Introduction to Digital Logic

Transcription

1 Introduction to Digital Logic Lecture 5 Simple CPU Overview Instruction Set

2 Software Process Software Program High Level Language Description if (x > ) x = x + y - z; a = b*x; Compiler JLEZ X,SKIP MOVE.W X,D ADD CMPI.W X,X,Y #,D SUB BLE X,X,Z SKIP SKIP: MUL ADD A,B,X Y,D SUB Z,D SKIP MUL Assembler Program Executing Mark Redekopp, All rights reserved.c/.cpp files Loader / OS Assembly (.asm/.s files) A compiler (i.e. gcc, VisualC++, etc.) also includes the assembler & linker Executable Binary Image Object/Machine Code (.o files) Linker

3 Hardware Components Computer hardware can be classified into three categories Input/Output Devices Supplies and consumes data Supplies the program Keyboard, Mouse, Monitor, Hard Drive (RAM/ROM) Temporary storage for data and program Performs operations on data as indicated by SW program Pentium, Celeron, etc. Input Devices Software Program Output Devices

4 Like the memories we learned about earlier Set of cells/rows that each store a group of bits (usually, byte = 8 bits) but can be larger like -bytes = 6-bits, or even -bytes = -bits, etc. Unique address assigned to each cell/row Used to reference the value in that location Address 5 FF 6 Device

5 Operations Memories perform operations Read: retrieves data value in a particular location (specified using the address) Write: changes data in a location to a new value To perform these operations a set of address, data, and control inputs/outputs are used Note: A group of wires/signals is referred to as a bus Thus, we say that memories have an address, data, and control bus. Read 5 Write Addr. 5 A Read Operation Addr. 5 Mark Redekopp, All rights reserved A Write Operation

6 Address Performs the same -step process over and over again Fetch an instruction from memory Decode/ the instruction Generate necessary control signals to control the datapath to execute the given instruction Execute the instruction Perform the specified operation These steps are known as the Instruction Cycle Fetch Instruction Arithmetic Circuitry Circuitry ADD SUB CMP Add the specified values Generate control signals

7 Primary Components of a ALU Registers Circuitry Connects to memory and I/O via address, data, and control buses Addr 5 6

8 Arithmetic and Logic Unit (ALU) Executes arithmetic operations like addition and subtraction along with logical operations (AND, OR, etc.) out op. ALU ADD, SUB, AND, OR in in Addr 5 6

9 Registers Some are for general use by software Registers provide fast, temporary storage locations within the processor (to avoid having to read/write slow memory) Others are required for specific purposes to ensure proper operation of the hardware out op. ALU ADD, SUB, AND, OR in in R-RF PC Addr 5 6

10 General Purpose Registers Registers available to software instructions for use by the programmer/compiler Instructions use these registers as inputs (source locations) and outputs (destination locations) out op. ALU ADD, SUB, AND, OR in in R-RF PC Addr 5 6

11 What if we didn t have registers? Example w/o registers: F = (X+Y) (X*Y) Requires an ADD instruction, MULtiply instruction, and SUBtract Instruction w/o registers ADD: Load X and Y from memory, store result to memory MUL: Load X and Y again from mem., store result to mem. SUB: Load results from ADD and MUL and store result to mem 9 memory accesses out op. ALU ADD, SUB, AND, OR in in R-RF PC Addr 5 6 X Y F

12 What if we have registers? Example w/ registers: F = (X+Y) (X*Y) Load X and Y into registers ADD: R + R and store result in R MUL: R * R and store result in R SUB: R R and store result in R Store R back to memory total memory access out op. ALU ADD, SUB, AND, OR in in R-RF PC X Y Addr 5 6 X Y F

13 Other Registers Some bookkeeping information is needed to make the processor operate correctly Example: Program Counter (PC) Recall that the processor must fetch instructions from memory before decoding and executing them PC register holds the address of the currently executing instruction out op. ALU ADD, SUB, AND, OR in in R-RF PC Addr 5 6

14 Fetching an Instruction To fetch an instruction Assume instructions are stored in a 56x6 bit memory (each instruction is stored using a -byte / 6-bit value PC contains the address of the instruction The value in the PC is placed on the address bus and the memory is told to read The PC is incremented, and the process is repeated for the next instruction out op. ALU ADD, SUB, AND, OR in in R-RF PC PC = Addr = Addr = inst. machine code = Read FF inst. inst. inst. inst. inst. 5

15 Fetching an Instruction To fetch an instruction Assume instructions are stored in a 56x6 bit memory (each instruction is stored using a -byte / 6-bit value PC contains the address of the instruction The value in the PC is placed on the address bus and the memory is told to read The PC is incremented, and the process is repeated for the next instruction out op. ALU ADD, SUB, AND, OR in in R-RF PC PC = Addr = Addr = inst. machine code = Read FF inst. inst. inst. inst. inst. 5

16 Circuitry circuitry is used to decode the instruction and then generate the necessary signals to complete its execution s the ALU Selects Registers to be used as source and destination locations out op. ALU ADD, SUB, AND, OR in in R-R PC Addr FF inst. inst. inst. inst. inst. 5

17 Circuitry Assume hex is machine code for an ADD instruction of R = R + R Logic will select the registers (R and R) tell the ALU to add select the destination register (R) ADD PC Addr inst. out ALU ADD in in R-RF FF inst. inst. inst. 5

18 Input / Output Keyboard, Mouse, Display, USB devices, Hard Drive, Printer, etc. can perform reads and writes on I/O devices just as it does on memory I/O devices have locations that contain data that the processor can access These locations are assigned unique addresses just like memory 7F a = 6 hex in ASCII Keyboard Interface 8 A D C READ

19 Input / Output Writing a value to the video adapter can set a pixel on the screen 7F Video Adapter A D C FE may signify a white dot at a particular location FE 6 FE WRITE Keyboard Interface 8 6

20 Our 8-bit Computer 8-bit data and addresses but 6-bit instructions I/O 8-bit input data from keyboard 8 LED s for output display Store program instructions For our design we ll use a separate 56x6 instruction memory (56 rows / instructions each being 6-bits) Stores program data For our design we ll use a separate 8x8 data memory (each data element is a byte-size value Registers Temporary storage locations inside processor for fast access For our design we ll use (6) 8-bit registers: R-RF

21 Address Space With 8-bit values we can make 56 unique addresses Mark Redekopp, All rights reserved Address range: -FF hex We said our data memory will only have 8 locations Address range: 7F hex What about the other 8? We will map our I/O devices to some of those unused addresses Read of address 8 hex will return keyboard data Write of address 8 hex will output to LED s 7F 8 FF Keyboard (Read) / LEDs (Write) 8 Unoccupied / Unused 8 x 8 RAM I/O and Unused Space

22 Instruction Set supports instructions Instruction Type Comment ADD Rd,Rs,Rt Rd = Rs + Rt SUB Rd,Rs,Rt Rd = Rs Rt MOVE Rd,Rs Rd = Rs XOR Rd,Rs,Rt Rd = Rs XOR Rt AND Rd,Rs,Rt Rd = Rs AND Rt OR Rd,Rs,Rt Rd = Rs OR Rt JEQZ Rt,WX If(Rt = ) jump to instruc. At address WX JLTZ Rt,WX If(Rt < ) jump to instruc. At address WX ST Rs,[Rt] Store to (MEM[Rt] = Rs) LDC Rd,XY Load Constant (Rd = XY) LD Rd,[Rt] Load from (Rd = MEM[Rt] )

23 Machine Code Form of Instructions Every instruction is represented by a 6-bit binary string: IW[5:] Basic Machine Code Format broken into four - bit fields of info Opcode: -bit field indicating what instruction it is d/w: d = Dest. register number or W: Upper -bits of jump address s/x: s = Operand A reg. number or X: Lower -bits of jump address or upper -bits of constant for load t/y: t = Operand B reg. number of Y: Lower -bits of constant for load

24 Machine Code Form of Instructions Mark Redekopp, All rights reserved IW[5:] IW[:8] IW[7:] IW[:] ADD d s t SUB d s t MOVE d s XOR d s t AND 5 d s t OR 6 d s t JEQZ 8 W X t JLTZ 9 W X t ST B s t LDC E d X Y LD F d t Examples: ADD R,R5,R6 => 56 hex; MOVE RC,R => c; JEQZ R,C => 8C hex; LDC R,FE => EFE; LD R7,[R] => F7;

25 Conditional (If) Example if(x==y) Z = X+Y Else Z = X; IMEM Addr Instruction Type Comment LDC R, Load constant for later use LDC R,8 Address of Keyboard Input LD R,[R] Read in X value from keyboard LD R,[R] Read in Y value from keyboard SUB R,R,R Do X-Y to check if X==Y 5 JEQZ R,8 Goto THEN portion 6 MOVE R5,R Z = X 7 JEQZ R,9 Skip the THEN portion 8 ADD R5,R,R Z = X + Y 9 ST R5,[R] Write value to LED s A next instruction

26 Loop Example char data[] = { }; int i, j=; for(i=9; i >= ; i--) j = j + data[i]; IMEM Addr Instruction Type Comment LDC R, Constant for later use LDC R, Constant for Decrement Op. LDC R, Based address of data array LDC R, J = LDC R,9 I = 9 5 JLTZ R,B I >=? 6 ADD R5,R,R Setup address to data[i] 7 LD R6,[R5] Get value of data[i] 8 ADD R,R,R6 J = j + data[i] 9 SUB R,R,R I-- A JEQZ R,5 Repeat B next instruction