A Brief Introduction to Memory

Memory_Intro

RAM, random access memory, is the ability to store/read values based on address inputs 
(versus say sequentially). It is also commonly referred to as voltile memory because it 
requires a constant supply of electricity to retain its contents.

The number of values that a RAM array can store is based on the number of address inputs.

With 1 address input we can store 2 values (0 or 1).
2 addresses = 4 values (00, 01, 10, 11)
3 addresses = 8 values (000, 001, 010, 100, 011, 101, 110, 111)
4 addresses = 16 values

num of values in RAM array = 2^num address inputs

An example RAM array:
10 addresses = 8196 bits organized as 1024(2^10) values of 8 bits each (1024x8)
Imagine this as a post office that has 1024 mailboxes that can store 1 byte 
(8 bits [00000000]) in each mail box.

1024(2^10) bytes is also known as a kilobyte (K or KB). In everyday life we typically 
deal with numbers based on the power of 10 where kilo would be a 1000 (10^3). Remember 
binary is based on powers of 2.

So for each address we add we double the amount of memory:
1 kilobyte = 1024 bytes = 2^10 bytes
2 kilobytes = 2048 bytes = 2^11 bytes
4 kilobytes = 4096 bytes = 2^12 bytes
8 kilobytes = 8192 bytes = 2^13 bytes
16 kilobytes = 16,384 bytes = 2^14 bytes
32 kilobytes = 32,768 bytes = 2^15 bytes
64 kilobytes = 65,536 bytes = 2^16 bytes
128 kilobytes = 131,072 bytes = 2^17 bytes
256 kilobytes = 262,144 bytes = 2^18 bytes
512 kilobytes = 524,288 bytes = 2^19 bytes
1024 kilobytes = 1,048,576 bytes = 2^20 bytes

1024 kilobytes (KB) = 1 megabyte (MB)

1 megabytes = 1,048,576 bytes = 2^20 bytes
2 megabytes = 2,097,152 bytes = 2^21 bytes
4 megabytes = 4,194,304 bytes = 2^22 bytes
8 megabytes = 8,388,608 bytes = 2^23 bytes
16 megabytes = 16,777,216 bytes = 2^24 bytes
32 megabytes = 33,554,432 bytes = 2^25 bytes
64 megabytes = 67,108,864 bytes = 2^26 bytes
128 megabytes = 134,217,728 bytes = 2^27 bytes
256 megabytes = 268,435,456 bytes = 2^28 bytes
512 megabytes = 536,870,912 bytes = 2^29 bytes
1024 megabytes = 1,073,741,824 bytes = 2^30 bytes

1024 megabytes (MB) = 1 gigabyte (GB)
1024 gigabytes (GB) [2^40] = 1 terabyte (TB)
1024 terabytes (TB) [2^50] = 1 petabyte (PB)
1024 petabytes (PB) [2^60] = 1 exabyte

Note: Sometimes you will hear referenced kilobits or megabits. This is not common when
talking about memory. This usually occurs in the discussion of data transmission over 
some medium like a wire "megabits per second".

Now for another example. For this example we have to remember that hexadecimal is base 16. 
So a hexadecimal digit (0-9,A-F) is equivilant to 4 bits. So a byte can be represented 
by 2 hexadecimal digits.

Binary(base 2) Hexadecimal(base 16) Decimal(base 10)
0000           0                    0
0001           1                    1
0010           2                    2
0011           3                    3
0100           4                    4
0101           5                    5
0110           6                    6
0111           7                    7
1000           8                    8
1001           9                    9
1010           A                    10
1011           B                    11
1100           C                    12
1101           D                    13
1110           E                    14
1111           F                    15

Since 2 hexadecimal digits can represent a byte we can use B6 to represent 10110110 
(1011 = B, 0110 = 6).

Lets take a quick look at converting hexadecimal to binary and then to decimal:
B6h
B = 1011, 6 = 0110
To convert 10110110 to decimal  1x128 + 0x64 + 1x32 + 1x16 + 0x8 + 1x4 + 1x2 + 0x1 = 182

So back to our memory example. Say we have a chip with:
16 addresses (2^16) = 65,536 bytes

Since the address is 2 bytes exactly we can address this with a hexadecimal range of 
0000h through FFFFh.