The document provides an overview of basic computer organization and design, focusing on instruction codes, computer registers, instruction types, and the instruction cycle. It describes the components of instruction codes, the roles of opcodes and addresses, various instruction formats, and how control units generate necessary signals. Additionally, it covers input/output operations and interrupt handling in a basic computer architecture.

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2. Basic Computer Organization and
Design
o Instruction Codes
o Computer Registers
o Computer Instructions
o Timing and Control
o Instruction Cycle
o Memory Reference Instructions
o Input-Output and Interrupt
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3. Instruction Codes
o A program is a set of instructions that specify
the operations, operands, and the sequence by
which processing has to occur.
o A computer instruction is a binary code that
specifies a sequence of micro-operations for
the computer.
o An instruction code is a group of bits that
instruct the computer to perform a specific
operation.
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4. Instruction Codes…
o Instruction format:
o Two major components of an instruction code:
oThe operation code (opcode)
oAddress
o An opcode specifies the operation for the instruction.
o An address specifies the registers and/or locations
in memory to use for that operation
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Opcode Address
Fig: Instruction Format
01115 14
I
12
Addressing
mode

5. Instruction Codes…
o Address Mode:
o The address field of an instruction represents
oDirect address: the address in memory of the data to use
(the address of the operand)
oIndirect address: the address in memory of the address
in memory of the data to use
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6. Basic Computer Registers
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Fig: Basic Processor
registers

7. Basic Computer Registers…
Fig: Registers
Connected to
Common Bus
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8. Computer Instructions
o The basic computer has three instruction code
formats:
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9. Computer Instructions…
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Fig: Basic Computer
Instructions

10. Computer Instructions…
o Instruction Types:
o Functional Instructions
o Arithmetic, logic, and shift instructions
o ADD, CMA, INC, CIR, CIL, AND, CLA
o Transfer Instructions
oData transfers between the main memory and the
processor registers
o LDA, STA
o Control Instructions
oProgram sequencing and control
o BUN, BSA, ISZ
o Input/Output Instructions
oInput and output
oINP, OUT
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11. Timing and Control
o Control Unit:
o Control units are implemented in one of two
ways:
o Hardwired Control
oCU is made up of sequential and combinational
circuits to generate the control signals
o Microprogrammed Control
oA control memory on the processor contains
microprograms that activate the necessary control
signals
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12. Timing and Control…
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Fig: Control unit of Basic Computer

13. Timing and Control…
o Timing Signal:
o Example: T0, T1, T2, T3, T4, T0, T1, . . .
Assume: At time T4, SC is cleared to 0 if decoder
output D3 is active.
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Clock
T0 T1 T2 T3 T4 T0
T0
T1
T2
T3
T4
D3
CLR
SC
D3T4: SC  0

14. Instruction Cycle
o In the basic computer each instruction cycle
consists:
i. Fetch an instruction from memory
ii. Decode the instruction
iii. Read the effective address from memory if the
instruction has an indirect address.
iv. Execute the instruction.
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15. Instruction Cycle…
o Fetch and Decode
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T0: AR PC (S0S1S2=010, T0=1)
T1: IR  M [AR], PC  PC + 1 (S0S1S2=111, T1=1)
T2: D0, . . . , D7  Decode IR(12-14), AR  IR(0-11), I  IR(15)

16. Instruction Cycle…
o Determine the Type of Instruction
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D'7IT3: AR M[AR]
D'7I'T3: Nothing
D7I'T3: Execute a register-reference instr.
D7IT3: Execute an input-output instr.

17. Instruction Cycle…
o Register Reference Instructions
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Instruction Micro-operation Description
D7l'T3: SC 0 Clear SC
CLA D7l'T3IR(11):AC0 Clear AC
CLE D7l'T3IR(10):E0 Clear E
CMA D7l'T3IR(9):AC (AC)’ Complement AC
CME D7l'T3IR(8): E (E)’ Complement E
CIR D7l'T3IR(7): ACshr AC, AC(I5)E, EAC(0) Circulate right
CIL D7l'T3IR(6): ACshl AC, AC(0) E, EAC(I5) Circulate left
INC D7l'T3IR(5):ACAC+1 Increment AC
SPA D7l'T3IR(4): If (AC(I5) = 0) then (PCPC +1) Skip if positive
SNA D7l'T3IR(3): If (AC(I5) = 1) then (PCPC +1) Skip if negative
SZA D7l'T3IR(2): If (AC = 0) then (PCPC +1) Skip if AC zero
SZE D7l'T3IR(1): If (E = 0) then (PCPC +1) Skip if E zero
HLT D7l'T3IR(0): S 0 (Sis a start-stop flip-flop) Halt computer

18. Instruction Cycle…
o Memory Reference Instruction
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Symbol
Operation
Decoder Symbolic Description
AND D0 AC  AC  M[AR]
ADD D1 AC  AC + M[AR], E  Cout
LDA D2 AC  M[AR]
STA D3 M[AR]  AC
BUN D4 PC  AR
BSA D5 M[AR]  PC, PC  AR + 1
ISZ D6 M[AR]  M[AR] + 1, if M[AR] + 1 = 0 then PC  PC+1
AND to AC
D0T4: DR  M[AR] Read operand
D0T5: AC  AC  DR, SC  0 AND with AC
ADD to AC
D1T4: DR  M[AR] Read operand
D1T5: AC  AC + DR, E  Cout, SC  0 Add to AC and store carry in E

19. Instruction Cycle…
o Memory Reference Instruction…
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LDA: Load to AC
D2T4: DR  M[AR]
D2T5: AC  DR, SC  0
STA: Store AC
D3T4: M[AR]  AC, SC  0
BUN: Branch Unconditionally
D4T4: PC  AR, SC  0
BSA: Branch and Save Return Address
M[AR]  PC, PC  AR + 1
D5T4: M[AR]  PC, AR  AR + 1
D5T5: PC  AR, SC  0
ISZ: Increment and Skip-if-Zero
D6T4: DR  M[AR]
D6T5: DR  DR + 1
D6T4: M[AR]  DR, if (DR = 0) then (PC  PC + 1), SC  0

20. Instruction Cycle…
o Flowchart For Memory Reference Instructions
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21. Input-output and Interrupt
o Input and output instructions are needed for
transferring information to and from AC
register.
o for checking the flag bits, and
o for controlling the interrupt facility.
o Note: Input-output instructions have an
operation code 1111 and are recognized by the
control when D7 = 1 and I = 1.
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22. Input-Output and Interrupts…
o Input-Output Instructions…
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D7IT3 = p
IR(i) = Bi, i = 6, …, 11
p: SC  0 Clear SC
INP pB11: AC(0-7)  INPR, FGI  0 Input char. to AC
OUT pB10: OUTR  AC(0-7), FGO  0 Output char. from AC
SKI pB9: if(FGI = 1) then (PC  PC + 1) Skip on input flag
SKO pB8: if(FGO = 1) then (PC  PC + 1) Skip on output flag
ION pB7: IEN  1 Interrupt enable on
IOF pB6: IEN  0 Interrupt enable off
INPR Input register - 8 bits
OUTR Output register - 8 bits
FGI Input flag - 1 bit
FGO Output flag - 1 bit
IEN Interrupt enable - 1 bit

23. Input-Output and Interrupts…
o External events can force the computer to quit
the normal program flow to react fast on the
events. Such event that requires fast response by
the computer is called interrupt.
o interrupt routine: The collection of
instructions that have to be executed to respond
to the interrupt
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o IEN (Interrupt-enable flip-flop)
o can be set and cleared by instructions
o when cleared, the computer cannot be interrupted

24. Input-Output and Interrupts…
o Flowchart for Interrupt Cycle
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25. Programming the Basic Computer
o Introduction
o Machine Language
o Assembly Language
o Programming Arithmetic and Logic Operations
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26. Introduction
o Instruction Set of the Basic Computer
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m: effective address
M: memory word
(operand) found at m

27. Introduction…
o Types of programming language:
o Machine language: 1’s and 0’s (Binary code),
Octal, and Hexadecimal code
o Assembly language: symbolic Code
o High-level language
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28. Machine Language
o Comparison of Programming Languages
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