THE MICROPROCESSOR Von Neumann's Architecture Model Input/Output unit Provides instructions and data Memory unit Stores both instructions and data Arithmetic and logic unit Processes everything Control unit Controls execution of instructions Central processing unit = ALU + CU Stored program 7! Program can be manipulated as if it is data 1 System Bus Architecture Model System bus Data bus carries transmitted data Address bus identi¯es location of transmitted data Control bus speci¯es how data is transmitted Power bus supplies power to units I/O bus identi¯es i/o devices 2 Central Processing Unit MAR M A MDR I N OpCode OpAddr A DS M E RW M PC O R Y Control HZN ALU 3 Instruction Cycle Program counter (PC) contains address of the in- struction being executed Instruction register (IR) contains the instruction being interpreted Fetch-execute cycle: The steps that the control unit carries out in executing a program are: 1. Fetch the next instruction to be executed from memory 2. Decode the opcode 3. Read operand(s) from main memory, if any 4. Execute the instruction and store results 5. Go to step 1 Opcode: machine language code (syntax) Decoding Determine the type, operation and ope- rand(s) of an instruction 4 Instruction Types Formats: Instructions are represented as sequence of ¯elds. 'Code' is usually 1 byte long and length of the others depends on 'Code' Format 1: Code (ex: HLT) Format 2: Code Address (ex: JMP $0123) Format 3: Code Data (ex: ADD R, $01) Format 4: Code Address1 Address2 (ex: MOV $0123, $0200) Register Transfer Language (RTL) Instructions are sequences of microinstructions that execute within one clock pulse. Types: ² Arithmetic: SUB R1, R2 ² Logic: XOR R, $1010 ² Transfer: MOV R1, R2 ² Branching: JNE $3210 ² Control: CLA 5 Fetch-Execute Cycle (Example) Step 0: Actual state of CPU MAR M A Program MDR I N ... $002E: ... OpCode OpAddr $05 $0030: ADD DS M $0032: $0036 E $0034: HLT RW M $0036: $0001 $0030 O $0038: ... R ... Y Control HZN ALU PC (Program Counter) = $0030, A = $0005 Step 1: Fetch the instruction MAR=$0030, PC = $0030 $0030 M A Program $0099 I N ... $002E: ... ADDOpAddr $0005 $0030: ADD DS M $0032: $0036 E $0034: HLT RW M $0036: $0001 $0032 O $0038: ... R ... Y Control HZN ALU MDR = $0099 (ADD), OpCode = $0099, PC = $0032 6 Fetch-Execute Cycle (Example) Step 2: Decode the OpCode $0099 = ADD, then needs one operand Step 3a: Fetch the operand MAR=$0032, PC = $0032 $0032 M A Program $0036 I N ... $002E: ... ADD$0036 $0005 $0030: ADD DS M $0032: $0036 E $0034: HLT RW M $0036: $0001 $0034 O $0038: ... R ... Y Control HZN ALU MDR = $0036, OpAddr = $0036, PC = $0034 7 Fetch-Execute Cycle (Example) Step 3b: Fetch operand value MAR=$0036 $0036 M A Program $0001 I N ... $002E: ... ADD$0036 $0005 $0030: ADD DS M $0032: $0036 E $0034: HLT RW M $0036: $0001 $0034 O $0038: ... R ... Y Control HZN ALU MDR = $0001 Step 4: Execute and store A = $0005 $0036 M A Program $0001 I N ... $002E: ... ADD $0036 $0006 $0030: ADD DS M $0032: $0036 E $0034: HLT RW M $0036: $0001 $0034 O $0038: ... R ... Y Control HZN ALU MDR = $0001, A = A + MDR = $0006 8 Control Unit and Status Register Control Unit controls the instruction cycle. De- codes the instructions: interpretes the opera- tions (OpCode) to perform according to the in- structions formats Status Register: ALU is connected to CU through the status register. Each bit (called status bit) of the status register conveys information about the last performed operation. Example C H N Z ... V C: Carry bit (1 if carry generated, 0 otherwise) H: Halt bit (1 if processor halted, 0 otherwise) N: Sign bit (1 if result negative, 0 otherwise) Z: Zero bit (1 if result is zero, 0 otherwise) ... (1 if . true, 0 otherwise) V: Overflow bit (1 if overflow, 0 otherwise) etc. 9 Implementations of Control Unit A control unit can be implemented in two ways: by Hardwired control: A synchronous sequential circuit that realizes all required control ac- tions of the CPU. That is: all functions of the CU are implemented in hardware. With this method, the CPU is very fast but com- plex, very expensive and di±cult to modify. Microprogrammed control: Binary control val- ues of the CU are stored in a special memory called control memory (CM). CM is usually a PLD (ROM, PLA or PAL). Each word of CM is a microinstruction and the set of mi- croinstructions is called microprogram. The microprogram implements operations on the registers and other parts of the CPU. It may contains program steps that collectively im- plement a single (macro)instruction. That is: all functions of the CU is de¯ned in a micro- program. With this method, the CPU is sim- ple, cheap and easy to modify but slow. Most current processors use microprogrammed con- trol. 10 Microprogramming Microprogram = sequence of microinstructions de- ¯ned to execute an instruction written in ma- chine language Example of microprogram: An addition instruc- tion (ADD) received by the CPU is reduced to a sequence of 3 microinstructions and 4 microin- structions, respectively during the fetch cycle and the execute cycle. Fetch cycle 1. Fetch the instruction (load in CPU) 2. Decode the instruction (interpret OpCode) 3. Increment Program Counter (PC++) Execute cycle 1. Fetch the ¯rst operand of addition 2. Fetch the second operand of addition 3. Add the operands (in accumulator register) 4. Store the result in memory 11 Register Transfer Language (RTL) Microoperations: A microinstruction is composed of microoperations: elementary operations per- formed on data stored in registers or memory. For instance, microinstruction 3 of execute cycle of instruction ADD is composed of 2 microop- erations: 1. Addition in A: A à A+ MDR 2. Update status bit N: N à An¡1 RTL is used to describe microoperations. Each microoperation involves transfering data from a source register S to a destination register R. In RTL, a microoperation is of the form D à S where à copies content of S into D. D is mod- i¯ed and S is not. 12 Register Transfer Language (RTL) (continued) Basic symbols for RTL Arithmetic microoperations Logic microoperations Shift microoperations (page 350) 13 Registers Addressing Modes Specify rules for accessing operations' operands that are stored in memory or registers, or directly pro- vided by the instructions. Addressing modes should be designed to ² Increase the programming flexibility and ease ² Reduce the size of generated compiler code ² Adapt the program to the operating system ² Allow easy access to operands everywhere E®ective address = absolute address of the oper- and obtained by the application of addressing 14 Addressing Modes Implied mode: Operand is in a register implied by the OpCode of the instruction. Example: ADD #31 Immediate mode: Operand is a constant value spe- ci¯ed in the instruction itself. Example: ADD R, #10 Register mode: Operand is in register speci¯ed in the instruction itself. Example: ADD S, D Register indirect mode: Operand are is in a mem- ory address that is content of a register speci¯ed by the instruction itself. Example: ADD (D), #3 15 Addressing Modes Direct mode: Absolute address of operand is ex- plicitly speci¯ed in instruction. Example: ADD @1234, S Indirect mode: Absolute address of operand is con- tent of a memory address. Example: ADD [@1234], #10 Relative mode: Content of PC + OpAddr. Exam- ple: ADD D, $S Indexed mode: Content of an index register + Op- Addr. Example: ADD D, @500(S) 16 Summary of Addressing Modes 17 Instruction Set Architecture Machine language: Binary language that de¯ne in- structions. Lowest level language. Very di±cult language. Example: in CISC architecture, the addition of 2 and 37 in machine language is 100010 00000010 00100101 Assembly language: Symbolic language in which codes of the machine language are replaced by symbolic names. Example: in RISC architec- ture, the addition of 2 and 37 (contents of reg- ister D and S) is ADD D; S Instruction set is the complete set of machine lan- guage instructions of a CPU. Two major types of instruction set architectures: Reduced Instruction Set Computers: Small set of simple instructions. Hardwired control Complex Instruction Set Computers: Large set of complex instructions. Microprogram- med control 18 Elementary Instruction Set Typical Data Transfer Instructions Typical Arithmetic Instructions 19 Elementary Instruction Set (continued) Typical Logical and Bit Manipulation Instructions Typical Shift Instructions 20 Elementary Instruction Set (continued) Typical Program Control Instructions Conditional Branch Instructions Relating to Status Bits 21 Elementary Instruction Set (continued) Conditional Branch Instructions for Unsigned Numbers Conditional Branch Instructions for Signed Numbers 22 RISC Architectures 1. Memory accesses are restricted to load and store instructions 2. Addressing modes are limited in number 3. Instruction formats are all of the same length 4. Small instruction set 5. Instructions perform elementary operations 6. Large number of registers 7. Size of a program is relatively large (memory) 8. Simple control unit 9. Hardwired control 10. Fast program execution 11. Data manipulation instructions are "register to register" 23 RISC Architectures (continued) 24 CISC Architectures 1. Memory access is directly available to most types of instructions 2. Addressing modes are substantial in number 3. Instruction formats are of di®erent lengths 4. Large instruction set 5. Instructions perform both elementary and com- plex operations 6. Small number of registers 7. Size of a program is relatively small (memory) 8. Complex control unit 9. Microprogrammed control 10. Slow program execution 25 CISC Architectures (continued) 26 CISC Architectures (continued) 27 Example of Assembly Language Programming under CISC architecture Example: Write a program that compare two pos- itive numbers x and y.