Basic Processing Unit UNIT5 Introduction l l The

Basic Processing Unit UNIT-5

Introduction l l The heart of any computer is the central processing unit (CPU). The CPU executes all the machine instructions and coordinates the activities of all other units during the execution of an instruction. This unit is also called as the Instruction Set Processor (ISP). By looking at its internal structure, we can understand how it performs the tasks of fetching, decoding, and executing instructions of a program.

Introduction l l l The processor is generally called as the central processing unit (CPU) or micro processing unit (MPU). An high-performance processor can be built by making various functional units operate in parallel. High-performance processors have a pipelined organization where the execution of one instruction is started before the execution of the preceding instruction is completed.

Introduction l l In another approach, known as superscalar operation, several instructions are fetched and executed at the same time. Pipelining and superscalar architectures provide a very high performance for any processor.

l l A typical computing task consists of a series of steps specified by a sequence of machine instructions that constitute a program. A program is a set of instructions performing a meaningful task. An instruction is command to the processor & is executed by carrying out a sequence of suboperations called as micro-operations. various blocks of a typical processing unit are PC, IR, ID, MAR, MDR, a set of register arrays for temporary storage, Timing and Control unit.

Some Fundamental Concepts

Fundamental Concepts l l l 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) or Instruction Pointer (IP) After fetching an instruction, the contents of the PC are updated to point to the next instruction in the sequence. But, when a branch instruction is to be executed, the PC will be loaded with a different (jump/branch address).

l l l Instruction register, IR is another key register in the processor, which is used to hold the opcodes before decoding. IR contents are then transferred to an instruction decoder (ID) for decoding. The decoder then informs the control unit about the task to be executed. The control unit along with the timing unit generates all necessary control signals needed for the instruction execution. Suppose that each instruction comprises 2 bytes, and that it is stored in one memory word.

l To execute an instruction, the processor has to perform the following three steps: 1. Fetch the contents of the memory location pointed to by the PC. The contents of this location are interpreted as an instruction code to be executed. Hence, they are loaded into the IR/ID. Symbolically, this operation can be written as IR ----- [(PC)]

2. Assuming that the memory is byte addressable, increment the contents of the PC by 2, that is, PC ------ [PC] + 2 3. Decode the instruction to understand the operation & generate the control signals necessary to carry out the operation. 4. Carry out the actions specified by the instruction in the IR.

l l l In cases where an instruction occupies more than one word, steps 1 and 2 must be repeated as many times as necessary to fetch the complete instruction. These two steps together are usually referred to as the fetch phase; step 3 constitutes the decoding phase; and step 4 constitutes the execution phase.

l l To study these operations in detail, let us examine the internal organization of the processor. The main building blocks of a processor are interconnected in a variety of ways. A very simple organization is shown in Figure

l l Figure shows an organization in which the arithmetic and logic unit (ALU) and all the registers are interconnected through a single common bus, which is internal to the processor. The data and address lines of the external memory bus are connected to the internal processor bus via l l l the memory data register, MDR, and the memory address register, MAR, respectively. Register MDR has two inputs and two outputs.

l l 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 block.

l l l This unit is responsible for issuing the signals that control the operation of all the units inside the processor and for interacting with the memory bus. The number and use of the processor registers RO through R(n - 1) vary considerably from one processor to another. Registers may be provided for general-purpose use by the programmer. Some may be dedicated as special-purpose registers, such as index registers or stack pointers.

l l Three registers, Y, Z, and TEMP are transparent to the programmer, they are used by the processor for temporary storage during execution of some instructions. The multiplexer 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. The two possible values of the MUX control input Select as Select 4 and Select Y for selecting the constant 4 or register Y, respectively.

l l As instruction execution progresses, data are transferred from one register to another, often passing through the ALU to perform some arithmetic or logic operation. The instruction decoder and control logic unit is responsible for implementing the actions specified by the instruction loaded in the IR register. The decoder generates the control signals needed to select the registers involved and direct the transfer of data. The registers, the ALU, and the interconnecting bus are collectively referred to as the data path.

l With few exceptions, an instruction can be executed by performing one or more of the following operations in some specified sequence: 1. Transfer a word of data from one processor register to another or to the ALU 2. Perform an arithmetic or a logic operation and store the result in a processor register 3. Fetch the contents of a given memory location and load them into a processor register 4. Store a word of data from a processor register into a given memory location

Timing diagram of memory read operation

Executing an Instruction l l l 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 MDR HAS TWO INPUTS AND TWO OUTPUTS Datapath Textbook Page 413

Executing an Instruction l l Transfer a word of data from one processor register to another or to the ALU. Perform an arithmetic or a logic operation and store the result in a processor register. Fetch the contents of a given memory location and load them into a processor register. Store a word of data from a processor register into a given memory location.

Register Transfers Internal processor bus Riin Ri Riout Yin Y Constant 4 Select MUX A B ALU Zin Z Zout Figure 7. 2. Input and output gating for the registers in Figure 7. 1.

Register Transfers l All operations and data transfers are controlled by the processor clock. Figure 7. 3. Input and output gating for one register bit.

Performing an Arithmetic or Logic Operation l l l 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 R 1 to those of R 2 and store the result in R 3? 1. 2. 3. R 1 out, Yin R 2 out, Select. Y, Add, Zin Zout, R 3 in

Fetching a Word from Memory l Address into MAR; issue Read operation; data into MDR. Figure 7. 4. Connection and control signals for register MDR.

Fetching a Word from Memory l l l Ø Ø Ø 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 (R 1), R 2 MAR ← [R 1] Start a Read operation on the memory bus Wait for the MFC response from the memory Load MDR from the memory bus R 2 ← [MDR]

Timing MAR ← [R 1] Assume MAR is always available on the address lines of the memory bus. Start a Read operation on the memory bus Wait for the MFC response from the memory Load MDR from the memory bus R 2 ← [MDR]

Execution of a Complete Instruction l l l Add (R 3), R 1 Fetch the instruction Fetch the first operand (the contents of the memory location pointed to by R 3) Perform the addition Load the result into R 1

Architecture Internal processor bus Riin Ri Riout Yin Y Constant 4 Select MUX A B ALU Zin Z Zout Figure 7. 2. Input and output gating for the registers in Figure 7. 1.

Execution of a Complete Instruction Add (R 3), R 1

Execution of Branch Instructions l l l 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

Execution of Branch Instructions Step Action 1 PCout , MAR in , Read, Select 4, Add, Z in 2 Zout , PCin , Yin , WMF C 3 MDR out , IR in 4 Offset-field-of-IRout, Add, Z in 5 Z out, PCin , End Figure 7. 7. Control sequence for an unconditional branch instruction.

Multiple-Bus Organization

Multiple-Bus Organization l Add R 4, R 5, R 6 Step Action 1 PCout, R=B, MAR in , Read, Inc. PC 2 WMFC 3 MDR out. B , R=B, IR in 4 R 4 out. A , R 5 out. B , Select. A, Add, R 6 in , End Figure 7. 9. Control sequence for the instruction. Add R 4, R 5, R 6, for the three-bus organization in Figure 7. 8.

Quiz l What is the control sequence for execution of the instruction Add R 1, R 2 including the instruction fetch phase? (Assume single bus architecture)

Hardwired Control

Overview l l l To execute instructions, the processor must have some means of generating the control signals needed in the proper sequence. Two categories: hardwired control and microprogrammed control Hardwired system can operate at high speed; but with little flexibility.

Control Unit Organization Clock CLK Control step counter External inputs IR Decoder/ encoder Condition codes Control signals Figure 7. 10. Control unit organization.

Detailed Block Description

Generating Zin l Zin = T 1 + T 6 • ADD + T 4 • BR + … Branch T 4 Add T 6 T 1 Figure 7. 12. Generation of the Zin control signal for the processor in Figure 7. 1.

Generating End l End = T 7 • ADD + T 5 • BR + (T 5 • N + T 4 • N) • BRN +…

A Complete Processor

Microprogrammed Control

Overview l l Control signals are generated by a program similar to machine language programs. Control Word (CW); microroutine; microinstruction

Overview

Overview l Control store One function cannot be carried out by this simple organization.

Overview l l 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. Address. Microinstruction 0 PCout , MAR in , Read, Select 4, Add, Z in 1 Zout , PCin , Y in , WMF C 2 MDRout , IR in 3 Branch to starting addressof appropriatemicroroutine. . . . 25 If N=0, then branch to microinstruction 0 26 Offset-field-of-IRout , Select. Y, Add, Z in 27 Zout , PCin , End Figure 7. 17. Microroutine for the instruction Branch<0.

Overview External inputs IR Clock Starting and branch address generator m. PC Control store Figure 7. 18. Condition codes CW Organization of the control unit to allow conditional branching in the microprogram.

Microinstructions l l 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.

Partial Format for the Microinstructions What is the price paid for this scheme?

Further Improvement l l l 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

Microprogram Sequencing l l Ø Ø l l If all microprograms require only straightforward sequential execution of microinstructions except for branches, letting a μPC governs the sequencing would be efficient. However, two disadvantages: Having a separate microroutine for each machine instruction results in a large total number of microinstructions and a large control store. 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).

- Bit-ORing - Wide-Branch Addressing - WMFC

Mode Contents of IR OP code 0 1 11 10 0 Rsrc 87 Address (octal) Microinstruction 000 4, Add, Zin PCout, MARin, Read, Select 001 Zout, PCin, Yin, WMFC 002 MDRout, IRin 003 m. Branch {m PC¬ m. PC 5, 4 ¬ Rdst 4 3 0 101 (from Instruction decoder); [IR 10, 9]; m. PC 3 ¬ [IR 10] × [IR 9] × [IR 8]} 121 Rsrcout , MARin , Read, Select 4, Add, in. Z 122 Zout, Rsrcin 123 m. Branch {m. PC¬ 170; m. PC 0 ¬ [IR 8]}, WMFC 170 MDRout, MARin, Read, WMFC 171 MDRout, Yin 172 Rdstout , Select. Y , Add, Zin 173 Zout, Rdstin, End Figure 7. 21. Microinstruction for Add (Rsrc)+, Rdst. Note: Microinstruction at location 170 is not executed for this addressing mode.

Microinstructions with Next. Address Field l l 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)

Microinstructions with Next. Address Field

Implementation of the Microroutine

bit-ORing

Further Discussions l l Prefetching Emulation