Basic Computer Organization and Design ..................................................................... The basic computer design represents all of the major concepts in CPU design without overwhelming students with the complexity of a modern commercial CPU.

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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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