Tag: "Skill"
The binary, octal and hexadecimal number systems pervade all of computing. Every command is reduced to a sequence of strings of 1s and 0s for the computer to interpret. These commands seem like noise, garbage, especially with the sheer length of the information. Becoming familiar with binary and other number systems can make it much simpler to interpret the data.
Once you become familiar with the relationships between the number systems, you can manipulate the data in more convenient forms. Your ability to reason, solve and implement solid programs will grow. Patterns will begin to emerge when you view the raw data in hexadecimal. Some of the most efficient algorithms are based on the powers of two. So do yourself a favor and become more familiar with hexadecimal and binary.
Number Base Conversion and Place Value
I am going to skip this since you can refer to my previous entry for a detailed review of number system conversions[^].
Continuous Data
Binary and hexadecimal are more natural number systems for use with computers because they both have a base of 2 raised to some exponent; binary is 2^{1} and hexadecimal is 2^{4}. We can easily convert from binary to decimal. However, decimal is not always the most useful form. Especially when we consider that we don't always have a nice organized view of our data. Learning to effectively navigate between the formats, especially in your head, increases your ability to understand programs loaded into memory, as well as streams of raw data that may be found in a debugger or analyzers like Wireshark.
The basic unit of data is a byte. While it is true that a byte can be broken down into bits, we tend to work with these bundled collections and process bytes. Modern computer architectures process multiple bytes, specifically 8 for 64bit computers. And then there's graphics cards, which are using 128, 192 and even 256bit width memory accesses. While these large data widths could represent extremely large numbers in decimal, the values tend to have encodings that only use a fraction of the space.
Recognize Your Surroundings
What is the largest binary value that can fit in an 8bit field?
It will be a value that has eight, ones: 1111 1111
. Placing a space after every four bits helps with readability.
What is the largest hexadecimal value that can fit in an 8bit field?
We can take advantage of the power of 2 relationship between binary and hexadecimal. Each hexadecimal digit requires four binary bits. It would be very beneficial to you to commit the following table to memory:
Dec  0  1  2  3  4  5  6  7  8  9  10  11  12  13  14  15 
Bin  0000  0001  0010  0011  0100  0101  0110  0111  1000  1001  1010  1011  1100  1101  1110  1111 
Hex  0x0  0x1  0x2  0x3  0x4  0x5  0x6  0x7  0x8  0x9  0xA  0xB  0xC  0xD  0xE  0xF 
Now we can simply take each grouping of four bits, 1111 1111
, and convert them into hexdigits, FF
.
What is the largest decimal value that can fit in an 8bit field? This isn't as simple, we must convert the binary value into decimal. Using the number base conversion algorithm, we know that the eighth bit is equal to 2^{8}, or 256. Since zero must be represented the largest value is 2^{8}  1, or 255
.
Navigating the Sea of Data
Binary and hexadecimal (ok, and octal) are all number systems whose base is a power of two. Considering that computers work with binary, the representation of binary and other number systems that fit nicely into the logical data blocks, bytes, become much more meaningful. Through practice and the relatively small scale of the values that can be represented with 8bits, converting a byte a to decimal value feels natural to me. However, when the data is represented with 2, 4 and 8 bytes, the values can grow quickly, and decimal form quickly loses its meaning and my intuition becomes useless as to what value I am dealing with.
For example:
What is the largest binary value that can fit in an 16bit field?
It will be a value that has sixteen, ones: 1111 1111 1111 1111
.
What is the largest hexadecimal value that can fit in an 16bit field? Again, let's convert each of the blocks of fourbits into a hexdigit, FFFF
.
What is the largest decimal value that can fit in an 16bit field?
Let's see, is it 2^{16}  1, so that makes it 65355
, 65535
, or 65555
, it's somewhere around there.
Here's a realworld example that I believe many people are familiar with is the RGB color encoding for pixels. You could add a fourth channel to represent an alpha channel an encode RGBA. If we use onebyte per channel, we can encode all four channels in a single 32bit word, which can be processed very efficiently.
Imagine we are looking at pixel values in memory and the bytes appear in this format: RR GG BB
. It takes two hexadecimal digits to represent a single byte. Therefore, the value of pure green could be represented as 00 FF 00
. To view this same value as a 24bit decimal, is much less helpful, 65,280
.
If we were to change the value to this, 8,388,608
, what has changed? We can tell the value has changed by roughly 8.3M. Since a 16bit value can hold ~65K, we know that the third byte has been modified, and we can guess that it has been increased to 120 or more (8.3M / 65K). But what is held in the lower two bytes now? Our ability to deduce information is not much greater than an estimate. The value in hexadecimal is 80 00 00
.
The difference between 8,388,608
and 8,388,607
are enormous with respect to data encoded at the bytelevel:
8,388,608 
8,388,607 

00  80  00  00  7F  FF  
Now consider that we are dealing with 24bit values in a stream of pixels. For every 12bytes, we will have encoded 4 pixels. Here is a representation of what we would see in the data as most computer views of memory are grouped into 32bit groupings:
4,260,948,991 
2,568,312,378 
3,954,253,066 

FD  F8  EB  FF  99  15  56  3A  8C  B1  1D  0A  
Binary
I typically try to use binary only up to 8bits. Anything larger than that, I simply skip the first 8bits (1byte), and focus on the next 8bits. For example: 1111 1111
1111 1111
. As I demonstrated with the RGB color encoding, values do not always represent a single number. In fact, it is a stream of data. So whether there is one byte, four bytes, or a gigabytes worth of data, we usually process it either one byte or one word at a time. We actually break down decimal numbers into groups by using a comma (or some other regional punctuation) to separate thousands, e.g. 4,294,967,295
, 294
of what? Millions.
Binary manipulation can be found in many contexts. One of the most common is storing a collection of flags in an unsigned buffer. These flags can be flipped on and off with the Boolean flag operations of your programming language. Using a mask with multiple bits allows an enumerated value with more than two options to be encoded within the binary buffer. I'm not going to go to describe the mechanics here, I simply want to demonstrate that data is encoded in many ways, and there are many reasons for you to become proficient at picking apart bytestreams down to the bit.
Hexadecimal
It is just as beneficial to be able to convert between hexadecimal and decimal in your head from 1 to 255, especially if you ever deal with selecting colors for webpages or you edit images in programs like Photoshop. It only takes two hexdigits to represent an 8bit byte. If you memorize the values that are defined by the highorder hexdigit, reading hexadecimal byte values becomes almost as easy as reading decimal values. There are two tables listed below. The table on the left indicates the value mapping for the highorder and loworder hexdigits of a byte. The table on the right contains a set of landmark values that you will most certainly encounter and find useful:


Some of the numbers listed in the table on the right are more obvious than others for why I chose to included them in the map. For example, 100, that's a nice round number that we commonly deal with daily. When you run into 0x64, now you can automatically map that in your mind to 100 and have a referencepoint for its value. Alternatively, if you have a value such as 0x6C, you could start by calculating the difference: 0x6C  0x64 = 8; add that to 100 to know the decimal value is 108.
Some of you will recognize the values 127, 168, 192, 224 and 238. For those that do not see the relevance of these values, they are common octets found in landmark network IP addresses. The table below shows the name and common dotteddecimal for as well as the hexadecimal form of the address in both bigendian and littleendian format:
Name  IPv4 Address  
bigendian  littleendian  
localhost  127.0.0.1  0x7F000001  0x0100007F  
private range  192.168.x.x  0xC0A80000  0x0000A8C0  
multicast base address  224.0.0.0  0xE0000000  0x000000E0  
multicast last address  239.255.255.255  0xEFFFFFFF  0xFFFFFFEF 
One additional fact related to the IPv4 multicast address range, is the official definition declares any IPv4 address with the leading four bits set to 1110
to be a multicast address. 0xE is the hexdigit that maps to binary 1110
. This explains why the full multicast range of addresses is from 0xE0000000 to 0xEFFFFFFF, or written in decimal dotted notation as 224.0.0.0 to 239.255.255.255.
Octal
I'm sure octal has uses, but I have never ran into a situation that I have used it.
Actually, there is one place, which is to demonstrate this equality:
Oct 31 = Dec 25
Landmark Values
Binary has landmark values similar to the way decimal does. For example, 1's up to 10's, 10's through 100, then 1000, 1M, 1B ... These values are focused on powers of 10, and arranged in groups that provide a significant range to be meaningful when you roundoff or estimate. Becoming proficient with the scale of each binary bit up to 2^{10}, which is equal to 1024, or 1K. At this point, we can use the powers of two in multiple contexts; 1) Simple binary counting, 2) measuring data byte lengths.
I have constructed a table below that shows landmark values with the power of 2. On the left I have indicated the name of the unit if we are discussing bits and the size of a single value; for example, 8bits is a byte. I haven't had the need to explore data values larger than 64bits yet, but it's possible that some of you have. To the right I have indicated the units of measure used in computers when we discuss sizes. At 1024 is a kilobyte, and 1024 kilobytes is officially a megabyte (not if you're a hard drive manufacturer...). I continued the table up through 2^{80}, which is known as a "yottabyte." Becoming familiar with the landmarks up to 32bits is probably enough for most people. To the far right I converted the major landmark values into decimal to give you a sense of size and scale for these numbers.
Unit (bits)  Binary Exponent  Unit (bytes)  Place Value  Largest Value  
Bit (b)  2^{0}  Byte (B)  1  
2^{1}  Word  2  
2^{2}  Doubleword  4  
Nibble  2^{3}  Quadword  8  
2^{4}  16  
2^{5}  32  
2^{6}  64  
Byte (B)  2^{7}  128  255  
2^{8}  256  
2^{9}  512  
2^{10}  Kilobyte (KB)  1024  
2^{20}  Megabyte (MB)  1024^{2}  1,048,576  
2^{30}  Gigabyte (GB)  1024^{3}  1,073,741,824  
Doubleword  2^{31}  1024^{3}·2  4,294,967,295  
2^{32}  1024^{3}·2^{2}  
2^{40}  Terabyte (TB)  1024^{4}  
2^{50}  Petabyte (PB)  1024^{5}  
2^{60}  Exabyte (EB)  1024^{6}  
Quadword  2^{63}  1024^{6}·2^{3}  9,223,372,036,854,775,807  
2^{70}  Zettabyte (ZB)  1024^{7}  
2^{80}  Yottabyte (YB)  1024^{8} 
Summary
Looking at a large mass of symbols, such as a memory dump from a computer, can appear to be overwhelming. We do have tools that we use to develop and debug software help us organize and make sense of this data. However, these tools cannot always display this data in formats that are helpful for particular situations. In these cases, understanding the relationships between the numbers and their different representations can be extremely helpful. This is especially true when the data is encoded at irregular offsets. I presented a few common forms that you are likely to encounter when looking at the values stored in computer memory. Hopefully you can use these identity and navigation tips to improve your development and debugging abilities.
There are many different philosophies with regards to how source code should be commented. The gamut of these philosophies range from "Every single statement must have a comment." to "Comments are useless; avoid them at all costs!" I am not even going to attempt to explain the disparity of range. However, I will attempt to address and suggest potential solutions the reasons that are often cited for these extreme approaches to commenting code.
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