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Modern computers use the stored program concept.
The stored program concept is that:
o machine code instructions are stored in main
memory,
o these are fetched and executed serially,
o by a processor that performs arithmetic and
logical operations.
The concept was introduced by John von Neumann.
Previously, “programs” were entered into a
computer by techniques such as setting switches
and plugging patch leads into panels.
The concept can be seen earlier in Alan Turing’s
development of the Universal Turing Machine.
Machine Architecture
Machine Architecture
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The Fetch-Execute cycle is the basic
operation cycle of a computer.
The cycle is repeated over and over again
from the moment a computer is turned on
until it is turned off.
Each time that the cycle is iterated, a single
machine code instruction if fetched from
main memory, decoded and executed.
The result (output) of an instruction is usually
stored into a general purpose register but
could also be written back to main memory.
Machine Architecture
The Fetch-Execute cycle has three stages:
1. Fetch: The instruction to execute is
retrieved from the main memory.
2. Decode: The op-code part of the instruction
is examined to determine which parts of the
CPU must be used to carry out the
instruction.
3. Execute: The appropriate hardware
components of the CPU are used to carry
out the instruction.
Machine Architecture
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The Fetch-Execute cycle makes use of some
specific purpose registers on the processor:
o Program Counter (PC): Stores the address in main
memory of the next instruction to execute.
o Memory Address Register (MAR): Stores the address
in main memory of the location that the processor
is currently reading data from or writing data to.
o Memory Buffer Register (MBR): Temporarily stores
the word of data that is currently being transferred
between the processor and the main memory.
o Current Instruction Register (CIR): Stores the
instruction that is currently being executed by the
processor whilst it is decoded and executed.
Machine Architecture
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The steps involved in the Fetch-Execute cycle are
often described in register transfer notation. This
has the format:
Register / Storage Location  Value
e.g.
MAR  [PC]
means that the contents of the Program Counter
are copied into the Memory Address Register
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The symbols [] are used to indicate the contents or
a register or storage location.
Machine Architecture
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The animation on the next slide shows one
iteration of the fetch-execute cycle, to carry
out the instruction stored in memory location
104.
The animation focuses in most detail on the
fetch stage of the cycle as this stage is the
same for all instructions.
Machine Architecture
1
MAR  [PC]
(Contents of Program Counter transferred to MAR)
2
(a) PC  [PC] + 1
(Increment contents of Program Counter)
(b) MBR  [Memory]addressed
(Contents of addressed memory location loaded into MBR)
3
• Memory address transferred along address bus to main
CIR  [MBR]
memory.
(Transfer contents of
• Contents of addressed location looked up in main memory.
Memory Buffer Register
• Looked up value transferred to processor along data bus.
into Current Instruction
Register)
Processor
Main Memory
4
Decode instruction
5
Execute instruction
Machine Architecture
+1
Simultaneous
PC
MAR
104
105
104
104
CIR
MBR
Data Bus
10000101
10000101
10000101
Address Bus
Address
Content
100
101
102
103
104
105
106
01110101
11010101
00101000
11011001
10000101
10101101
01001101
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During the decode stage, the control unit
splits the instruction in the CIR into two parts:
instruction
10000101
1000 0101
op-code
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operand
Op-code: The binary pattern which identifies
the instruction to be carried out e.g. add, or,
branch.
Operand: The data that the instruction will
operate on.
Machine Architecture
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The operand could be a number such as 5,
which the instruction should use directly.
Alternatively it might be the address in main
memory where the data to use is stored.
How to interpret the operand will depend
upon the addressing mode that the
instruction is using.
Instructions may have zero, one or more
operands.
Machine Architecture
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What happens in the execute stage of the cycle is
determined by the op-code of the instruction that is
being executed.
The op-code will determine which circuitry within the
processor should be used to carry out the instruction.
Temporary results and the final result of executing
instructions are usually stored in registers instead of
main memory as registers can be accessed much
more quickly.
Modern computers have many general purpose
registers, but it can be convenient to think of a
simplified model of a computer with just one general
purpose register, known as the accumulator.
Machine Architecture
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The example below shows the execute stage for an ADD
instruction that adds the value in a specified memory location
onto the current value in the accumulator register, storing the
result in the accumulator.
Action
Explanation
MAR  [CIR (Operand)]
Transfer the value in the
instruction’s operand into the MAR.
This is the address that the value to
add on is stored at.
MBR  [Memory]addressed
Fetch the value to add from main
memory into the MBR.
ALU  MBR, [ACC]
In the ALU, add together the
contents of the MBR and the
accumulator.
ACC  [ALU]
Write the output of the instruction
into the accumulator.
Machine Architecture
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After an instruction has been executed, the
contents of a special purpose register known as the
status register are updated.
Each bit of the status register is known as a flag
and has a specific purpose.
Some example status register bits are:
Bit
Purpose
Carry
Set if the last instruction produced a carry.
Zero
Set if the last instruction produced a result of zero.
Overflow
Set if the last instruction produced an overflow, i.e. a
result outside the allowable range.
The status register stores information about the
result of previous operations as these may affect
future operations.
Machine Architecture