The effect of the # wires in the data bus

Recall on how the data bus is used:

  • The data bus (= wires) is used to transfer data (binary digits) between the CPU and memory

    Example: data stored in memory address 4 (11111111 00000000) is transfered over the data bus to the CPU

The effect of the # wires in the data bus

Important fact about electrical wires:

  • Each bit of the data must be transfered on a different wire of the data bus !!!

    (When you send more than 1 electrical signal on a wire, it will cause a short circuit !)

 
 

The width of the databus:

  • Width of the databus = # wires used in the data bus      

    (The more wires you have, the more costly the computer system will become.)

The effect of the # wires in the data bus
 

Effect of the width (= number of wires) of the databus:

  • A databus that consists of 8 bits (= 8 wires), can transfer 1 byte of data per read/write operation

  • A databus that consists of 16 bits (= 16 wires), can transfer 2 bytes of data per read/write operation

  • A databus that consists of 32 bits (= 32 wires), can transfer 4 bytes of data per read/write operation

  • And os on...
 

 

Analogy:   the number of wires in a data bus is similar to the number of lanes on a high way

The effect of the # wires in the data bus - an analogy

One wire in the data bus can transfer one bit of data - a wire lane in a high way:

 

More wires in the data bus allow the Computer System to transfer more data bits in a single data transfer operation

Width of the data bus in present day computer systems
 

Current trend:

  • All computers made in 2019 has 64 bits data buses (i.e.: 64 bits machines)

  • Back in 2012, computers used to have 32 bits data buses
 
 

Note:

  • The width of the data bus is always a power of 2 (i.e.: 2n)

    (You will need to take CS355 to understand why.... It has something to do with how the memory selection circuitry is constructed...)

The effect of the # wires in the address bus

Recall on how the address bus is used:

  • The address bus (= wires) is used to send the address (location) information to memory

    Example: when the CPU want to read data stored in memory location (address) 4, the CPU sends the value 4 (in binary - explained later) on the address bus

The effect of the # wires in the address bus

  • Recall that:

      • Each wire of the address bus can transmit 1 bit of the address

     

     

     

     

     

     

     

     

The effect of the # wires in the address bus

  • Recall that:

      • Each wire of the address bus can transmit 1 bit of the address

  • Therefore:

      • An address bus that contains n wires can transmit an n bits address

     

     

     

     

     

The effect of the # wires in the address bus

  • Recall that:

      • Each wire of the address bus can transmit 1 bit of the address

  • Therefore:

      • An address bus that contains n wires can transmit a n bits address

  • Because of the fact that:

      • Each memory byte is identified by a unique memory address

     

     

The effect of the # wires in the address bus

  • Recall that:

      • Each wire of the address bus can transmit 1 bit of the address

  • Therefore:

      • An address bus that contains n wires can transmit a n bits address

  • Because of the fact that:

      • Each memory byte is identified by a unique memory address

  • Therefore:

      • A address bus that consists of n wires (= bits), can identify 2n different memory locations

    I.e.: the # wires in the address bus determines the maximum size of the computer memory !

The effect of the # wires in the address bus - Summary
 

Summary:

  • The width of the address bus will determine the maximum size of the memory that a computer can access (= use)

      • The wider (= more wires) the address bus, the more memory a computer can use.

        (More memory allows the computer to store more data and solve larger size problems, e.g., sort more data)

Historical fact
 

The first generation of Personal Computers (around 1980) has an address bus with 16 wires:

  • A address bus that consists of 16 wires (= bits), can transmit 216 (= 64K) different addresses

  • Consequently:

      • The largest memory that such a computer system can use (= access) is 64 K bytes

 

 

Historical fact: the Apple II personal computer - very popular in around 1980 - had a 16 bit address bus and can have at most 64 K byte RAM memory

Address buses of present day computers
 

  • 1 G byte = 1,073,741,824 bytes = 230 bytes

    You need 30 wires in the address bus to identify 1 G bytes of memory

  • All PCs has at least 33 wires address buses and can access (= use) 8 G (= 233) bytes memory

  • Some (high end) PCs has more than 34 or 35 bits address bus and can use maximum 16 or 32 GBytes memory
 
 

An infamous quote uttered by Bill Gates:

  • "When we set the upper (memory) limit of PC-DOS at 640K, we thought nobody would ever need that much memory." (1985)

    source of the quote