Computer Architecture 2 Patrick Marshall

Gates per CPU Vacuum tube Transistor Small Scale integration –1965 on –Up to 100 devices on a chip Medium Scale integration – to 1971 –100-3,000 devices on a chip Large Scale integration –3, ,000 devices on a chip Very Large Scale integration – 1978 to date –100,000 – 100,000,000 devices on a chip Ultra large Scale integration –Over 100,000,000 devices on a chip

Moore’s Law Increased density of components on chip Gordon Moore – cofounder of Intel Number of transistors on a chip will double every year Since 1970’s development has slowed a little –Number of transistors doubles every 18 months Cost of a chip has remained almost unchanged Higher packing density means shorter electrical paths, giving higher performance Smaller size gives increased flexibility Reduced power and cooling requirements Fewer interconnections increase reliability

Moore’s Law

CPU ( revisited) Control Unit ALU Bus Registers –Program Counter Register –Intruction Register –General Purpose Registers

Instructions within a CPU 1. Add the number stored in box 9 to your secret agent code number 2. Divide the previous result by the number stored in box 10 3 Subtract the number stored in box 8 4 If the previous result is not equal to 30, go to box 7. Otherwise to the nexy box 5 Subtract 13 from the previous result 6 Return to HQ for more information 7 Bomb… too bad

Instructions within a CPU Three different classes of instructions –Boxes 1,2,3,5 are arithmetic operations –Box 4 contains a conditional jump or conditional branch –Box 6 contains the return instruction, which is a control instruction which contains no data and refers to no other address Each arithmetic and conditional jump consists of two parts an operation and an address

Von-Neumann Architecture RegistersALU Control Unit CPU Commmunication Bus Memory (for Intructions and date) I/O The Von- Neumann Bottleneck

Registers Not all registers are the same size –Data or Instructions need to be same size as a memory location –In an 8 bit Microprocessoer Data registers are 8 bit wide Address registers are 16 bit wide –Giving 64K addressable Memory Locations need to be large enough to hold the highest memory location –Intel has 20 bit memory register –Max addressable size was 1024k (1Mb) –Motorola has 32 bit memory register Max addressable size should be 2 32 which is 4096 Mb But is only 2 24 which is 16Mb

ALU –Performs arithmetic and logic operations –Data can come from memory or input devices or registers –Most CPU has 8+ registers, 8 bit CPU’s have 1 register called the accumulator ACC ALU Flags (CCR) Control Unit

To connect registers on the bus a control signal is required Everthing happens on a clock cycle, like the computers heartbeat Fetches and executes intructions –Translates macroinstructions into microinstructions Smallest event that can take place inside a CPU –Clocking a flip-flop –Moving data from one register to another

Register Transfer Language A shorthand for expressing what happens inside a CPU. 1 or more letters followed by numbers indicates registers or memory location Square brackets means “the contents of” Left arrow means transfer MS means the computers memory Main Store MS(X) means the valuein the main store at memory location X E.G –[MAR] [PC] –[PC] [PC] + 1 –[MS(20)] [PC]

Register Transfer Language MAR –Memory Address Register MBR- Memory Buffer Register MDR – Memory Data Register

Operation of a computer Known as the Fetch-Execute cycle or Machine Cycle –Fetch the next instruction from memory into the control unit –Decode instruction –Obey the instruction –Go back to 1

Fetch Execute in Detail Program counter contains (points to) the address of the next instruction in memory Step 1: [MAR] [PC] ‘ Contents of PC are moved into the MAR –MAR holds the address of a location in memory into which data is being written or read –MAR now holds the contents of the PC Step 2: [PC] [PC]+ 1 ‘The program is incremented Step3: [MBR] [MS([MAR]) –MBR takes the value of the memory location held in MAR –MBR is temporary holding place for data received from memory –MBR contains the bit pattern of the instruction to be executed

Fetch Execute in Detail Step 4: [IR] [MBR] ‘Instruction register takes value of MBR –IR now contains the bit pattern of the instruction to be executed –Divided into fields, operator and operand –Single address instructions Step 5: CU [IR(op-code)] –The control unit executes the instruction

Lets look at the Add instruction Example: –P=Q+R Single Accumulator Move Q,DO Add R,DO Move DO,P –More complex instruction set,single clock cycle Add Q,R,P

Lets look at the Add instruction Step 1: [MAR] [IR(address)] –[MAR] takes the address part of the instruction register Step 2:[MBR] [MS([MAR])] –Memory buffer takes the value of the data stored in memory at the address in [MAR] Step 3: ALU [MBR],ALU [D0] –ALU adds the contents of the [MBR] with the contents of data (accumulator register D0 Step 4: [D0] ALU

More on Operation Fetch cycle is always the same Different execution cycles depending on the instruction, one per instuction Instruction Groups –ALU instructions –Move or copy instructions –Transfer of control

Branching/Jumping Branch forces the CPU to execute instructions out of the normal sequence Conditional Branch allows high level contructs such as IF THEN ELSE to be executed –Based on values in the CCR CCR- Condition Code Register –After the CPU executes an instruction, it updates the bits of the CCR –C= Carry Flag –Z=Zero Flag –N=Negative Flag –V= Overflow

CCR- Condition Control Unit CCR as we saw is connected to the Control Unit BCC Branch on carry clear C=0 BCS Branch on carry set C=1 BEQ Branch on zero result Z=1 BNE Branch on non-zero result Z=0 BMI Branch on minus result (two’s comp) N=1 BPL Branch on positive result (tow’s comp) N=0 BVC Branch on overflow clear (two’s comp) V=0

CCR- Condition Control Unit BVS Branch on overlfow set (two’s comp) V= 1 BGE Branch on greater than or equal to N.V+N.V=1 used in two’s compliment arithmetic BGT Branch on greater than N.V.Z+N.V.Z=1 BHI Branch if higher than C.Z=1 Used in unsigned arithemtic BLE Branch if less than Z+N.V+N.V=1 used in tow’s comp arithmetic BLS Branch if lover than or the same C+Z=1 Used in unsigned arithmetic

CCR- Condition Control Unit BCC adrs: IF [C]=0 THEN [PC] [IR(adrs)] Branch on carry clear ( jump to the target address of the carry bit in the CCR is 0)