![Chapter 7
Basic processing Unit
Chapter Objectives
• How a processor executes instructions
• Internal functional units and how they are connected
• Hardware for generating internal control signals
• The micro programming approach
• Micro program organization
Fundamental Concepts
• Processor fetches one instruction at a time, and perform the operation specified.
• Instructions are fetched from successive memory locations until a branch or a jump
instruction is encountered.
• Processor keeps track of the address of the memory location containing the next
instruction to be fetched using Program Counter (PC).
• Instruction Register (IR)
Executing an Instruction
• Fetch the contents of the memory location pointed to by the PC. The contents of this
location are loaded into the IR (fetch phase).
IR ← [[PC]]
• Assuming that the memory is byte addressable, increment the contents of the PC by 4
(fetch phase).
PC ← [PC] + 4
• Carry out the actions specified by the instruction in the IR (execution phase).
Processor Organization
lines
Data
Address
lines
bus
Memory
Carry-in
ALU
PC
MAR
MDR
Y
Z
Add
XOR
Sub
bus
IR
TEMP
R0
control
ALU
lines
Control signals
R n 1-( )
Instruction
decoder and
Internal processor
control logic
A B
Figure 7.1. Single-bus organization of the datapath inside a processor.
MUXSelect
Constant 4](https://image.slidesharecdn.com/310471266-chapter-7-notes-computer-organization-160702183830/85/310471266-chapter-7-notes-computer-organization-1-320.jpg)
![• The response time of each memory access varies (cache miss, memory-mapped
I/O,…).
• To accommodate this, the processor waits until it receives an indication that the
requested operation has been completed (Memory-Function-Completed, MFC).
• Move (R1), R2
MAR ← [R1]
Start a Read operation on the memory bus
Wait for the MFC response from the memory
Load MDR from the memory bus
R2 ← [MDR]
• The output of MAR is enabled all the time.
• Thus the contents of MAR are always available on the address lines of the
memory bus.
• When a new address is loaded into MAR, it will appear on the memory bus at the
beginning of the next clock cycle.(in fig)
• A read control signal is activated at the same time MAR is loaded.
• This means memory read operations requires three steps, which can be described
by the signals being activated as follows
R1out,MARin,Read
MDRinE,WMFC
MDRout,R2in
Figure 7.5. Timing of a memory Read operation.
1 2
Clock
Address
MR
Data
MFC
Read
MDR inE
MDR out
Step 3
MAR in](https://image.slidesharecdn.com/310471266-chapter-7-notes-computer-organization-160702183830/85/310471266-chapter-7-notes-computer-organization-5-320.jpg)
310471266 chapter-7-notes-computer-organization
- 1. Chapter 7 Basic processing Unit Chapter Objectives • How a processor executes instructions • Internal functional units and how they are connected • Hardware for generating internal control signals • The micro programming approach • Micro program organization Fundamental Concepts • Processor fetches one instruction at a time, and perform the operation specified. • Instructions are fetched from successive memory locations until a branch or a jump instruction is encountered. • Processor keeps track of the address of the memory location containing the next instruction to be fetched using Program Counter (PC). • Instruction Register (IR) Executing an Instruction • Fetch the contents of the memory location pointed to by the PC. The contents of this location are loaded into the IR (fetch phase). IR ← [[PC]] • Assuming that the memory is byte addressable, increment the contents of the PC by 4 (fetch phase). PC ← [PC] + 4 • Carry out the actions specified by the instruction in the IR (execution phase). Processor Organization lines Data Address lines bus Memory Carry-in ALU PC MAR MDR Y Z Add XOR Sub bus IR TEMP R0 control ALU lines Control signals R n 1-( ) Instruction decoder and Internal processor control logic A B Figure 7.1. Single-bus organization of the datapath inside a processor. MUXSelect Constant 4
- 2. • ALU and all the registers are interconnected via a single common bus. • The data and address lines of the external memory bus connected to the internal processor bus via the memory data register, MDR, and the memory address register, MAR respectively. • Register MDR has two inputs and two outputs. • Data may be loaded into MDR either from the memory bus or from the internal processor bus. • The data stored in MDR may be placed on either bus. • The input of MAR is connected to the internal bus, and its output is connected to the external bus. • The control lines of the memory bus are connected to the instruction decoder and control logic. • This unit is responsible for issuing the signals that control the operation of all the units inside the processor and for increasing with the memory bus. • The MUX selects either the output of register Y or a constant value 4 to be provided as input A of the ALU. • The constant 4 is used to increment the contents of the program counter. Register Transfers
- 3. • Instruction execution involves a sequence of steps in which data are transferred from one register to another. • For each register two control signals are used to place the contents of that register on the bus or to load the data on the bus into register.(in figure) • The input and output of register Riin and Riout is set to 1, the data on the bus are loaded into Ri. • Similarly, when Ri out is set to 1, the contents of register Ri are placed on the bus. • While Riout is equal to 0, the bus can be used for transferring data from other registers. Example • Suppose we wish to transfer the contents of register R1 to register R4. This can be accomplished as follows. • Enable the output of registers R1 by setting R1out to 1. This places the contents of R1 on the processor bus. • Enable the input of register R4 by setting R4out to 1. This loads data from the processor bus into register R4. • All operations and data transfers with in the processor take place with in time periods defined by the processor clock. • The control signals that govern a particular transfer are asserted at the start of the clock cycle. Figure 7.3. Input and output g ating for one register bit. D Q Q Clock 1 0 Riout Riin Bus Performing an Arithmetic or Logic Operation • The ALU is a combinational circuit that has no internal storage. • ALU gets the two operands from MUX and bus. The result is temporarily stored in register Z. • What is the sequence of operations to add the contents of register R1 to those of R2 and store the result in R3? o R1out, Yin
- 4. o R2out, SelectY, Add, Zin o Zout, R3in • All other signals are inactive. • In step 1, the output of register R1 and the input of register Y are enabled, causing the contents of R1 to be transferred over the bus to Y. • Step 2, the multiplexer’s select signal is set to Select Y, causing the multiplexer to gate the contents of register Y to input A of the ALU. • At the same time, the contents of register R2 are gated onto the bus and, hence, to input B. • The function performed by the ALU depends on the signals applied to its control lines. • In this case, the ADD line is set to 1, causing the output of the ALU to be the sum of the two numbers at inputs A and B. • This sum is loaded into register Z because its input control signal is activated. • In step 3, the contents of register Z are transferred to the destination register R3. This last transfer cannot be carried out during step 2, because only one register output can be connected to the bus during any clock cycle. Fetching a Word from Memory • The processor has to specify the address of the memory location where this information is stored and request a Read operation. • This applies whether the information to be fetched represents an instruction in a program or an operand specified by an instruction. • The processor transfers the required address to the MAR, whose output is connected to the address lines of the memory bus. MDR Memory-bus Figure 7.4. Connection and control signals for register MDR. data lines Internal processor busMDRoutMDRoutE MDRinMDRinE • At the same time , the processor uses the control lines of the memory bus to indicate that a Read operation is needed. • When the requested data are received from the memory they are stored in register MDR, from where they can be transferred to other registers in the processor.
- 5. • The response time of each memory access varies (cache miss, memory-mapped I/O,…). • To accommodate this, the processor waits until it receives an indication that the requested operation has been completed (Memory-Function-Completed, MFC). • Move (R1), R2 MAR ← [R1] Start a Read operation on the memory bus Wait for the MFC response from the memory Load MDR from the memory bus R2 ← [MDR] • The output of MAR is enabled all the time. • Thus the contents of MAR are always available on the address lines of the memory bus. • When a new address is loaded into MAR, it will appear on the memory bus at the beginning of the next clock cycle.(in fig) • A read control signal is activated at the same time MAR is loaded. • This means memory read operations requires three steps, which can be described by the signals being activated as follows R1out,MARin,Read MDRinE,WMFC MDRout,R2in Figure 7.5. Timing of a memory Read operation. 1 2 Clock Address MR Data MFC Read MDR inE MDR out Step 3 MAR in
- 6. Storing a word in Memory Writing a word into a memory location follows a similar procedure. The desired address is loaded into MAR. Then , the data to be written are loaded into MDR, and a write command is issued. Example Executing the instruction Move R2,(R1) requires the following steps 1 R1out,MARin 2.R2out,MDRin,Write 3.MDRoutE,WMFC Execution of a Complete Instruction Add (R3), R1 Fetch the instruction Fetch the first operand (the contents of the memory location pointed to by R3) Perform the addition Load the result into R1 Step Action 1 PCout , MARin , Read,Select4,Add, Zin 2 Zout , PCin , Yin , WMFC 3 MDRout , IRin 4 R3out , MARin , Read 5 R1out , Yin , WMFC 6 MDRout , SelectY,Add, Zin 7 Zout , R1in , End Figure7.6. Control sequencefor executionof the instruction Add (R3),R1.
- 7. lines Data Address lines bus Memory Carry-in ALU PC MAR MDR Y Z Add XOR Sub bus IR TEMP R0 control ALU lines Control signals R n 1-( ) Instruction decoder and Internal processor control logic A B Figure 7.1. Single-bus organization of the datapath inside a processor. MUXSelect Constant 4 Execution of Branch Instructions • A branch instruction replaces the contents of PC with the branch target address, which is usually obtained by adding an offset X given in the branch instruction. • The offset X is usually the difference between the branch target address and the address immediately following the branch instruction. • Conditional branch
- 8. Figure 7.7. Control sequence for an unconditional branch instruction. Multiple-Bus Organization Memory b us data lines Figure 7.8. Three-b us or g anization of the datapath. Bus A Bus B Bus C Instruction decoder PC Re gister file Constant 4 ALU MDR A B R MUX Incrementer Address lines MAR IR
- 9. Example : Add R4, R5, R6 Hardwired Control • To execute instructions, the processor must have some means of generating the control signals needed in the proper sequence. • Two categories: hardwired control and micro programmed control • Hardwired system can operate at high speed; but with little flexibility. Control Unit Organization n
- 10. Detailed Control design External inputs Figure 7.11. Separation of the decoding and encoding functions. Encoder Reset CLK Clock Control signals counter Run End Condition codes decoder Instruction Step decoder Control step IR T1 T2 Tn INS1 INS2 INSm Generating Zin • Zin = T1 + T6 • ADD + T4 • BR + …
- 11. Generating End • End = T7 • ADD + T5 • BR + (T5 • N + T4 • N) • BRN +… Figure 7.13. Generation of the End control signal. T7 Add Branch Branch<0 T5 End NN T4T5 A Complete Processor Instruction unit Inte ger unit Floating-point unit Instruction cache Data cache Bus interf ace Main memory Input/ Output System b us Pr ocessor Figure 7.14. Block diagram of a complete processor .
- 12. Microprogrammed Control • Control signals are generated by a program similar to machine language programs. • Control Word (CW); microroutine; microinstruction PCin PCout MARin Read MDRout IRin Yin Select Add Zin Zout R1out R1in R3out WMFC End 0 1 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 0 0 1 0 0 0 1 0 0 1 0 0 0 0 0 1 0 0 1 0 0 0 1 0 0 0 0 0 1 0 0 1 0 0 1 0 0 0 0 0 0 1 0 0 0 0 1 0 1 0 0 0 0 1 0 0 1 0 0 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 0 0 1 0 0 1 0 0 Micro - instruction 1 2 3 4 5 6 7 Figure 7.15An example of microinstructions for Figure 7.6. Step Action 1 PC out , MAR in , Read, Select4, Add, Z in 2 Z out , PC in , Y in , WMF C 3 MDR out , IR in 4 R3 out , MAR in , Read 5 R1 out , Y in , WMF C 6 MDR out , SelectY, Add, Z in 7 Z out , R1 in , End Figure 7.6. Con trol sequence for execution of the instruction Add (R3),R1.
- 13. Figure 7.16. Basic organization of a microprogrammed control unit. store Control generator Starting address CW Clock µPC IR • The previous organization cannot handle the situation when the control unit is required to check the status of the condition codes or external inputs to choose between alternative courses of action. • Use conditional branch microinstruction.
- 14. Microinstructions • A straightforward way to structure microinstructions is to assign one bit position to each control signal. • However, this is very inefficient. • The length can be reduced: most signals are not needed simultaneously, and many signals are mutually exclusive. • All mutually exclusive signals are placed in the same group in binary coding. F2 (3 bits) 000: No transfer 001: PC in 010: IR in 011: Z in 100: R0 in 101: R1 in 110: R2 in 111: R3 in F1 F2 F3 F4 F5 F1 (4 bits) F3 (3 bits) F4 (4 bits) F5 (2 bits) 0000: No transfer 0001: PC out 0010: MDR out 0011: Z out 0100: R0 out 0101: R1 out 0110: R2 out 0111: R3 out 1010: TEMP out 1011: Offset out 000: No transfer 001: MAR in 010: MDR in 011: TEMP in 100: Y in 0000: Add 0001: Sub 1111: XOR 16 ALU functions 00: No action 01: Read 10: Write F6 F7 F8 F6 (1 bit) F7 (1 bit) F8 (1 bit) 0: SelectY 1: Select4 0: No action 1: WMFC 0: Continue 1: End Figure 7.19. An example of a partial format for field-encoded microinstructions. Microinstruction
- 15. Further Improvement • Enumerate the patterns of required signals in all possible microinstructions. Each meaningful combination of active control signals can then be assigned a distinct code. • Vertical organization • Horizontal organization Micro program Sequencing • If all micro programs require only straightforward sequential execution of microinstructions except for branches, letting a µPC governs the sequencing would be efficient. • However, two disadvantages: o Having a separate micro routine for each machine instruction results in a large total number of microinstructions and a large control store. o Longer execution time because it takes more time to carry out the required branches. • Example: Add src, Rdst • Four addressing modes: register, autoincrement, autodecrement, and indexed (with indirect forms).
- 17. Microinstructions with Next-Address Field Figure 7.22. Microinstruction-sequencing organization. Condition codes IR Decoding circuits Control store Next address Microinstruction decoder Control signals Inputs External µ A R µ I R
- 18. • The microprogram we discussed requires several branch microinstructions, which perform no useful operation in the datapath. • A powerful alternative approach is to include an address field as a part of every microinstruction to indicate the location of the next microinstruction to be fetched. • Pros: separate branch microinstructions are virtually eliminated; few limitations in assigning addresses to microinstructions. • Cons: additional bits for the address field (around 1/6) F1 (3 bits) 000: No transfer 001: PCout 010: MDRout 011: Zout 100: Rsrcout 101: Rdstout 110: TEMPout F0 F1 F2 F3 F0 (8 bits) F2 (3 bits) F3 (3 bits) 000: No transfer 001: PCin 010: IRin 011: Zin 100: Rsrcin 000: No transfer 001: MARin F4 F5 F6 F7 F5 (2 bits)F4 (4 bits) F6 (1 bit) 0000: Add 0001: Sub 0: SelectY 1: Select4 00: No action 01: Read Microinstruction Address of next microinstruction 101: Rdstin 010: MDRin 011: TEMPin 100: Yin 1111: XOR 10: Write F8 F9 F10 F8 (1 bit) F7 (1 bit) F9 (1 bit) F10 (1 bit) 0: No action 1: WMFC 0: No action 1: ORindsrc 0: No action 1: ORmode 0: NextAdrs 1: InstDec Figure 7.23. Format for microinstructions in the example of Section 7.5.3.
- 19. Implementation of the Microroutine (See Figure 7.23 for encoded signals.) Figure 7.24. Implementation of the microroutine of Figure 7.21 using a 1 0 1 11110 0111110 001 001 1 21 0 00 0 00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 00 0 0 0101 110 37 7 00000000 0 1111 110 0 0 0 17 07 F9 0 0 0 0 0 0 F10 0 0 0 0 0 0 00 0 0 0 0 0 0 F8F7F6F5F4 000 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 1 0 0 0 10000 0000 1100000 10 0 0 0 0 0 0 1 0 0 0 0 0 0 0 00 01 000 000 001 110 100 10 F2 1 110 0 0 0 0 0 1 1 221 0 11110 111 00 1 1 2 0 21 0 00 address Octal 111 00000 1 0000000 10000000 F0 F1 0 0 0 10 0 010 010 0 11 001 110 100 0 0 0 1 1 0 1 F3 next-microinstruction address field. 011000 0 0 0 0 00 00 00000 0 0 0 0 030 0 00 0 0 decoder Microinstruction Control store Next address F1 F2 Other control signals F10F9F8 Decoder Decoder circuits Decoding Condition External codes inputs Rsrc RdstIR Rdst out Rdst in Rsrc out Rsrc in µ A R InstDec out OR mode OR indsrc R15 in R15 out R0 in R0 out Figure 7.25. Some details of the control-signal-generating circuitry.
- 20. Further Discussions • Prefetching • Emulation