Download The von Neumann Model – Chapter 4

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The von Neumann Model –
Chapter 4
COMP 2620
Dr. James Money
COMP 2620
The LC-3 as a von Neumann Machine
Memory
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Memory contains the storage elements along
with the MAR for addressing the memory
elements
The MDR holds the contents of a memory
location to/from storage
MAR is 16 bits, so the address space is 216
The MDR is 16 bits indicating each memory
location is 16 bits
Input/Output

There are two:
–
–
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Keyboard
Monitor
For the keyboard, there are two registers:
–
–
KBDR – keyboard data register which has the
ASCII code if the keys struck
KBSR – keyboard status register for status about
the keys struck
Input/Output

The monitor has also two registers:
–
–
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DDR – ASCII code of the character to be
displayed on the screen
DSR – the associated status information for the
character to be printed on the screen
We will talk more about these in detail in
Chapter 8
Processing Unit

This consists of the
–
–
ALU – arithmetic and logic unit
Eight registers
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R0,R1,…,R7
Use to store temporary values
Also used as operands for operations
Processing Unit

The ALU only has three operations:
–
–
–

Addition
Bitwise AND
Bitwise NOT
We will have to spend some time
implementing the other functions to achieve
full assembly language
Control Unit
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This contains the parts to control flow of the program
executing
It contains the finite state machine, which directs all
activity
Processing is carried out step-by-step and not clock
cycle by clock cycle
There is the CLK input to the finite state machine
which specifies how long each clock cycle lasts
Control Unit
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The Instruction Register (IR) is input to finite
state machine and has the current instruction
This is input to the finite state machine since
it determines what activities must be carried
out
The Program Counter (PC) keeps track of
the next instruction to be executed
Instruction Processing
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
The most basic unit of processing is the
instruction
There are two parts:
–
–
Opcode – what the instruction does
Operands – the parameters to the opcode, for
example

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Registers
Constants
Instruction Processing
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Each instruction is 16 bits, or one word on
the LC-3
The bits are numbered left to right from [15]
to [0].
Bits [15:12] contain the opcode
There are at most 24 = 16 opcodes
Bits [11:0] are operands
Instruction Cycle

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Each instruction is handled in a systematic
way through a sequence of steps call the
instruction cycle
Each step is called a phase
There are six phases to the cycle
Instruction Cycle

The six phases of the instruction cycle are:
–
–
–
–
–
–
Fetch
Decode
Evaluate Address
Fetch Operands
Execute
Store Result
Example: LC-3 ADD Instruction

LC-3 has 16-bit instructions.
–

Each instruction has a four-bit opcode, bits [15:12].
LC-3 has eight registers (R0-R7) for temporary storage.
–
Sources and destination of ADD are registers.
“Add the contents of R2 to the contents of R6,
and store the result in R6.”
Example: LC-3 LDR Instruction
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Load instruction -- reads data from memory
Base + offset mode:
–
–
add offset to base register -- result is memory address
load from memory address into destination register
“Add the value 6 to the contents of R3 to form a
memory address. Load the contents of that
memory location to R2.”
Instruction with all six phases

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Consider the Intel x86 instruction
ADD [eax], edx
It loads data from the address located in eax
and then adds it to the value in edx
The result is stored in the address located in
eax
Instruction with all six phases
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This instruction requires all 6 phases
Fetch and Decode are required by all
instructions
The address is eax is Evaluated and then it is
Fetched
The add performs during Execute
The result is stored at the end
Instruction types

So far, we’ve considered two instruction
types:
–
–

Operate instruction – processes data, such as
ADD
Data movement instruction – moves data from
one place to another, such as LDR
There is a third type of instruction called the
control instruction
Instruction types
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The control instruction changes the
sequence of execution in the processing unit
Normally, the MAR is loaded from the PC at
the beginning of processing
Thus, the control instruction must affect the
PC value during the EXECUTE phase
We use this generate loops and branches
Example: LC-3 JMP Instruction

Set the PC to the value contained in a register.
This becomes the address of the next instruction to
fetch.
“Load the contents of R3 into the PC.”