William Stallings Computer Organization and Architecture

1. William Stallings Computer Organization and Architecture Chapter 15 Micro-programmed Control 微程序控制

2. 15.1 Basic Concepts • 1951 Wilkes • 1964.4 IBM 360 • Use sequences of instructions to control complex operations Called micro-programming or firmware 微程序 固件

3. 1. Microinstructions • Microprogramming 微程序 • Microinstructions微指令

4. Implementation (1) • All the control unit does is generate a set of control signals • Each control signal is on or off • Represent each control signal by a bit • Have a control word for each micro-operation • Have a sequence of control words for each machine code instruction • Add an address to specify the next micro-instruction, depending on conditions

5. Implementation (2) • Today’s large microprocessor • Many instructions and associated register-level hardware • Many control points to be manipulated • This results in control memory控制存储器that • Contains a large number of words （p581） coresponding to the number of instructions to be executed • Has a wide word width Due to the large number of control points to be manipulated

6. Micro-instruction Word Length • Based on 3 factors • Maximum number of simultaneous micro-operations supported 同时执行的微操作数 • The way control information is represented or encoded微指令表示形式 • The way in which the next micro-instruction address is specified下址表示形式

7. Micro-instruction Types微指令类型 • Each micro-instruction specifies single (or few) micro-operations to be performed (vertical micro-instruction) 垂直 • Each micro-instruction specifies many different micro-operations to be performed in parallel (horizontal micro- instruction) 水平

8. Vertical Micro-instruction垂直微指令 • Width is narrow • n control signals encoded into log2 n bits • Limited ability to express parallelism • Considerable encoding of control information requires external memory word decoder to identify the exact control line being manipulated

9. Vertical Micro- instruction diag Micro-instruction Address Function Codes Jump Condition

10. Horizontal Micro- instruction水平微指令 • Wide memory word • High degree of parallel operations possible • Little encoding of control information

11. Horizontal Micro- instruction diag Internal CPU Control Signals Micro-instruction Address System Bus Control Signals Jump Condition

12. Compromise折衷 / 混合型 • Divide control signals into disjoint groups • Implement each group as separate field in memory word • Supports reasonable levels of parallelism without too much complexity Micro-instruction Address

13. Control Memory 控制存储器 . Jump to Indirect or Execute Fetch cycle routine . Jump to Execute Indirect Cycle routine . Jump to Fetch Interrupt cycle routine Execute cycle begin Jump to Op code routine . Jump to Fetch or Interrupt AND routine . Jump to Fetch or Interrupt ADD routine

14. 2. Control Unit 控制单元 • Sequence login 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 login loads new address into control buffer register based on next address information from control buffer register and ALU flags

15. Control Unit Function控制单元功能 • Sequence login 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 login loads new address into control buffer register based on next address information from control buffer register and ALU flags

16. Advantages and Disadvantages微程序控制优缺点 • Simplifies design of control unit • Cheaper便宜 • Less error-prone 不易出错 • Slower 速度慢

17. Tasks Done By Microprogrammed Control Unit微程序控制单元任务 • Microinstruction sequencing 微指令排序 • Microinstruction execution 微指令执行 • Must consider both together

18. Design Considerations • Size of microinstructions微指令大小 • Address generation time 地址生成时间 • Determined by instruction register 指令寄存器定 Once per cycle, after instruction is fetched • Next sequential address下一顺序地址 Common in most designed • Branches 转移 Both conditional and unconditional

19. 15.2 Sequencing Techniques排序技术 • Based on current microinstruction, condition flags, contents of IR, control memory address must be generated • Based on format of address information • Two address fields双地址字段/场 P589 • Single address field 单地址字段 P590 • Variable format可变格式 （略）

20. Address Generation ExplicitImplicit • Two-field Mapping • Unconditional Branch Addition • Conditional branch Residual control

21. 15.3 Execution微指令执行 • The cycle is the basic event • Each cycle is made up of two events • Fetch 取指 Determined by generation of microinstruction address • Execute执行

22. Execute • Effect is to generate control signals • Some control points internal to processor Rest go to external control bus or other interface

23. Control Unit Organization

24. Required Reading • Stallings chapter 15

25. Homework • P617 15.7