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In the following paragraph its talking about memory and it throws M into the L X W of memory notation and i'm confused on how 4M = 2^22. Thanks in advance

PARAGRAPH:

Memory is built from random access memory (RAM) chips. (We cover memory in detail in Chapter 6.) Memory is often referred to using the notation length X width (L X W). For example, 4M X 8 means the memory is 4M long (it has 4M = 2^2 X 2^20 = 2^22 items) and each item is 8 bits wide (which means that each item is a byte). To address this memory (assuming byte addressing), we need to be able to uniquely identify 2 ^ 22 different items.

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M in this context is just defined to be $2^{20}$, so if you have M items (bits, bytes, whatever), you'll have $2^{20}$ of them. The letter was chosen since it's the first letter in the term mega, meaning a million. Since $2^{10}=1024$ is close to $1000$, we'll have $2^{20}=(2^{10})^2≈(1000)^2=1000000$. M is sometimes called a "binary million".

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