Thus the MINIX 3 source code to support partitions on IBM computers is put in drvlib.c, rather than being included in driver.c, for two reasons. First, not all disk types support partitions. As noted earlier, the memory driver links to driver.o but does not use the functions compiled into drvlib.o. Second, this makes it easier to port MINIX 3 to different hardware. It is easier to replace one small file than to edit a large one with many sections to be conditionally compiled for different environments.
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 keyborads display nothing when keys are struck. It is therefore up to the software to display the input. This process is called echoing.
P226
Why does it need a internal buffer? There are two reasons. First, by doing internal buffering, the disk controller can verify the checksum before starting a transfer. If the checksum is incorrect, an error is signaled and no transfer to memory is done.
The second reason is that once a disk has started, the bits keep arriving from the disk at a constant rate, whether the controller is ready for them or not. If the controller tried to write data directly to memory, it would have to go over the system bus for each word transferred. If the bus were busy due to some other device using it, the controller would have to wait. If the next disk word arrived before the previous one had been stored, the controller would have to store it somewhere. If the bus were very busy, the controller might end up storing quite a few words and having a lot of administration to do as well. When the block is buffered internally, the bus is not needed until the DMA begins, so the design of the controller is much simpler because the DMA transfer to memory i not time critical.
P480
File Systems
From the user's standpoint, the most import aspect of a file system is how it appers to them, that is, what consitutes a file, how files are named and pretected, what operations are allowed on files, and so on. The details of whether linked lists or bitmaps are used to keep track of free storage and how many sectors there are in a logical block are of less interest, although they are of great importance to the designers of the file system.