一个由内部总线得到，另一个必须通过其他的数据源来得到，不能都在内部总线上。需要增加一个Y寄存器，用于临时保存源操作数

• ALU还需要有一个寄存器，来临时保存输出结果。这个结果不能总线上，因为如果放总线上的话，它又会当作ALU的输入

Implementation of control unit

• The control unit implementation can be done by two methods

  • Hardwired implementation

  • Microprogrammed implementation

• Hardwired implementation: the control unit is a combinational circuit

  • The input signals are transferred into a set of output control signals

• 时钟发生器：产生时钟信号，并提供计数器

• 译码器：根据指令寄存器中的指令操作码，生成操作码对应的唯一输出，提供给输出控制单元

• 标志：为控制单元提供相关的标志信号

• 控制单元：是控制器的核心，负责产生控制信号

Control matrix

• Programmable arrays may also be numerous

• Large control matrices are implemented hierarchically for speed

Problems With Hard Wired

• Complex micro-operation logic

• Difficult to design and test

• Inflexible design

• Difficult to add new instructions

Summary

• The instructions are divided into a sequence of stages

• Fetch, indirect, execute, interrupt

• Each stage is further divided into a sequence of microoperations

  • T1, T2, T3, …

• Each micro-operation occupies one clock unit

• The function of the control unit is to produce a sequence of timed control signals to control the data paths and ALU operations

• The control unit is a combinational circuit that transfers inputs to a group of control signals

  • Inputs: flags, instruction registers, clock, control signals from system bus

  • Outputs: control signals to system bus and within CPU

Microprogrammed Control

Basic Concepts

• Control unit can be implemented by a more flexible technique called microprogrammed control unit

  • The logic of the control unit is specified by a microprogram

  • Each line in microprogram describes a group of micro-operations in one time，called microinstruction

  • Construct a control word corresponding to this microinstruction

  • The control word determines the opening or closing of all doors

• 微指令中包括CPU内控制信号、系统总线控制信号、跳转条件、微指令地址

• 执行这条微指令的效果是，打开CPU内外所有为1的控制线，关闭所有为0的控制线

• 如果条件条件为假，自动执行下一个顺序的微指令

• 如果跳转条件为真，则执行“微指令地址”中对应的微指令

Micro-instruction Types

• horizontal microinstruction

  • 水平微指令包括四个部分

    • 内部控制信号：CPU内的每一个控制线都有相应的1位
    • 总线控制信号：每一个系统控制总线都有相应的1位
    • 条件字段：指示转移发生条件的字段
    • 微指令地址：转移的目标指令地址

  • 打开位值为1的控制线，关闭所有位值为0的控制线。执行一个或多个微操作

  • 如果跳转条件为假，顺序执行下一个指令。如果跳转条件为真，执行“微指令地址” 指向的微指令

  • Characteristic

    • Each control line inside the CPU and bus needs to have a separate control bit

      • The width of the control word is very large

    • Each line is a separate control bit

      • Each control line can be controlled independently
      • More kinds of micro operations that can be supported

  • The control lines are controlled by one bit of the control word independently

    • The coding of control signal is not compact

• vertical microinstruction

  • 垂直微指令也包括条件和跳转微指令地址，跟水平微指令类似

  • 在控制方面，和水平微指令的每一位对应一个控制线不同，垂直微指令又做了一次编码

  • 解决水平微指令的控制字太长的问题

  • 编码后，由一个解码器再翻译成控制线的控制信号

  • Characteristic

    • Width is narrow

      • n control signals encoded into $\log_2 n$ bits($n \rightarrow 2^n$)

      • Limited number of micro operations supported

    • Additional steps required

      • External memory word decode is needed
      • Identify the exact control line being manipulated
      • Send control signal

Micro-program Word Length

• Word length is an important factor in microprogram

• Word length is determined by three factors\

  • Maximum number of simultaneous micro-operations supported

  • The way control information is represented or encoded

  • The way in which the next micro-instruction address is specified

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Compromise

• Divide control signals into disjoint groups

• Implement each group as separate field in memory word

• Supports reasonable levels of parallelism without too much complexity

The control memory

• The control words are put into a special memory block called control memory, with each word having a unique address

• The microinstructions are organized as different routines

  • The microinstructions in each routine are executed sequentially

  • Each routine ends with a branch instruction points to the next routine

• 存储控制器中存储了取指例程、间接例程、中断例程、执行例程等，这些例程规定了在每个周期内需要执行的微操作序列

• 例程中还规定了在执行结束后转移到下面哪一个例程，也就是指定了周期的执行顺序

• 例如，取指周期后，可能会到间接周期，也可能到执行周期，需要根据取指周期来确定

• Sequencing logic

• Control address register

• Control memory

• Control buffer register

• 控制存储器：存储微指令

• 控制地址寄存器：包含下一个将要被读取的微指令地址。类似于PC

• 控制缓冲寄存器：微指令由控制存储器读取之后，放到控制缓冲寄存器中。控制缓冲寄存器的左半部分直接和控制线连接，读取微指令直接生成控制信号，控制门的开或关

• 定序逻辑：负责为控制地址寄存器提供地址，并发出读命令

Control Unit Function

• Sequence logic unit issues read command

  • Word specified in control address register is read into control buffer register

  • Control buffer register contents generates control signals and next address information

• Sequence logic loads new address into control address register based on next address information from control buffer register and ALU flags

Next Address Decision

• Sequencing logic needs to determine the next microinstruction

  • At the end of each microinstruction, the next microinstruction address is loaded into the control address register

• Depending on ALU flags and control buffer register

  • Next microinstruction in sequence

  • Jump to a new routine based on jump instructions

  • Jump to a machine instruction routine

Next Address

• Next microinstruction in sequence

  • Add 1 to control address register

• Jump to new routine based on jump microinstruction

  • Load address field of control buffer register into control address register

• Jump to machine instruction routine

  • Load control address register based on opcode in IR

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1. 指令译码器是将指令寄存器中的操作码翻译成控制寄存器地址

2. 控制地址寄存器到控制存储器中找到控制字，并放到控制缓冲寄存器中

3. 控制缓冲寄存器进行控制操作。如果是垂直微指令，则需要译码器进行译码后生成控制信号

4. 控制信号译码器用于垂直微指令，将控制字翻译成控制线的控制信号，发送到CPU内部和系统总线

5. 对于垂直微指令，有一个附加逻辑和时间延迟

6. 下一个微指令地址给定序逻辑，以确定执行哪一个微指令

Simple summary

• All the control unit does is generate a set of control signals

  • Control the opening and closing of each gate

  • Use one control bit to represent the control signal of a door

  • Control bits of all gates constitute control words

• Have a control word for each group of micro-operations in one time

• The next microinstruction can be specified in the microinstruction and executed conditionally

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• For machine code instruction

  • It is generally divided into multiple cycles, such as fetch cycle, execution cycle, etc

  • Multiple microoperations need to be completed

  • Each microoperation corresponds to one control word

  • A group of control words realizes the control function of a machine code

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• Today’s large microprocessor

  • Many instructions and associated register-level hardware

  • Many control points to be manipulated

• This results in control memory that

  • Contains a large number of words，co-responding to the number of instructions to be executed

  • Has a wide word width，due to the large number of control points to be manipulated

Advantages and Disadvantages

• Advantages

  • Simplifies design of control unit

  • Cheaper

  • Less error-prone

• Disadvantages

  • Additional processing required

  • Slower

Microinstruction Sequencing

• Microinstruction controller has two basic tasks

  • Microinstruction sequencing： Get the next microinstruction from the control memory

  • Microinstruction execution : Through control word generates control signal for executing the micro instruction

• Must consider both together

  • Affects instruction format and controller timing

Sequencing Techniques

• Two problems must be considered

  • Size of microinstructions

  • Address generation time

• Size of microinstructions

  • The larger the microinstruction, the larger the control memory, and the more expensive
  • Size of microinstructions should be reduced

• Address generation time

  • The requirement to execute microinstructions as quickly as possible

Address generation time

• Determined by instruction register

  • Once per cycle, after instruction is fetched

• Next sequential address

  • Use in most cases

• Branch

  • Both conditional and unconditional

  • Generally, there is a jump after 3-4 microinstructions

  • Very important

Sequencing Techniques

• Based on current microinstruction, condition flags, contents of IR, generate control memory address of next microinstruction

• The jump of microinstructions can be divided into three modes according to the address information format in the microinstructions

  • Two address fields

    • 对于双地址字段模式，微指令中提供了2个地址字段

    • 控制缓冲寄存器读取微指令后，第一部分产生控制信号，第二部分和第三部分是2个地址，这2个地址，加上指令寄存器的内容，连接到一个多路器中。多路器选择一个送到控制地址寄存器中

    • 控制地址寄存器通过译码，产生下一个微指令的地址。地址选择的输入是由转移逻辑模块来完成的，它根据微指令中的控制部分，以及标志位，向多路器提供选择信号

    • 双地址字段方式比较简单，微指令中有2个地址，需要更多的位

  • Single address field

    • 单地址字段方式中，微指令中只有一个地址。下一个地址的选择方法是：

      • 地址字段中指定的地址

      • 指令寄存器中的代码

      • 下一个顺序地址

    • 下一个顺序地址是用上一次的控制地址寄存器中的地址+1来得到的

  • Variable format

    • 可变格式提供两种不同格式的指令。用1位来标识当前是哪种格式。

    • 在一种格式中，所有位都用于产生控制信号。所以，下一个微指令的地址，或者是下一顺序地址，或者是由指令寄存器来指定

    • 另一种格式中，有一些位用于启动转移逻辑模块，其余的位来提供地址。这样就可以进行条件或者无条件转移

    • 这种方法的缺点是转移微指令需要花费一个时钟周期

Microinstruction Execution

• Microinstruction execution is a periodic basic even

• Each cycle is made up of two events

  • Fetch

  • Execute

• Fetch

  • Get the microinstruction from the control memory according to the generated microinstruction address

• Execute

  • Execute microinstructions and generate control signals

Execution effect

• Effect is to generate control signals

• Internal

  • Control signals inside the processor

  • Control the internal components of the processor

• External

  • Control signals to external control bus or other interface

  • Control disk read-write, I/O read-write etc

• With the function to generate next address

Summary

• A microprogrammed control unit is a relatively simple logic circuit that is capable of

  • Sequencing through microinstructions

  • Generating control signals to execute each microinstruction