Project 2 Preliminary Documentation
1. Introduction
Design and implement a simple file system using the disk driver from previous
project. I/O system interface – provided by my driver project: read_block(int i,
char *p)
write_block(int i, char *p). FS can access the emulated disk using only these
functions. Also, develop test/presentation shell
2. Data Structures
I’ve used a double array for Descriptor: DES[index][n]; and for OFT table, I use
array, which looks like this: OFT[j] (this gives us the oft entry at index j. then, for
bitmap, I use an arrary of two integer, as what professor suggested.
3. System Architecture


What are the main functions?
create(symbolic_file_name): return ok/error
destroy(symbolic_file_name) : return ok/error
open(symbolic_file_name): return index
close(index): return ok/error
read(index, mem_area, count): return #bytes read
write(index, mem_area, count): return #bytes written
lseek(index, pos) : return ok/error
directory: return ok/error
findnewblock: return index of new block or error
What is the calling hierarchy?
create calls lseek, write, read
destroy calls lseek, write, read
open calls lseek, read
write calls find new block
4. Test Cases
The tests of the file system will assume the following sizes of the disk and the
internal data structures:
 Disk: 4 cylinders, 2 surfaces, 8 sectors/track, 64 Bytes/sector
 File name: maximum of 4 characters, no extension
 Number of files: not more than 10
 Number of open files: 3 plus the directory, i.e., the size of the OFT is exactly
4 entries. (The directory should be opened automatically when the file
system starts and remain open until the file system is shut down.).
I’ll use the test cases provided first to test my design.
If my program reads the following input file:
in 4 2 8 64 disk
cr foo
op foo
wr 1 x 60
wr 1 y 10
sk 1 55
rd 1 10
cl 1
dr
in 4 2 8 64 disk
op foo
q
it should produce a file that looks something like this: output
Warning: disk does not exist, initialized to blank
file foo created
file foo opened, index=1
60 bytes written
10 bytes written
current position is 55
10 bytes read: xxxxxyyyyy
file 1 closed
files: foo 70
Warning: disk does not exist, initialized to blank
error: file foo does not exist
program terminated
5. Pseudo Code (for preliminary documentation)
Create(int file_name):
int DesIndex = 1;
char memory[100];
/* find a free file Descriptor between DES[1] to DES[23] */
while (true) do
if DesIndex is 24 then
return ERROR: NO FREE FILE DESCRIPTOR;
end if;
if DES[DesIndex][0] is negative then
Des[DesIndex][0] = 0; /* set length of file to 0 */
break;
end if;
DesIndex++;
end while;
/* start reading from OFT[0] 4 bytes at a time into memory */
/* Directory is already in OFT, so we don’t need to open it */
/* check if there already exits a file with file_name */
lseek(0, 0);
while (OFT[0].currentPos< OFT[0].length) do
read(0, m, 4);
if m is file_name then
return ERROR:FILE ALREADY EXITS;
end while;
/* find an empty directory entry */
lseek(0, 0);
while (OFT[0].currentPos< OFT[0].length) do
read(0, m, 4)
if m is EMPTY AND m%2 is 0 then /* first int of the pair */
lseek(0, OFT[0].currentPos-4);
write(0, file_name, 4);
write(0, DesIndex, 4);
return OK;
end if;
end while;
return ERROR: NO EMPTY DIRECTORY ENTRY;
end Create;
Destroy (int file_name):
int fname;
int dIndex;
boolean fdFound = false;
/* find index of file descriptor of file_name and remove dir entry*/
lseek(0, 0);
while (OFT[0].currentPos< OFT[0].length) do
read(0, (char*)fname, 4);
if fname is file_name and OFT[0].currentPos%2=0 then
read(0, (char*)dIndex, 4); /* get the file descriptr index */
lseek(0, OFT[0].currentPos-8);
write(0, EMPTY, 4); /* remove directory entry */
write(0, EMPTY, 4); /* remove directory entry */
fdFound = true;
break while;
end if;
end while;
if fdFound is false then
return ERROR: NO FILE DIRECTORY FOUND;
/* update bit map */
totalBlock = DES[dIndex][0] / 64 bytes + 2;
blockNumInDF = 1;
while blockNumInFD <= totalBlock do
BitMapNum = DES[dIndex][blockNumInFD] / 64
BitMapOffset = DES[dIndex][blockNumInFD] % 64
BM[BitMapNum] = BM[BitMapNum] & MASK2[BitMapOffset]
end while;
/* free file descriptor */
Des[dIndex][1] = EMPTY;
Des[dIndex][2] = EMPTY;
Des[dIndex][3] = EMPTY;
return OK;
end Destroy;
Open (int file_name):
int fname;
int dIndex;
boolean fdFound = false;
/* search directory to find index of file descriptor of file_name */
lseek(0, 0);
while (OFT[0].currentPos< OFT[0].length) do
read(0, (char*)fname, 4);
if fname is file_name and OFT[0].currentPos%2=0 then
read(0, (char*)dIndex, 4); /* get the file descriptr index */
fdFound = true;
break while;
end if;
end while;
if fdFound is false then
return ERROR: NO FILE DIRECTORY FOUND;
/* allocate and update a free OFT entry */
oftIndex = 1;
while (true) do
if oftIndex is 4 then
return ERROR: NO FREE OFT ENTRY
end if;
if OFT[oftIndex].length is EMPTY then
/* update OFT entry */
if DES[dIndex][1] is not EMPTY then
read_block(DES[dIndex][1], OFT[oftIndex].buffer);
end if;
OFT[oftIndex].currentPos = 0;
OFT[oftIndex].index = dIndex;
OFT[oftIndex].length = DES[dIndex][0];
break while;
end if;
oftIndex++;
end while;
return oftIndex;
end Open
Close (int oftIndex):
if oftIndex > 3 then
return ERROR: OFT INDEX IS LARGER THAN 3
/* calculate the block pointer in FD */
blockNumberInFD = OFT[oftIndex].length / bufferSize + 2;
/* write buffer to disk */
writeblock(DES[OFT[oftIndex].index][blockNumberInFD],
OFT[oftIndex].buffer);
/* update file length */
DES[OFT[oftIndex].index][0] = OFT[oftIndex].length;
/* free OFT entry */
OFT[oftIndex].length = EMPTY
return OK;
end Close;
Read (oftIndex, memory, count):
int startCount = 0;
/* computer position in buffer */
posInBuffer = OFT[oftIndex].currentPos % bufferSize;
while(true) do
/* count reached */
if startCount == count or
OFT[oftIndex].currentPos == OFT[oftIndex].length then
return OK;
end if;
if posInBuffer is 64 bytes then /* end of buffer is reached */
/* calculate the block pointer in FD */
blockNumberInFD = OFT[oftIndex].currentPos / bufferSize + 1;
/* set buffer position to start of the buffer */
posInBuffer = 0;
/* write buffer to disk */
writeblock(DES[OFT[oftIndex].index][blockNumberInFD++],
OFT[oftIndex].buffer);
readblock( DES[OFT[oftIndex].index][blockNumberInFD],
OFT[oftIndex].buffer);
end if;
memory[startCount] = (OFT[oftIndex].buffer)[posInBuffer];
posInBuffer ++;
OFT[oftIndex].currentPos++;
startCount++;
End while;
End Read;
Write (oftIndex, memory, count):
int startPos = 0；
/* computer position in buffer */
posInBuffer = OFT[oftIndex].currentPos % bufferSize;
while(true) do
/* check to see if count is reached */
if startPos is count then
return OK;
end if;
/* if current position in buffer exceeds bufferSize */
if posInBuffer = 64 then
/* update current buffer position */
posInBuffer = 0;
/* calculate the block number in file directory */
blockNumberInFD = OFT[optIndex].length / bufferSize + 1;
/* check if block already exists in FD */
if DES[OFT.index][blockNumberInFD] is EMPTY then
/* find a new block using bit map */
newDiskBlockIndex = findNewBlock();
else
newDiskBlockIndex = DES[OFT.index][blockNumberInFD];
end if;
/* write buffer into block */
writeblock(newDiskBlockIndex, OFT[oftIndex].buffer);
end if;
/* write one byte from memory to buffer */
(OFT[oftIndex].buffer)[posInBuffer] = memory[startPos];
startPos++;
OFT[optIndex].length++;
OFT[optIndex].currentPos++;
end while;
end write;
Lseek (oftIndex, newPos) :
/* calculate the block number in FD for newPos */
blockNumberOfNewPos = newPos / bufferSize + 1;
/* calculate the block number in buffer */
blockNumberOfBuffer = OFT[oftIndex].currentPos / bufferSize +1;
if blockNumberOfNewPos != blockNumberOfBuffer then
/* write the buffer to disk */
writeblock(blockNumberOfBuffer, OFT[oftIndex].buffer);
/* read buffer from disk */
readblock(blockNumberOfNewPos, OFT[oftIndex].buffer);
end if;
OFT[oftIndex].currentPos = newPos;
return OK;
end lseek;
Directory :
int file_name;
int file_index_in_FD;
/* read the directory file */
lseek(0,0);
while(OFT[0].currentPos < OFT[0].length) do
/* print file_name and file length for each directory entry */
read(0, file_name, 4);
read(0, file_index_in_FD, 4);
print file_name;
print DES[file_index_in_FD][0];
end while;
end Directory;
findNewBlock():
for (i=0; i<2; i++)
for (j=0; j<16; j++)
/* search BM from the beginning
/* check each bit in BM[i] for “0”
test = BM[i] & MASK[j])
if (test == 0) then /* bit j of BM[i] is “0” */
return i*16+J;
end if;
end for;
end for;
return ERROR: NO FREE DISK SPACE;
end findNewBlock;
Lin He (52924739)
Project 2: File System
Preliminary Documentation
ICS145B SQ2005
Due. 04/25/05