Final Year Project:
Using Linux Filesystems Under Windows
Chris Bryden
BEng. Electronics and Software Engineering
School of Computer Science
University of Birmingham
38
5.10 Layer 0: The ReadLogicalSector Function
The ReadLogicalSector function provides the interface between layer 0
and layer 1. It reads a logical sector from a specified drive number and partition
number. It accepts five arguments:
·
=bDrive - The Drive number, 0x80 for 1
st
HDD, 0x81 for 2nd, etc.
·
=bPart - The partition number, 0 for 1
st
partition, 1 for the 2nd, etc.
·
=chSecNum - The sector number of the sector to be read.
·
=lpBuff - The buffer into which to read the sector data.
·
=chBuffSize - The size of the buffer
This function must calculate the CHS reference that corresponds to the
logical sector to be read, then calls ReadPhysicalSector to perform the disk read.
First the absolute sector number (sectors numbered from 0 from the 1st
sector on the disk) is calculated. This is simply done by adding the ulStartSec
value for the specified partition to the logical sector number, ulSecNum. This
absolute sector number must then be converted to a CHS reference for use by
the ReadLogicalSector function. This is done using the calculation shown below:
For more information on how the absolute sector number translates to a CHS
reference, please see section 2. For more information on the DiskInfo array,
please see section 5.7
Once the CHS reference has been calculated, the ReadPhysicalSector
function is called to perform the disk read.
5.11 Layer 1: The ReadBlock Function
This function reads a logical block from the mounted filesystem. It must
take into account the block size, which can vary from filesystem to filesystem.
Typical values for the block size are 1024, 2048 or 4096 bytes. This function
provides the interface between layer 0 and layer 1 and is used by any operation
in layer 2 that needs to access the filesystem on disk. It calls the layer 0 function
ReadLogicalSector to perform the disk reads, one or more of which may be
necessary to read a complete block. To read the correct sectors from disk, two
values need to be calculated. The start sector (ulSector) and the number of
sectors to read (uNumSec). These are calculated as shown below:
//perform sector # to CHS translation
ulPhSec = chSecNum % DiskInfo[iDrive].rgDskParams[3] + 1;
chSecNum /= DiskInfo[iDrive].rgDskParams[3];
ulPhHead = chSecNum % (DiskInfo[iDrive].rgDskParams[2] + 1);
ulPhCyl = chSecNum / (DiskInfo[iDrive].rgDskParams[2] + 1);
ulSector = ulBlkSize * ulBlock / SECTOR_SIZE;
uNumSec = ulBlkSize / SECTOR_SIZE;
