CPU & Memory
Basic | Advanced (using 80386)

CPU & Memory Interaction

In order to illustrate how the three main components of the Central Processing Unit interact i.e.; the Control Unit, the ALU and Memory, we will now look at how a CPU;

accepts keyboard input
multiplies two numbers
prints the result.

[Dual processor CPU]

(1). First Instruction

Control Unit recognises program is in memory, and executes first step.

         
+CPU---------Input-----------------------------------+
| +---------+  v  |----------Primary Storage--------||
| | Control |     |    Programs        |    Data    ||
| | Unit    | <-> |--------------------|------------||
| +---------+     |1 Enter    |2 Enter |7     |8    ||
|      ^          |  First    |  Second|      |     ||
|      v          |  Number   |  Number|      |     ||
|+------------+   |-----------|--------|------|-----||
|| Arithmetic |   |3 Multiply |4 Print |9     |10   ||
|| Logic Unit |<->|  1st & 2nd|  Result|      |     ||
||------------|   |  Numbers  |        |      |     ||
||            |   |-----------|--------|------|-----||
||            |   |5 End      |6       |10    |11   ||
|+------------+   |           |        |      |     ||
|               | +---------------------------------+|
+------------Output----------------------------------+
                v

(2). Request first number

Program tells the user to "Enter First Number".

          
+CPU---------Input-----------------------------------+
| +---------+  v  |-----------Primary Storage-------||
| | Control |     |    Programs        |    Data    ||
| | Unit    | <-> |--------------------|------------||
| +---------+     |1 Enter    |2 Enter |7     |8    ||
|      ^          |  First    |  Second|      |     ||
|      v          |  Number   |  Number|      |     ||
|+------------+   |-----------|--------|------|-----||
|| Arithmetic |   |3 Multiply |4 Print |9     |10   ||
|| Logic Unit |<->|  1st & 2nd|  Result|      |     ||
||------------|   |  Numbers  |        |      |     ||
||            |   |-----------|--------|------|-----||
||            |   |5 End      |6       |10    |11   ||
|+------------+   |           |        |      |     ||
|               | +---------------------------------+|
+------------Output----------------------------------+
                v

(3). Input first number

user enters 25 on keyboard. Electronic signal sent to CPU, and sent to Address 7 in Primary Storage.

          
+CPU------------Input--------------------------------+
| +---------+  v  |-----------Primary Storage-------||
| | Control |     |    Programs        |    Data    ||
| | Unit    | <-> |--------------------|------------||
| +---------+     |1 Enter    |2 Enter |7 25  |8    ||
|      ^          |  First    |  Second|      |     ||
|      v          |  Number   |  Number|      |     ||
|+------------+   |-----------|--------|------|-----||
|| Arithmetic |   |3 Multiply |4 Print |9     |10   ||
|| Logic Unit |<->|  1st & 2nd|  Result|      |     ||
||------------|   |  Numbers  |        |      |     ||
||            |   |-----------|--------|------|-----||
||            |   |5 End      |6       |10    |11   ||
|+------------+   |           |        |      |     ||
|               | +---------------------------------+|
+------------Output----------------------------------+
                v

(4). Request second number

control unit executes second instruction "Enter Second Number".

          
+CPU---------Input-----------------------------------+
| +---------+  v  |-----------Primary Storage-------||
| | Control |     |    Programs        |    Data    ||
| | Unit    | <-> |--------------------|------------||
| +---------+     |1 Enter    |2 Enter |7 25  |8    ||
|      ^          |  First    |  Second|      |     ||
|      v          |  Number   |  Number|      |     ||
|+------------+   |-----------|--------|------|-----||
|| Arithmetic |   |3 Multiply |4 Print |9     |10   ||
|| Logic Unit |<->|  1st & 2nd|  Result|      |     ||
||------------|   |  Numbers  |        |      |     ||
||            |   |-----------|--------|------|-----||
||            |   |5 End      |6       |10    |11   ||
|+------------+   |           |        |      |     ||
|               | +---------------------------------+|
+------------Output----------------------------------+
                v

(5). Input second number

40 is entered on the Keyboard, and an electronic signal is sent to the CPU. This signal is routed by the control unit to address 8 in primary storage.

          
+CPU---------Input-----------------------------------+
| +---------+  v  |-----------Primary Storage-------||
| | Control |     |    Programs        |    Data    ||
| | Unit    | <-> |--------------------|------------||
| +---------+     |1 Enter    |2 Enter |7 25  |8 40 ||
|      ^          |  First    |  Second|      |     ||
|      v          |  Number   |  Number|      |     ||
|+------------+   |-----------|--------|------|-----||
|| Arithmetic |   |3 Multiply |4 Print |9     |10   ||
|| Logic Unit |<->|  1st & 2nd|  Result|      |     ||
||------------|   |  Numbers  |        |      |     ||
||            |   |-----------|--------|------|-----||
||            |   |5 End      |6       |10    |11   ||
|+------------+   |           |        |      |     ||
|               | +---------------------------------+|
+------------Output----------------------------------+
                v

(6). CU issues multiply

control unit executes third instruction "Multiply first and second Numbers".

          
+CPU---------Input-----------------------------------+
| +---------+  v  |-----------Primary Storage-------||
| | Control |     |    Programs        |    Data    ||
| | Unit    | <-> |--------------------|------------||
| +---------+     |1 Enter    |2 Enter |7 25  |8 40 ||
|      ^          |  First    |  Second|      |     ||
|      v          |  Number   |  Number|      |     ||
|+------------+   |-----------|--------|------|-----||
|| Arithmetic |   |3 Multiply |4 Print |9     |10   ||
|| Logic Unit |<->|  1st & 2nd|  Result|      |     ||
||------------|   |  Numbers  |        |      |     ||
||            |   |-----------|--------|------|-----||
||            |   |5 End      |6       |10    |11   ||
|+------------+   |           |        |      |     ||
|               | +---------------------------------+|
+------------Output----------------------------------+
                v

(7). ALU sets up multiplication

the control unit informs the ALU that two numbers are coming, and that it is to multiply them. The control unit sends the ALU copies of addresses 7 and 8.

          
+CPU---------Input-----------------------------------+
| +---------+  v  |-----------Primary Storage-------||
| | Control |     |    Programs        |    Data    ||
| | Unit    | <-> |--------------------|------------||
| +---------+     |1 Enter    |2 Enter |7 25  |8 40 ||
|      ^          |  First    |  Second|      |     ||
|      v          |  Number   |  Number|      |     ||
|+------------+   |-----------|--------|------|-----||
|| Arithmetic |   |3 Multiply |4 Print |9     |10   ||
|| Logic Unit |<->|  1st & 2nd|  Result|      |     ||
||------------|   |  Numbers  |        |      |     ||
||    25      |   |-----------|--------|------|-----||
||    40  x   |   |5 End      |6       |10    |11   ||
||            |   |           |        |      |     ||
|+------------+ | +---------------------------------+|
+------------Output----------------------------------+
                v

(8). ALU performs multiplication

the ALU performs the multiplication: (25 x 40 = 1000)

          
+CPU---------Input-----------------------------------+
| +---------+  v  |-----------Primary Storage-------||
| | Control |     |    Programs        |    Data    ||
| | Unit    | <-> |--------------------|------------||
| +---------+     |1 Enter    |2 Enter |7 25  |8 40 ||
|      ^          |  First    |  Second|      |     ||
|      v          |  Number   |  Number|      |     ||
|+------------+   |-----------|--------|------|-----||
|| Arithmetic |   |3 Multiply |4 Print |9     |10   ||
|| Logic Unit |<->|  1st & 2nd|  Result|      |     ||
||------------|   |  Numbers  |        |      |     ||
||    25      |   |-----------|--------|------|-----||
||    40  x   |   |5 End      |6       |10    |11   ||
||  1000      |   |           |        |      |     ||
|+------------+ | +---------------------------------+|
+------------Output----------------------------------+
                v

(9). Results to memory

the control unit sends a copy of the multiplied results (1000) to address 9 of primary storage.

          
+CPU---------Input-----------------------------------+
| +---------+  v  |-----------Primary Storage-------||
| | Control |     |    Programs        |    Data    ||
| | Unit    | <-> |--------------------|------------||
| +---------+     |1 Enter    |2 Enter |7 25  |8 40 ||
|      ^          |  First    |  Second|      |     ||
|      v          |  Number   |  Number|      |     ||
|+------------+   |-----------|--------|------|-----||
|| Arithmetic |   |3 Multiply |4 Print |9 1000|10   ||
|| Logic Unit |<->|  1st & 2nd|  Result|      |     ||
||------------|   |  Numbers  |        |      |     ||
||    25      |   |-----------|--------|------|-----||
||    40  x   |   |5 End      |6       |10    |11   ||
||  1000      |   |           |        |      |     ||
|+------------+ | +---------------------------------+|
+------------Output----------------------------------+
                v

(10). Print instruction

the control unit executes the fourth instruction "Print Result".

          
+CPU---------Input-----------------------------------+
| +---------+  v  |-----------Primary Storage-------||
| | Control |     |    Programs        |    Data    ||
| | Unit    | <-> |--------------------|------------||
| +---------+     |1 Enter    |2 Enter |7 25  |8 40 ||
|      ^          |  First    |  Second|      |     ||
|      v          |  Number   |  Number|      |     ||
|+------------+   |-----------|--------|------|-----||
|| Arithmetic |   |3 Multiply |4 Print |9 1000|10   ||
|| Logic Unit |<->|  1st & 2nd|  Result|      |     ||
||------------|   |  Numbers  |        |      |     ||
||    25      |   |-----------|--------|------|-----||
||    40  x   |   |5 End      |6       |10    |11   ||
||  1000      |   |           |        |      |     ||
|+------------+ | +---------------------------------+|
+------------Output----------------------------------+
                v

(11). Print result

the control unit sends the result (1000) to the printer, and the printer prints this value.

          
+CPU---------Input-----------------------------------+
| +---------+  v  |-----------Primary Storage-------||
| | Control |     |    Programs        |    Data    ||
| | Unit    | <-> |--------------------|------------||
| +---------+     |1 Enter    |2 Enter |7 25  |8 40 ||
|      ^          |  First    |  Second|      |     ||
|      v          |  Number   |  Number|      |     ||
|+------------+   |-----------|--------|------|-----||
|| Arithmetic |   |3 Multiply |4 Print |9 1000|10   ||
|| Logic Unit |<->|  1st & 2nd|  Result|      |     ||
||------------|   |  Numbers  |        |      |     ||
||    25      |   |-----------|--------|------|-----||
||    40  x   |   |5 End      |6       |10    |11   ||
||  1000      |   |           |        |      |     ||
|+------------+ | +---------------------------------+|
+------------Output----------------------------------+
                v

(12). End instruction

the control unit then executes the final instruction 'End'.

          
+CPU---------Input-----------------------------------+
| +---------+  v  |-----------Primary Storage-------||
| | Control |     |    Programs        |    Data    ||
| | Unit    | <-> |--------------------|------------||
| +---------+     |1 Enter    |2 Enter |7 25  |8 40 ||
|      ^          |  First    |  Second|      |     ||
|      v          |  Number   |  Number|      |     ||
|+------------+   |-----------|--------|------|-----||
|| Arithmetic |   |3 Multiply |4 Print |9 1000|10   ||
|| Logic Unit |<->|  1st & 2nd|  Result|      |     ||
||------------|   |  Numbers  |        |      |     ||
||    25      |   |-----------|--------|------|-----||
||    40  x   |   |5 End      |6       |10    |11   ||
||  1000      |   |           |        |      |     ||
|+------------+ | +---------------------------------+|
+------------Output----------------------------------+
                v

Advanced (using Intel 80386)

[80386 CPU internals]

(Components explained below) [Ref 79,80]

Adding two numbers (summarised version follows)

Step 1	1. The prefetch unit asks the Bus Interface Unit to retrieve the instruction to add two numbers. 1.1. At the same time the segment and paging units convert the instruction's location from a virtual address to physical address, for the Bus Interface unit.
Step 2	The Bus Interface Unit gets the instruction from RAM and sends it to the prefetch unit.
Step 3	The Prefetch Unit forwards the instruction to the Decode Unit which forwards it to the Execution Unit.
Step 4	The Control Unit sends a virtual address of the first number to be added (stored in RAM) to the Protection Test Unit.
Step 5	The Protection Test Unit verifies that the control unit can access the first number and forwards it to the segment and paging units, where the virtual address is translated into a physical address for the bus interface unit.
Step 6	The Bus Interface unit retrieves the number stored at that address in RAM. This number then travels through the Protection Test Unit to the Execution Unit and is stored in one of the chip's internal registers.
Step 7	The second number undergoes a similar process.
Step 8	The Arithmetic Unit adds the second number retrieved from RAM and the first from the internal register.
Step 9	The Control unit tells the Bus interface unit to store the number in RAM. The Memory Management Unit translates the virtual address to a physical RAM address, completing the instruction.

Summarised

Prefetch Unit gets an instruction from RAM and passes it to the Control unit (CU).
CU gets first number from RAM and places it into a register.
CU gets second number from RAM and places it into a register.
CU directs the Arithmetic and Logic unit (ALU) to add the contents of the registers and puts the result into a register.
CU sends the result to RAM.

Notes

Bus Unit - Handles communication with devices external to the microprocessor chip
Code Prefetch Unit -Fetches instructions from memory before the microprocessor actually requests them
Instruction Decode Unit - Decodes the instruction prior to passing it to the Execution Unit for execution.
Execution Unit - Executes the instruction and includes;
- Control Unit - Coordinates the steps necessary to complete each instruction, and tells each part of the Execution unit what to do and when.
- Protection Test Unit - makes sure the operations performed by the Execution Unit are legal.
- Registers - working memory for the execution unit.
Memory Management unit (MMU) - The "virtual" location of an instruction (understood by software) is converted to a physical location (understood by the Bus Interface Unit).

CPU & Memory Interaction

(1). First Instruction

(2). Request first number

(3). Input first number

(4). Request second number

(5). Input second number

(6). CU issues multiply

(7). ALU sets up multiplication

(8). ALU performs multiplication

(9). Results to memory

(10). Print instruction

(11). Print result

(12). End instruction

Advanced (using Intel 80386)

Adding two numbers (summarised version follows)

Step 1

Step 2

Step 3

Step 4

Step 5

Step 6

Step 7

Step 8

Step 9

Summarised

Notes