Rather than starting with binary, let’s first examine a number system we’re already comfortable using, the decimal number system, which uses a base of 10. Base 10 implies two important properties for the interpretation and representation of decimal values.

For the example value 8425, the 5 in the "ones" place contributes 5 (5 × 10⁰). The 2 in the "tens" place contributes 20 (2 × 10¹). The 4 in the "hundreds" place contributes 400 (4 × 10²), and, finally, the 8 in the "thousands" place contributes 8000 (8 × 10³). More formally, one could express 8425 as

This pattern of increasing exponents applied to a base of 10 is the reason why it’s called a base 10 number system. Assigning position numbers to digits from right to left starting with d₀ implies that each digit d_i contributes 10ⁱ to the overall value. Thus, the overall value of any N-digit decimal number can be expressed as:

Fortunately, as we’ll soon see, a very similar pattern applies to other number systems.

While you may never have considered the specific formula describing decimal numbers as powers of 10, the concept of { ones, tens, hundreds, etc. } places should hopefully feel comfortable. Luckily, similar terminology applies to other number systems, like binary. Of course, the base is different in other number systems, so each digit position contributes a different amount to its numerical value.

A binary number system uses a base of 2 instead of decimal’s 10. Analyzing it the same way that we just did for decimal reveals several parallels (with 2 substituted for 10):

The first point implies that counting in binary follows the same pattern as decimal: by simply enumerating the values and adding digits (bits). Because this section focuses on unsigned numbers (zero and positives only), it’s natural to start counting from zero. Table 1 shows how to count the first few natural numbers in binary. As you can see from the table, counting in binary quickly increases the number of digits. Intuitively, this growth makes sense, since each binary digit (two possible values) represents less information than a decimal digit (10 possible values).

The second point about labeling digits looks really familiar! In fact, it’s so similar to decimal that it leads to a nearly identical formula for interpreting a binary number. Simply replace the 10 at the base of each exponent with a 2:

Applying this formula yields the unsigned interpretation of any binary number. For example, take 0b1000:

Here’s a longer one-byte example, 0b10110100:

Thus far, we’ve examined two number systems, decimal and binary. Decimal is notable due to its comfort for humans, whereas binary matches the way data is stored in hardware. It’s important to note that they are equivalent in their expressive power. That is, there’s no number you can represent in one system that you can’t represent in the other. Given their equivalence, it may surprise you that we’re going to discuss one more number system: the base 16 hexadecimal system.

With two perfectly good number systems, you may wonder why we need another. The answer is primarily convenience. As shown in Table 1, binary bit sequences quickly grow to a large number of digits. Humans tend to have a tough time making sense of long sequences containing only 0’s and 1’s. And whereas decimal is more compact, its base of 10 is a mismatch with binary’s base 2.

Decimal doesn’t easily capture the range that can be expressed using a fixed number of bits. For example, suppose that an old computer uses 16-bit memory addresses. Its valid addresses range from 0b0000000000000000 to 0b1111111111111111. Represented in decimal, the addresses range from 0 to 65535. Clearly, the decimal representations are more compact than the long binary sequences, but unless you memorize their conversions, it’s more difficult to reason about the decimal numbers. Both problems only get worse on modern devices, which use 32- or 64-bit addresses!

These long bit sequences are where hexadecimal’s base 16 shines. The large base allows each digit to represent enough information for hexadecimal numbers to be compact. Furthermore, because the base is itself a power of two (2⁴ = 16), it’s easy to map hexadecimal to binary, and vice versa. For the sake of completeness, let’s analyze hexadecimal in the same way as decimal and binary:

Unsurprisingly, the same trusty formula for interpreting a number applies to hexadecimal, with 16 as the base:

For example, to determine the decimal value of 0x23C8:

Conceptually, there are infinitely many unsigned integers. In practice, a programmer must choose how many bits to dedicate to a variable prior to storing it, for a variety of reasons:

The number of bits used to store an integer dictates the range of its representable values. Figure 2 depicts how we might conceptualize infinite and finite unsigned integer storage spaces.

Attempting to store a larger value to a variable than the variable’s size allows is known as integer overflow. This chapter defers the details of overflow to a later section. For now, think of it like a car’s odometer that "rolls over" back to zero if it attempts to increase beyond its maximum value. Similarly, subtracting one from zero yields the maximum value.

At this point, a natural question to ask about unsigned binary is "What’s the largest positive value that N bits can store?" In other words, given a sequence of N bits that are all 1, what value does the sequence represent? Reasoning about this question informally, the analysis in the previous section shows that N bits yield 2^N unique bit sequences. Since one of those sequences must represent the number 0, that leaves 2^N - 1 positive values ranging from 1 to 2^N - 1. Thus, the maximum value for an unsigned binary number of N bits must be 2^N - 1.

For example, 8 bits provide 2⁸ = 256 unique sequences. One of those sequences, 0b00000000, is reserved for 0, leaving 255 sequences for storing positive values. Therefore, an 8-bit variable represents the positive values 1 through 255, the largest of which is 255.