respond to the need for flexibility of keyboard layouts to provide for

Respond to the need for flexibility of keyboard

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respond to the need for flexibility of keyboard layouts to provide for different languages, many operating systems provide for loadable keymaps or code pages, which make it possible to choose the mapping between keyboard codes and codes delivered to the application, either when the system is booted or later. [Page 309]
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If the terminal is in canonical (i.e., cooked) mode, characters must be stored until an entire line has been accumulated, because the user may subsequently decide to erase part of it. Even if the terminal is in raw mode, the program may not yet have requested input, so the characters must be buffered to allow type ahead. (System designers who do not allow users to type far ahead ought to be tarred and feathered, or worse yet, be forced to use their own system.) Two approaches to character buffering are common. In the first one, the driver contains a central pool of buffers, each buffer holding perhaps 10 characters. Associated with each terminal is a data structure, which contains, among other items, a pointer to the chain of buffers for input collected from that terminal. As more characters are typed, more buffers are acquired and hung on the chain. When the characters are passed to a user program, the buffers are removed and put back in the central pool. The other approach is to do the buffering directly in the terminal data structure itself, with no central pool of buffers. Since it is common for users to type a command that will take a little while (say, a compilation) and then type a few lines ahead, to be safe the driver should allocate something like 200 characters per terminal. In a large-scale timesharing system with 100 terminals, allocating 20K all the time for type ahead is clearly overkill, so a central buffer pool with space for perhaps 5K is probably enough. On the other hand, a dedicated buffer per terminal makes the driver simpler (no linked list management) and is to be preferred on personal computers with only one or two terminals. Figure 3-28 shows the difference between these two methods. Figure 3-28. (a) Central buffer pool. (b) Dedicated buffer for each terminal. (This item is displayed on page 310 in the print version)
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Although the keyboard and display are logically separate devices, many users have grown accustomed to seeing the characters they have just typed appear on the screen. Some (older) terminals oblige by automatically displaying (in hardware) whatever has just been typed, which is not only a nuisance when passwords are being entered but greatly limits the flexibility of sophisticated editors and other programs. Fortunately, PC keyboards display nothing when keys are struck. It is therefore up to the software to display the input. This process is called echoing. Echoing is complicated by the fact that a program may be writing to the screen while the user is typing. At the very least, the keyboard driver has to figure out where to put the new input without it being overwritten by program output.
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