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Device Drivers in Xinu

What is a device driver ?

Device driver is a piece of software associated with device to

provide support for using that device. Xinu has device drivers

for tty devices (terminal & keyboard), disk, network, files and

windows. In Xinu files and windows are treated as pseudo-

devices.

What support does a device drivers provide?

Convenience: a simple interface to the device which makes

the user programming simple. Users are spared of many

programming details for using the device. For example, lowerlevel details of the device hardware are handled by the device

driver.

Portability of user programs: Since the device specific

details are hidden in the device driver, the user programs can

operate with a range of devices without requiring

modifications. For example, print command works correctly

whether it is a Panasonic or Epson printer as long as the correct

driver for the printer is installed.

Resource Management: Each device is managed as a

resource by the device driver to take care of issues such as

resource sharing, security.

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How device drivers in Xinu work ?

U

s

e

r

system

call

Device switch table

devtab[ ]

Device

specific

upper-

half

routine

Device

specific

lower-

half

routine

Device

An Example: Read operation

system call upper-half routine lower-half routine

read( descrp, buff, count ) ttyread(devptr, buff, count) ttyiproc( )

devptr = &devtab[descrp]

Data transfer during a read operation for tty device:

buff

user buffer

*buff++ = ttygetc(devptr) ttyiproc( )

called within ttyread()

system buffer

Uniform interface across devices: The system call is the same across different devices,

but the parameters are interpreted differently.

read( descrp, buff, count )

interpreted as the number of

characters for a tty device

interpreted as the block number for a

disk device

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Device Switch Table

int dvnum;

long int (*dvinit)();

long int (*dvshutdown)();

long int (*dvopen)();

long int (*dvclose)();

long int (*dvread)();

long int (*dvwrite)();

long int (*dvseek)();

long int (*dvgetc)();

long int (*dvputc)();

long int (*dvcntl)();

long int (*dviint)();

long int (*dvoint)();

long int dvcsr;

long int dvivec;

long int dvovec;

char *dvioblk;

int dvminor;

device switch table entry

( struct devsw)

device switch table

struct devsw devtab[ ]

Ptrs to device

specific high-level routines

Ptr to device

control block

The structure of the control block varies from device to device. The control block

contains information associated with the device.

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Data Structures for Disk Device Driver

devtab[ ]

dvioblk

dstab[ ]

dstab[ ] : array of

disk control block.

One control block

per disk,

*dreq : ptr to list of pending

disk requestsdnum : device number

dibsem : semaphore for i-block

ddirsem : semaphore for directory

dflsem : semaphore for free list

dnfiles : # currently open files

*ddr : ptr to incore directory

fields of each dstab entry

List of pending requests

dreq

structure of each node in the

list of pending requests

drdba : disk block #

drpid : id of process making the requestdrbuff : buffer address

drstat : return address

drnext : ptr to next request node

Requests are addid to the list by the high-level device driver routine (DDR) and

requests are removed from the list by the low-level DDR.

Req

list

upper-half DDR lower-half DDR

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How readand write calls works

dsread( n , buff , m )

disk block numberdevice numberptr to user buffer

buff

user buffer

drbuff

read

dsread( )

1. allocate memory for a request

node using getbuf( ).

2. fill pid and other fields of the

request node.

3. enqueue the request

4. suspend the process

dswrite( )

1. allocate memory for a request

node using getbuf( ).

2. fill pid and other fields of the

request node.

3. enqueue the request

5. lower-half routine executes to

read the data from disk

4. lower-half routine executes to

write the data to disk

5. deallocate memory used for

request node

6. deallocate memory used for

request node

The process is blocked in case ofread.

Exception: readis optimized

read

write

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Disk Scheduling Algorithm in Xinu

List of pending requests

dreqnull

First request on the list is

the one that hardware is

performing; hardware is

idle when the list is empty.

10 15 9 20 40 2

23

New request

for block 23

45

New request

for block 45

Optimization of disk requests

Situation :New request is for the same block as one of the pending requests

ld request New request ptimization (Y/N)AD AD

AD W IT

W IT AD

W IT W IT

S K AD (or W IT )AD (or W IT ) S K

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Basics of Xinu File System

File : A sequence of zero or more bytes; any further structure is to be enforced by user level

programs

Index Mechanism : To support file growth and random movement within a file Xinu uses

an indexing mechanism. Each file is stored as a link-list of index nodes. Each

index node points to a subset of datablock associated with the file.

Layout on the Disk : Block0 contains the directory, next few disk blocks are reserved for

storing the index node, the remaining blocks are used for storing the data

blocks.

directory

3 blocks for index

nodes

data blocks

A logical view of an hypothetical disk

0 1 2 3 4 5 6 7

Example:A file with index node #2, and data blocks: 5, 4, 7

first index node of the file

54 7

On our system we simulate a disk where:

Each disk block is 512 bytes

Each index node is 64 bytes

Each data block is 512 bytes

Each index node can point to 29 data blocks

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More About Index Nodes

Struct iblk{

long ib_byte; /* first data byte */

IBADDR ib_next; /* next iblock # */

IBADDR ib_dba[IBLEN]; /* ptrs data

block */ }

On our system:

sizeof(data block) = sizeof(disk block) =512 bytes

sizeof (iblk) = 64 bytes

sizeof(ib_byte) = 4 bytes

sizeof(ib_next) = 2 bytes

sizeof(ptr) = 2 bytes

IBLEN = 29

For the first index node of a file ib_byte = 0; for the n-th index node

ib_byte = (n - 1) sizeof(data block)

ibtodb(ib) : calculates the address of the disk block containing the index node ib .

For example, ibtodb(37) = 5

ibdisp(ib) : calculates the displacement of the index node within the disk block.

For example, ibdisp(37) = 5 * 64

Operations on index nodes:

ibclear(ibptr, ibbyte) : initializes an in-core index node

ibptr->ib_dba[i] = DBNULL; ibptr->ib_byte = ibbyte; ibptr -> ib_next = IBNULL;

ibget(diskdev, inum, loc) : copy the index node inum from the disk to the memory

location pointed by the pointerloc .

Note a temporary buffer is needed to read the disk block containing the specified index

block. Since a disk block is bigger than a index block, the disk block should not be

read into the location pointed by loc .

Use ibtodb( ) to calculate the disk block #, copy the disk block into a temporary

buffer. Use ibdisp( ) to calculate the location where the specified index block is located

within the temporary buffer.

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fd = open(diskdev, sample , orw); /*open file sample */

How a file gets opened in Xinu

Device Table

disk entry

file entry

m

n

dvioblk ptr

dvioblk ptr

dstab[ ] : an array of

disk control blocks

dir

ptr

incore

directory

file

entries

fdptr

dstab[ ] : an array of

disk control blocks

file entry

file entry

file entry

n mfl_id

(static)

fl_dev

(dynamic) fl-dent ptr

This ptr is

established when

a file is opened

dfdsrch : locates the file in the incore directory, returns fdptr

dfalloc : allocates an unused file control block, returns findex

dsopen : returns n, index of the dev table entry for file

dsopen( ) calls : dfckmd( ), dfdsrch( ), and dfallaoc( ).

mn