The format of the request packet for the WRITE, READ, and WRITE VERIFY commands is:

┌──────────────────────────────┬──────────────────────────────┐
│Field                         │Length                        │
├──────────────────────────────┼──────────────────────────────┤
│Request Header                │32 BYTES                      │
├──────────────────────────────┼──────────────────────────────┤
│Start Block                   │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│Block Count                   │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│BlocksXferred                 │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│Flags                         │WORD                          │
├──────────────────────────────┼──────────────────────────────┤
│SGDescrCount                  │WORD                          │
├──────────────────────────────┼──────────────────────────────┤
│Reserved                      │DWORD                         │
├──────────────────────────────┼──────────────────────────────┤
│SGDescriptors                 │ARRAY                         │
└──────────────────────────────┴──────────────────────────────┘

Request Header

The fixed length request header.
Start Block
The starting disk block for the data transfer operation. A disk block is defined as a 512-byte logical disk sector.
Block Count
The number of 512-byte blocks to be transferred.
BlocksXferred
The number of 512-byte blocks successfully transferred by the driver. This field is updated before the request and request list notification routines are called and before the Status field is marked as Done.
Flags
A bit field of command-specific control flags. The following flags have been defined:

Bit 0

If set, write through. Defeats any lazy-write caching performed by the physical device driver. Notice that lazy-write, through battery-backed RAM, is permitted, even if this bit is set.
Bit 1
If set, cache request on outboard controller cache.
Bits 2-15
Reserved, set to 0.
SGDescrCount
The number of scatter/gather descriptors in the SGDescriptors field.
SGDescriptors
An array of scatter/gather descriptors describing the buffers for data transfer specified by the command.

The File System Driver (FSD) guarantees the following to be true:

──────────────────────────────────────────────────────────────────────────

BlockCount * 512 equals the sum of the BufferSize fields in SGDescriptors.

──────────────────────────────────────────────────────────────────────────

In addition, buffers are typically DWORD aligned. The physical device driver should be optimized for this case, but should not rely upon it.

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