Input a hex digit and return its' decimal equivalent as output
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An assembly language is a programming language that can be used to directly tell the computer what to do. An assembly language is almost exactly like the machine code that a computer can understand, except that it uses words in place of numbers.
A computer cannot really understand an assembly program directly. However, it can easily change the program into machine code by replacing the words of the program with the numbers that they stand for. A program that does that is called an assembler. Programs written in assembly language are usually made of continuing on binary input and output in assembly languagewhich are small tasks that the computer performs when it is running the program. They are called instructions because the programmer uses them to instruct the computer what to do.
The part of the computer that follows the instructions is the processor. The assembly language of a computer is a low-level language, which means that it can only be used to do the simple tasks that a computer can understand directly.
In order to perform more complex tasks, one must tell the computer each of the simple tasks that are part of the complex task. For example, a computer does not understand how to print a sentence on its screen. Instead, a program written in assembly continuing on binary input and output in assembly language tell it how to do all of the small steps that are involved in printing the sentence.
Such an assembly program would be composed of many, many instructions, that together do something that seems very simple and basic to a human. This makes it hard for humans to read an assembly program.
In contrast, a high-level programming language may have a single instruction such as PRINT "This is a sentence" that will tell the computer to perform all of the small tasks for you.
When computer scientists first built programmable machines, they programmed them continuing on binary input and output in assembly language in machine code, which is a series of numbers that instructed the computer what to do. Writing machine language was very hard to do and took a long time, so eventually assembly language was made.
Assembly language is easier for a human to read and can be written faster, but it is still much harder for a human to use than a high-level programming language which tries to mimic human language. To program in machine code, the programmer needs to know what each instruction looks like in binary or hexadecimal.
Although it is easy for a computer to quickly figure out what machine code means, it is hard for a programmer. Each instruction can have several forms, all of which just look like a bunch of numbers to people. Any mistake that someone makes while writing machine code will only be noticed when the computer does the wrong thing. Figuring out the mistake is hard because most people cannot tell what machine code means by looking at it. An example of what machine code looks like:.
This hexadecimal machine code tells an x86 computer processor to add 42 to the accumulator. It is very difficult for a person to read and understand it continuing on binary input and output in assembly language if that person knows machine code.
With assembly language, each instruction can be written as a short word, called a mnemonicfollowed by other things like numbers or other short words. The mnemonic is used so that the programmer does not have to remember the exact numbers in machine code continuing on binary input and output in assembly language to tell the computer to do something.
Examples of mnemonics in assembly language include addwhich adds data, and movwhich moves data from one place to another.
Because 'mnemonic' is an uncommon word, the phrase instruction type or just instruction is sometimes used instead, often incorrectly. The words and numbers after the first word give more information about what to do. For instance, things following an add might be what two things to add together and the things following mov say what to move and where to put it. Assembly language also allows programmers to write the actual data the program uses in easier ways.
Most assembly languages have support for easily making numbers and text. In machine code, each different type of number like positive, negative or decimal, would have to be manually converted into binary and text would have to be defined one letter at a time, as numbers.
Assembly language provides what is called an abstraction of machine code. When using assembly, programmers do not need to know the details of what numbers mean to the computer, the assembler figures that out instead. Assembly language actually still lets the programmer use all the features of the processor that they could with machine code.
In this sense, assembly language has a very good, rare trait: Because of this, machine code is almost never used as a programming language. When programs are finished, they have already been transformed into machine code so that the processor can actually run them. Sometimes, however, if the program has a bug mistake in it, programmers will want to be able to tell what each part of the machine code is doing.
Disassemblers are programs that help programmers do that by transforming the machine code of the program back into assembly language, which is much easier to understand. Disassemblers, which turn machine code into assembly language, do the opposite of assemblers, which turn assembly language into machine code. An understanding of how computers are organized, how they seem to work at a very low level, is needed to understand how an assembly language program works.
At the most simplistic level, computers have three main parts:. In most computers, memory is divided up into bytes. Each byte contains 8 bits. Each byte in memory also has an address which is a number that says where the byte is in memory. The first byte in memory has an address of 0, the next one has an address of 1, and so on. Dividing memory into bytes makes it byte addressable because each byte gets a unique address. Addresses of byte memories cannot be used to refer to a single bit of a byte.
A byte is the smallest piece of memory that can be addressed. Even though an continuing on binary input and output in assembly language refers to a particular byte in memory, processors allow for using several bytes of memory in a row. The most common use of this feature is to use either 2 or 4 bytes in a row to represent a number, usually an integer.
Single continuing on binary input and output in assembly language are sometimes also used to represent integers, but because they are only 8 bits long, they can only hold 2 8 or different possible values. Using 2 or 4 bytes in a row raises the number of different possible values to be 2 16or 2 32, respectively.
When a program uses a byte or a number of bytes in a row to represent something like a letter, number, or anything else, those bytes are called an object because they are all part of the same thing. Even though objects are all stored in identical bytes of memory, they are treated as though they have a 'type', which says how the bytes should be understood: Machine code can also be thought of as a type that is interpreted as instructions.
For instance, it is not valid to store a negative number in a positive number object and it is not valid to store a fraction in an integer. An address that points to is the address of a multi-byte object is the address to the first byte of that object — the byte that has the lowest address. An assembly language program needs to keep track of which memory addresses hold which objects, and how big those objects are. A program that does so is type safe because it only does things to objects that are safe to do on their type.
Note that most programs do not actually explicitly store what the type of an object is, they just access objects consistently - the same object is always treated as the same type. The processor runs executes instructions, which are stored as machine code in main memory. As well as being able to access memory for storage, most processors have a few small, fast, fixed-size spaces for holding objects that are currently being worked with.
These spaces are called registers. Processors usually execute three types of instructions, although some instructions can be a combination of these types. Below are some examples of each type in x86 assembly language. The following x86 assembly language instruction reads loads a 2-byte object from the byte at address 0x in hexadecimal into a bit register called 'ax':.
In this assembly language, square brackets around a number or a register name mean that the number should be used as an address to the data that should be used. The use of an address to point to data is called indirection. In this next example, without the square brackets, another register, bx, actually gets the value 20 loaded into it. If the operands the things that come after the mnemonicappear in the reverse order, an instruction that loads something from memory instead writes it to memory:.
Here, the memory at address h gets the value of ax. If this example is executed right after the previous one, the 2 bytes at h continuing on binary input and output in assembly language h will be a 2 byte integer with the value of Here, 42 and ax are added together and the result is stored back in ax.
In x86 assembly it is also possible to combine a memory access and mathematical operation like this:. This instruction adds the value of the 2 byte integer stored at h to ax and stores the answer in ax.
This instruction computes the or of the contents of the registers ax and bx and stores the result back into ax. Usually, instructions are executed in the order they appear in memory, which is the order they are typed in the assembly code. The processor just executes them one after another. However, in order for processors to do complicated things, they need to execute different instructions based on what the data they were given is.
The ability of processors to execute different instructions depending on something's outcome is called branching. Instructions that decide what the next instruction should be are called branch instructions. In this example, suppose someone wants to calculate the amount of paint they will need to paint a square with a certain side length. However, due to economy of scale the paint store will not sell them any less than amount of paint needed continuing on binary input and output in assembly language paint a x square.
To figure continuing on binary input and output in assembly language the amount of paint they will need to get based on the length of the square they want to paint, they come up with this set of steps:. This example introduces several new things, but the first two instructions are familiar.
They copy the value of ax into bx and then subtract from bx. One of the new things in this example is called a label, a concept found in assembly languages in general. Labels can be anything the programmer wants unless it is the name of an instruction, which would confuse the assembler. In this example, the label is 'continue'. It is interpreted by the assembler as the address of an instruction. In this case, it is the address of mult ax. Another new concept is that of flags.
On x86 processors, many instructions set 'flags' in the processor that can be used by the next instruction to decide what to do. In this case, if bx was less thansub will set a flag that says the result was less than zero.