What is Input Buffering?
With reference to computer memory, input buffering refers to the process of storing all incoming data and information in the input block or the input area before these are sent for further processing by the Central Processing Unit.
Technically, it also refers to the process used in designing the compilers which is necessary to process and transfer input characters.
Understanding Input Buffering
The input buffering process stores the data of the input string in a particular region called the input area before sending it for processing to the CPU.
This process helps the analyzer to scan the input string from left to right using one character at a time while identifying a token.
The input buffering process typically involves examining the character or blank space past the token or ‘int’ before determining the token itself.
For this, the analyzer uses two specific pointers, such as:
- The begin pointer or bptr – This pointer points to the start of the string that is to be read.
- The lookahead pointer or lptr – This pointer moves ahead, which is why it is also called the forward pointer, to search for the end of a particular token.
When the processing is completed in the input buffer, both the begin and lookahead pointers will search for the subsequent token in it. This process will continue through the input buffering operation.
Working Process
Ideally, in the input buffering process, the input buffer is split into two halves, and each of these halves supports the overall process in different ways, depending on the movement of the lptr, as explained below:
- When the lookahead pointer moves to the halfway point of the first half, the other half is filled with fresh characters that must be read.
- When the lookahead pointer moves to the right side of the input buffer in the second half, the first half will be filled with new characters.
This Process Continues
Ideally, in the input buffering process, two specific things are used for specific purposes such as:
- Sentinels – This helps in making a check and ensuring that the other half of the buffers is not converted while every time the forward pointer is converted. If that happens, then it has to be reloaded.
- Buffer pairs – This helps in decreasing the overhead amount. There are two buffers of N-character size each which are reloaded alternately.
The input buffering process is quite efficient and offers a lot of benefits, which are explained in the later section of this article, but it also has a serious downside.
The lookahead amount is limited. This makes it quite hard to distinguish tokens if the lookahead pointer has to traverse a greater distance than the length of the buffer.
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In that case it has to wait for the character following the exact parenthesis to find out whether the DECLARE is an array name or a keyword.
What is the Importance of Input Buffering?
The most important role played by the input buffering technique is that it helps in finding the precise lexeme when it needs to send more than one character to the next lexeme.
Ideally, this special buffering method helps with a lot of things such as:
- In handling large lookaheads securely
- In expediting the operations of the lexical analyzer
- In using sentinels for marking the buffer end
- In decreasing the volume of overhead
- In processing an input character
- In transferring those processed characters
- In improving the involvement of sentinels
Overall, the input buffering technique helps in saving a considerable amount of time for checking the buffer ends.
This is because in the input buffering process only one test is usually done to find out whether the pointer is pointing to the end of an input string.
Further tests are carried out only when the pointer reaches the halfway point of the buffer.
Also, the average number of tests carried out in the input buffering process for each input character is very close to 1 because there are N-number of input characters run into while reaching to the end of the string.
Why Input Buffering is Required in Lexical Analysis?
Lexical Analysis needs input buffering because it needs to access secondary memory every time it has to identify tokens, which takes a lot of time.
Input buffering helps in storing the input strings in the buffers so that they can be scanned as and when needed quite readily and quickly by Lexical Analysis.
The process is costly and time-consuming. This is because each and every character of the input string is needed to be scanned by the analyzer from left to right to read it from the memory.
It needs a lot of system calls, and the number may vary based on the size of the specific program.
This will make the system very slow, over and above the fact that the whole process will be much less efficient, which is why the input buffering technique is required for Lexical Analysis.
Since all the incoming information and data is stored in the specific input buffer before it is sent to the Central Processing Unit (CPU) for processing, it will reduce the overall time.
Conclusion
Input buffering, as you can see, is a very useful process that helps in determining the correct character in an input string which can be done before these are sent to the CPU for processing.
Therefore, this process makes the entire operation much more secure, efficient and quick, apart from saving significant overhead.