Exam2 Review Dr. Bernard Chen Ph.D. University of Central Arkansas Spring 2009

Floating-Point Representation 32-bit floating point format. Leftmost bit = sign bit (0 positive or 1 negative). Exponent in the next 8 bits. Use a biased representation. Final portion of word (23 bits in this example) is the significant (sometimes called mantissa).

Example Convert the following number;37.75 into floating point format to fit in 32 bit register. Convert the number from decimal into binary Normalize all digits including the fraction to determine the exponent x sign EXP Significant

Bus Transfer For register R0 to R3 in a 4 bit system S1 S0 4-line common bus Register DRegister CRegister BRegister A Used for highest bit from each register Used for lowest bit

Question For register R0 to R63 in a 16 bit system: What is the MUX size we use? How many MUX we need? How many select bit?

Memory Transfer The transfer of information from a memory word to the outside environment is called a read operation The transfer of new information to be stored into the memory is called a write operation

Memory Read and Write AR: address register DR: data register Read: DR  M[AR] Write: M[AR]  R1

Arithmetic Microoperations A single circuit does both arithmetic addition and subtraction depending on control signals. Arithmetic addition: R3  R1 + R2 (Here + is not logical OR. It denotes addition)

Arithmetic Microoperations Arithmetic subtraction: R3  R1 + R2 + 1 where R2 is the 1’s complement of R2. Adding 1 to the one’s complement is equivalent to taking the 2’s complement of R2 and adding it to R1.

BINARY ADDER Binary adder is constructed with full- adder circuits connected in cascade.

BINARY ADDER-SUBTRACTOR

4-bit Binary Incrementer B3 B2 B1 B0 1 Always added to 1 C4 S3 S2 S1 S0

Shift Microoperations Symbolic designation Description R ← shl R Shift-left register R R ← shr R Shift-right register R R ← cil R Circular shift-left register R R ← cir R Circular shift-right register R R ← ashl R Arithmetic shift-left R R ← ashr R Arithmetic shift-right R TABLE 4-7. Shift Microoperations

Logical Shift A logical shift transfers 0 through the serial input The bit transferred to the end position through the serial input is assumed to be 0 during a logical shift (Zero inserted) 00

Circular Shift The circular shift circulates the bits of the register around the two ends without loss of information

Arithmetic Shift An arithmetic shift shifts a signed binary number to the left or right An arithmetic shift-left multiplies a signed binary number by 2 An arithmetic shift-right divides the number by 2 In arithmetic shifts the sign bit receives a special treatment

Arithmetic Shift Right Arithmetic right-shift: Rn-1 remains unchanged; Rn-2 receives Rn-1, Rn-3 receives Rn-2, so on. For a negative number, 1 is shifted from the sign bit to the right. A negative number is represented by the 2’s complement. The sign bit remained unchanged.

Arithmetic Shift Left LSB Carry out Sign bit R n-1 R n-2 Vs=1 : Overflow Vs=0 : use sign bit LSB 0 insert The operation is same with Logic shift-left The only difference is you need to check overflow problem

Purpose of Chapter5 In this chapter we introduce a basic computer and show how its operation can be specified with register transfer statements.

Instruction Codes A process is controlled by a program A program is a set of instructions that specify the operations, data, and the control sequence An instruction is stored in binary code that specifies a sequence of microoperations Instruction codes together with data are stored in memory (Stored Program Concept)

Program statements and computer instructions Computer instruction Field specifying the operation to be executed Field specifying the data To be operated on

Instruction code format Instruction code format with two parts : Op. Code + Address Op. Code : specify 16 possible operations(4 bits) Address : specify the address of an operand(12 bits) If an operation in an instruction code does not need an operand from memory, the rest of the bits in the instruction(address field) can be used for other purpose Op. Code Address instruction data Not an instruction

Direct address 2. Address is selected in memory and its Data placed on the bus to be loaded into the Data Register to be used for requested instructions Occurs When the Operand Part Contains the Address of Needed Data. 1. Address part of IR is placed on the bus and loaded back into the AR

Direct address

Indirect address 3. New Address is selected in memory and placed on the bus to be loaded into the DR to use later 2. Address is selected in memory and placed on the bus to be loaded Back into the AR Occurs When the Operand Contains the Address of the Address of Needed Data. 1. Address part of IR is placed on the bus and loaded back into the AR

Indirect address

Effective address: Effective address: Address where an operand is physically located Effective address: 457 Effective address: 1350

Accumulator(AC) : takes input from ALU The ALU takes input from DR, AC and INPR : ADD DR to AC, AND DR to AC Note) Input register is not connected to the bus. The input register is connected only to the ALU Computer Registers

5-2 Computer Registers Data Register(DR) : hold the operand(Data) read from memory Accumulator Register(AC) : general purpose processing register Instruction Register(IR) : hold the instruction read from memory Temporary Register(TR) : hold a temporary data during processing Address Register(AR) : hold a memory address, 12 bit width

5-2 Computer Registers Program Counter(PC) : hold the address of the next instruction to be read from memory after the current instruction is executed Instruction words are read and executed in sequence unless a branch instruction is encountered A branch instruction calls for a transfer to a nonconsecutive instruction in the program The address part of a branch instruction is transferred to PC to become the address of the next instruction To read instruction, memory read cycle is initiated, and PC is incremented by one(next instruction fetch)

5-2 Computer Registers Input Register(INPR) : receive an 8-bit character from an input device Output Register(OUTR) : hold an 8-bit character for an output device

Mano's simple Computer: Instructions 000 AND 100BUN 001ADD (Branch Unconditional) 010LDA 101BSA (Load Accumulator) (Branch and Store Address) 011STA 110ISZ (Store Accumulator) (Increment and Skip if Zero) I Any bits other than 0111 and 1111 are called memory reference instructions

3 Instruction Code Formats : Fig. 5-5 Memory-reference instruction Opcode = 000  110 I=0 : 0xxx ~ 6xxx, I=1: 8xxx ~Exxx Register-reference instruction 7xxx (7800 ~ 7001) : CLA, CMA, Input-Output instruction Fxxx(F800 ~ F040) : INP, OUT, ION, SKI, I Opcode Address I=0 : Direct, I=1 : Indirect Register Operation I/O Operation Computer Instruction

Bus s1s1 s2s2 s0s0 16-bit common bus Clock LD INR OUTR IR INPR LD INR CLR LD INR CLR LD INR CLR LD INR CLR WRITE Address Adder & Logic E DR PC AR CLR Computer System Architecture, Mano, Copyright (C) 1993 Prentice-Hall, Inc. AC Mano’s Computer Figure 5-4 READ Memory Unit 4096x16 TR

A 4-bit binary sequence counter (SC) to count from 0 to 15 to achieve time sequencing; > A 4x16 decoder to decode the output of the counter into 16 timing signals, T0,..., T15  A digital circuit with inputs  D0,..., D7, T0,..., T15, I,  and address bits in IR (11-0)  to generate control outputs  supplied to control inputs and select signals of registers, bus.

Instruction and Interrupt cycles Fetch, decode Next Instruction Fetch, decode Next Instruction Execute Instruction Execute Instruction START HALT Instruction cycle Interrupt cycle Interrupt cycle Interrupt Cycle Interrupts Enabled Interrupts Disabled

REGISTER-REFERENCE INSTRUCTIONS The 12 register-reference instructions are recognized by I = 0 and D7 = 1 (IR(12-14) = 111). Each operation is designated by the presence of 1 in one of the bits in IR(0-11). Therefore D7I`T3  r = 1 is common to all register-transfer instructions.

For example B7 = 007 (in hexadecimal)., In binary this is equivalent to: (CIR) B6 = 006 (in hexadecimal)., In binary this is equivalent to: (CIL)

For example B3 = 008 (in hexadecimal)., In binary this is equivalent to: (Complement E) B4 = 010 (Bi=bit in position i =4) in binary is (skip if positive)

5.6 Memory Reference Instructions Opcode ( ) or the decoded output Di (i = 0,..., 6) are used to select one memory-reference operation out of 7.

5.7 IO and Interrupt Input-Output Configuration : –Input Register(INPR), Output Register(OUTR) These two registers communicate with a communication interface serially and with the AC in parallel Each quantity of information has eight bits of an alphanumeric code

IO and Interrupt Input Flag(FGI), Output Flag(FGO) –FGI : set when INPR has information, clear when INPR is empty –FGO : set when operation is completed, clear when output device is active (for example a printer is in the process of printing)

Program Interrupt Demonstration of the interrupt cycle : –The memory location at address 0 is the place for storing the return address –Interrupt Branch to memory location 1 –Interrupt cycle IEN=0 256(return address) 0 BUN 1120 Main Program Interrupt Service Routine 1 BUN 0 0 PC = Interrupt Here Save Return Address(PC) at 0 Jump to 1(PC=1)

Program Interrupt Demonstration of the interrupt cycle : The memory location at address 0 is the place for storing the return address Interrupt Branch to memory location 1 Interrupt cycle IEN=0 256(return address) 0 BUN 1120 Main Program Interrupt Service Routine 1 BUN 0 0 PC = Interrupt Here Save Return Address(PC) at 0 Jump to 1(PC=1)

8-2. General Register Organization Bus organization for 7 CPU registers: l 2 MUX l BUS A and BUS B l ALU l 3 X 8 Decoder

8-2. General Register Organization Bus organization for 7 CPU registers: 2 MUX: select one of 7 register or external data input by SELA and SELB BUS A and BUS B : form the inputs to a common ALU ALU : OPR determine the arithmetic or logic microoperation The result of the microoperation is available for external data output and also goes into the inputs of all registers 3 X 8 Decoder: select the register (by SELD) that receives the information from ALU

l Encoding of Register Selection Fields: »SELA or SELB = 000 (External Input) : MUX selects the external data »SELD = 000 (None) : no destination register is selected but the contents of the output bus are available in the external output

(Example 2) 1. Micro-operation R1  R2 - R3 2. Control word Field: SELA SELB SELDOPR Symbol: R2 R3 R1 SUB Control word: Example

Mano's Computer: RTL

Register Transfer Statement EACDRPCARM[AR]IR Initial Values 02A3400C Instru 102A3400C A Instru 202A A Instru 302A B Instru 402A A34B200 Instru 502A A Instru 60D5CB2A Instru 70151A2A Not shown 7080