1. Computer Organization and Architecture
CPU Structure and Function

2. CPU Structure CPU must:
Fetch instructions: The CPU reads an instruction from memory.
Interpret instructions: The instruction is decoded to determine what action is required.
Fetch data: The execution of an instruction may require reading data from memory or an I/O module.
Process data: The execution of an instruction may require performing some arithmetic or logical operation on data.
Write data: The results of an execution may require writing data to memory or an I/O module. 22

3. CPU With Systems Bus

4. CPU Internal Structure

5. 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
The registers in the CPU perform two roles:
User-visible registers
Control and status registers 23

6. User Visible Registers
A user-visible register is one that may be referenced by means of the machine language that the CPU executes. Can be characterized into the following categories:
General Purpose
Data
Address
Condition Codes (flags) 24

7. General Purpose Registers (1)
General purpose registers can be assigned to a variety of functions by the programmer.
May be true general purpose
May be restricted
May be used for data or addressing
Data
Accumulator
Addressing
Segment 25

8. General Purpose Registers (2)
Make them general purpose
Increase flexibility and programmer options
Increase instruction size & complexity
Make them specialized
Smaller (faster) instructions
Less flexibility 26

9. Fewer = more memory references
How Many GP Registers?
Between
Fewer = more memory references
More does not reduce memory references and takes up processor real estate
There is, however, a new approach which finds advantage of the use of hundreds of registers exhibited in some RISC systems. 27

10. Address registers should be large enough to hold full address
How big?
Address registers should be large enough to hold full address
Data registers should be large enough to hold full word
Often possible to combine two data registers
C programming
double int a;
long int a; 28

11. 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 29

12. Control & Status Registers
There are a number of CPU registers employed to control its operation.
Are not visible to the user on most machines. Some may be visible to to machine instructions executed in a control or operating system mode.
Four registers are essential to instruction execution:
Program Counter
Instruction Decoding Register
Memory Address Register
Memory Buffer Register 30

13. Control & Status Registers
Program Counter (PC): Contains the address of an instruction to be fetched.
Instruction Decoding Register or Instruction Register (IR): Contains the instruction most recently fetched.
Memory Address Register (MAR): Contains the address of a location in memory.
Memory Buffer Register (MBR): Contains a word of data to be written to memory or the word most recently read.
In a bus-organized system, the MAR connects directly to the address bus, and the MBR connects directly to the data bus.

14. Program Status Word
All CPU designs include a register or set of registers, often known as the program status word (PSW), that contain status information.
The PSW typically contains condition codes plus other status information.
Common fields or flags include the following:
Sign of last result
Zero
Carry
Equal
Overflow
Interrupt enable/disable
Supervisor (Indicates if CPU is executing in supervisor or user mode. Certain privileged instructions can be executed in only supervisor mode, and certain areas memory can be accessed only in supervisor mode.) 31

15. Example Register Organizations

16. Instruction Cycle
Indirect Cycle
May require memory access to fetch operands
Indirect addressing requires more memory accesses
Can be thought of as additional instruction subcycle

17. Instruction Cycle with Indirect

18. Instruction Cycle State Diagram

19. Data Flow (Instruction Fetch)
Depends on CPU design
In general:
Fetch
PC contains address of next instruction
Address moved to MAR
Address placed on address bus
Control unit requests memory read
Result placed on data bus, copied to MBR, then to IR
Meanwhile PC incremented by 1

20. IR is examined by the control unit
Data Flow (Data Fetch)
IR is examined by the control unit
If indirect addressing, indirect cycle is performed
Right most N bits of MBR, which contain the address reference, transferred to MAR
Control unit requests memory read
Result (address of operand) moved to MBR

21. Data Flow (Fetch Diagram)

22. Data Flow (Indirect Diagram)

23. Depends on instruction being executed May include
Data Flow (Execute)
May take many forms
Depends on instruction being executed
May include
Memory read/write
Input/Output
Register transfers
ALU operations

24. Data Flow (Interrupt)
Simple
Predictable
Current PC saved to allow resumption after interrupt
Contents of PC copied to MBR
Special memory location (e.g. stack pointer) loaded to MAR
MBR written to memory
PC loaded with address of interrupt handling routine
Next instruction (first of interrupt handler) can be fetched

25. Data Flow (Interrupt Diagram)

26. Fetch accessing main memory
Prefetch
Fetch accessing main memory
Execution usually does not access main memory
Can fetch next instruction during execution of current instruction
Called instruction prefetch 36

27. Add more stages to improve performance
Improved Performance
But not doubled:
Fetch usually shorter than execution
Prefetch more than one instruction?
Any jump or branch means that prefetched instructions are not the required instructions
Add more stages to improve performance 37

28. Calculate operands (i.e. EAs) Fetch operands Execute instructions
Pipelining
Fetch instruction
Decode instruction
Calculate operands (i.e. EAs)
Fetch operands
Execute instructions
Write result
Overlap these operations 38

29. Two Stage Instruction Pipeline

30. Timing Diagram for Instruction Pipeline Operation39

31. The Effect of a Conditional Branch on Instruction Pipeline Operation40

32. The logic needed for Six Stage Instruction Pipeline

33. Alternative Pipeline Depiction

34. Prefetch Branch Target Loop buffer Branch prediction Delayed branching
Dealing with Branches
A variety of approaches have been taken for dealing with conditional branches:
Multiple Streams
Prefetch Branch Target
Loop buffer
Branch prediction
Delayed branching 41

35. Prefetch each branch into a separate pipeline Use appropriate pipeline
Multiple Streams
Have two pipelines
Prefetch each branch into a separate pipeline
Use appropriate pipeline
Leads to bus & register contention
Multiple branches lead to further pipelines being needed 42

36. Prefetch Branch Target
Target of branch is prefetched in addition to instructions following branch
Keep target until branch is executed
Used by IBM 360/91 43

37. Maintained by fetch stage of pipeline
Loop Buffer
Very fast memory
Maintained by fetch stage of pipeline
Check buffer before fetching from memory
Very good for small loops or jumps
c.f. cache
Used by CRAY-1 44

38. Loop Buffer Diagram

39. Branch Prediction (1) Predict never taken Predict always taken
Assume that jump will not happen
Always fetch next instruction
68020 & VAX 11/780
VAX will not prefetch after branch if a page fault would result (O/S v CPU design)
Predict always taken
Assume that jump will happen
Always fetch target instruction 45

40. Taken/Not taken switch
Branch Prediction (2)
Predict by Opcode
Some instructions are more likely to result in a jump than others
Can get up to 75% success
Taken/Not taken switch
Based on previous history
Good for loops 46

41. Branch Prediction (3) Delayed Branch
Do not take jump until you have to
Rearrange instructions 47