Addressability

2. Addressability
Memory
11110011
Address
00000000
00000001
00000010
00000011
00000100
00000101
00000110
00000111
00001000
00001001
Each memory location has to
have an address
Each address is a unique
identifier for that specific
memory location
The way in which the computer
identifies memory locations is
known as addressability

3. Memory locations
Memory Address
0
1
The width of the address bus determines
how many memory locations can be
addressed
A 1 bit address bus can access 2 memory
locations
Processor
1 Bit
Address Bus

4. Memory locations
Memory Address
10
11
The width of the address bus determines
how many memory locations can be
addressed
A 2 bit address bus can access 4 memory
locations
Processor
00
012 Bit
Address Bus

5. Memory locations
Memory Address
110
111
The width of the address bus determines
how many memory locations can be
addressed
A 3 bit address bus can access 8 memory
locations
Processor
100
101
010
011
000
0013 Bit
Address Bus

6. Memory locations
How many memory locations could a computer with an 8 bit address bus
access?
How many memory locations could a computer with an 16 bit address bus
access?
How many memory locations could a computer with an 32 bit address bus
access?

7. Memory locations
How many memory locations could a computer with an 8 bit address bus
access?
28
= 256 memory locations
How many memory locations could a computer with an 16 bit address bus
access?
How many memory locations could a computer with an 32 bit address bus
access?

8. Memory locations
How many memory locations could a computer with an 8 bit address bus
access?
28
= 256 memory locations
How many memory locations could a computer with an 16 bit address bus
access?
216
= 65,536 memory locations
How many memory locations could a computer with an 32 bit address bus
access?

9. Memory locations
How many memory locations could a computer with an 8 bit address bus
access?
28
= 256 memory locations
How many memory locations could a computer with an 16 bit address bus
access?
216
= 65,536 memory locations
How many memory locations could a computer with an 32 bit address bus
access?
232
= 4,294,967,296 memory locations

10. Calculating addressable memory
If a computer has an 3 bit address bus, we know that it has 23
= 8 memory locations
But how much actual memory is that?
Memory Address
110
111
Processor
100
101
010
011
000
0013 Bit
Address Bus

11. Calculating addressable memory
In order to answer that question, we need to know how many bits can be
stored in each memory location.
This is determined by the width of the data bus.
Memory Address
110
111
Processor
100
101
010
011
000
0013 Bit
Address Bus
Data Bus

12. Calculating addressable memory
A computer with a data bus 2 bits wide would store 2 bits in each memory
location.
Memory Address
110
111
Processor
100
101
00
00
11
00
10
11
01
01
010
011
000
0013 Bit
Address Bus
2 Bit
Data Bus

13. Calculating addressable memory
Therefore, a computer with a 2 bit data bus and a 3 bit address bus would
have an addressable memory of
8 x 2 = 16 bits
= 2 bytes
Memory Address
110
111
Processor
100
101
00
00
11
00
10
11
01
01
010
011
000
0013 Bit
Address Bus
2 Bit
Data Bus

14. Calculating addressable memory
Total addressable memory =
the number of storage locations x the size of each storage location

15. Calculating addressable memory
Example:
Calculate the total addressable memory of a computer with a 16 bit address
bus and an 8 bit data bus.

16. Calculating addressable memory
Example:
Calculate the total addressable memory of a computer with a 16 bit address
bus and an 8 bit data bus.
addressable memory =
Number of storage locations x the size of each location
= 216
bits x 8 bits

17. Calculating addressable memory
Example:
Calculate the total addressable memory of a computer with a 16 bit address
bus and an 8 bit data bus.
addressable memory =
Number of storage locations x the size of each location
= 216
x 8 bits
= 65536 x 1 byte
= 65536 bytes
= 64 kb

18. Credits
Higher Computing – Computer Structure – Addressability
Produced by M. Cunningham for the City of Edinburgh Council 2010
All images licenced under Creative Commons 3.0
• Metropolitan Vickers, Trafford Park stationery by sludgegulper on Flickr
• Calculator Dreams by Keith Ramsey (RambergMediaImages on Flickr)