The test will always fail with the version of MINIX 3 described here This is

The test will always fail with the version of minix 3

This preview shows page 99 - 101 out of 140 pages.

enabled. The test will always fail with the version of MINIX 3 described here. This is good, because no code is provided to be executed if the test succeeds. Keep in mind if you decide to modify MINIX 3 yourself to use LBA48 that you need to do more than just add some code here. You will have to make changes in many places to handle the 48-bit addresses. You might find it easier to wait until MINIX 3 has been ported to a 64-bit processor, too. But if a 128 GB disk is not big enough for you, LBA48 will give you access to 128 PB (Petabytes). Now we will briefly look at how a data transfer takes place at a higher level. W_prepare, which we have already discussed, is called first. If the transfer operation requested was for multiple blocks (that is, a DEV_GATHER or DEV_SCATTER request), w_transfer line 12848 is called immediately afterward. If the transfer is for a single block (a DEV_READ or DEV_WRITE request), a one element scatter/gather vector is created,
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and then w_transfer is called. Accordingly, w_transfer is written to expect a vector of iovec_t requests. Each element of the request vector consists of a buffer address and the size of the buffer, constrained that the size must be a multiple of the size of a disk sector. All other information needed is passed as an argument to the call, and applies to the entire request vector. [Page 295] The first thing done is a simple test to see if the disk address requested for the start of the transfer is aligned on a sector boundary (line 12863). Then the outer loop of the function is entered. This loop repeats for each element of the request vector. Within the loop, as we have seen many times before, a number of tests are made before the real work of the function is done. First the total number of bytes remaining in the request is calculated by summing the iov_size fields of each element of the request vector. This result is checked to be sure it is an exact multiple of the size of a sector. Other tests check that the starting position is not at or beyond the end of the device, and if the request would end past the end of the device the size of the request is truncated. All calculations so far have been in bytes, but on line 12876 a calculation is made of the block position on the disk, using 64 bit arithmetic. Note that although the variable used is named block, this is a number of disk blocks, that is, 512 byte sectors, not the "block" used internally by MINIX 3, normally 4096 bytes. After this one more adjustment is made. Every drive has a maximum number of bytes that can be requested at one time, and the request is scaled back to this quantity if necessary. After verifying that the disk has been initialized, and doing so again if necessary, a request for a chunk of data is made by calling do_transfer (line 12887).
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