02 02 Operating System Services P2 — Transcript

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1. 0:08[Music]
2. 0:20okay
3. 0:21next system calls
4. 0:23so system calls provide a means for a
5. 0:26user or application programs to call
6. 0:28upon the services provided by the
7. 0:30operating systems from the previous
8. 0:32slides okay
9. 0:34so every time a user or a program
10. 0:38needs services coming from the operating
11. 0:40systems like program execution
12. 0:42communication etc
13. 0:44they are just using a system call so
14. 0:47it's just like we are calling a function
15. 0:50of an operating system so that we can
16. 0:52use it
17. 0:56all right so mostly accessed by programs
18. 1:00by a high-level application programming
19. 1:02interface or api
20. 1:04rather than direct system called use
21. 1:07okay
22. 1:08so three most common apis are the win32
23. 1:11api for windows pho 6 api for posix
24. 1:15based systems including unix and linux
25. 1:18and mac os and java api
26. 1:21for java virtual machine or jvm
27. 1:24okay
28. 1:28okay so next this is our example or this
29. 1:30is an example of a system call
30. 1:33okay
31. 1:34so generally written in c or c plus plus
32. 1:37although some are written in assembly
33. 1:39for optimal performance
34. 1:40okay so this figure illustrates the
35. 1:43sequence of system calls all right so
36. 1:45these are the system calls here
37. 1:50okay so this requires to copy a file
38. 1:54okay
39. 1:56so you can use strass
40. 1:58or stress
41. 1:59to see more examples of a large number
42. 2:01of system calls invoked by a single
43. 2:04simple command
44. 2:06so read the man page for strays
45. 2:09and try some simple examples
46. 2:12strays mkdir
47. 2:14temp stray cd temp stress date
48. 2:17okay and so on
49. 2:20so basically
50. 2:21with this one the system call sequence
51. 2:23to copy the contents
52. 2:25of one file to another so source file
53. 2:28could be
54. 2:29a dot cpp that java but this one is of
55. 2:32course that's what i've said earlier
56. 2:34it's in crc plus okay
57. 2:36and then
58. 2:38of course that would be converted or
59. 2:40translated into a destination file which
60. 2:43okay
61. 2:44mostly
62. 2:46this file
63. 2:47is basically
64. 2:50an executable or
65. 2:54okay any other forms or in this case
66. 2:58the source file
67. 2:59okay
68. 3:00and the destination file would be the
69. 3:03the other file okay or the copy of the
70. 3:05other file
71. 3:07all right
72. 3:10next
73. 3:11example of standard api so most
74. 3:14programmers do not use the low level
75. 3:16system calls directly but instead use
76. 3:19an api
77. 3:21or
78. 3:22the application programming interface
79. 3:25okay
80. 3:27so the following sidebar shows read
81. 3:30call available in the api or unix-based
82. 3:34systems
83. 3:35okay
84. 3:39next system call implementation so
85. 3:41typically
86. 3:42a number is associated with each system
87. 3:44call okay the system call interface
88. 3:47maintains a table indexed according to
89. 3:49these numbers
90. 3:51and this system call interface invokes
91. 3:54the intended system called an os kernel
92. 3:57and return status of the system call and
93. 4:00any return values
94. 4:03okay so the caller needs to know nothing
95. 4:06about how the system call is implemented
96. 4:09so basically if you are a programmer we
97. 4:10don't care about how the system call was
98. 4:13implemented
99. 4:14we simply call it and use it that's it
100. 4:17okay so just needs to obey the api and
101. 4:20understand the os will do us
102. 4:24a result call
103. 4:26so most details of the os interface
104. 4:28hidden from the programmer by the api
105. 4:31so managed by runtime support library
106. 4:34these are the set of functions built
107. 4:36into a libraries including with the
108. 4:37compiler
109. 4:39okay
110. 4:41so
111. 4:42the use of api instead of indirect
112. 4:44system calls provides for greater
113. 4:46program portability between different
114. 4:48systems
115. 4:51the api then makes the appropriate
116. 4:53system calls
117. 4:54through the system called interface
118. 4:56using a table lookup to access a
119. 4:59specific number of system calls as shown
120. 5:01here in the figure
121. 5:04alright
122. 5:07next
123. 5:08system called parameter passing
124. 5:10so often
125. 5:12more information is required than simply
126. 5:15identity
127. 5:16of the research system call
128. 5:18so the general methods used to pass
129. 5:20parameters to the os the simplest is
130. 5:23pass the parameters in registers
131. 5:26right or parameters is stored in block
132. 5:28or table in memory
133. 5:30and address of block passed as parameter
134. 5:33in a register so this approach is taken
135. 5:35by linux and solaris okay
136. 5:39so
137. 5:39also parameters placed or pushed onto
138. 5:43the stock by the program and popped off
139. 5:45the stock by the operating systems
140. 5:48we'll be covering this okay in
141. 5:50the latter chapters and discussions so
142. 5:54black and stock methods do not limit the
143. 5:56number over length
144. 5:58of parameters being passed
145. 6:02okay
146. 6:03so this one is parameter passing by a
147. 6:05table okay parameters are generally
148. 6:07passed to system calls via registers or
149. 6:10less commonly by values pushed onto the
150. 6:13stock
151. 6:14so large blocks of data are generally
152. 6:17accessed indirectly
153. 6:19through a memory address passed in
154. 6:21register
155. 6:22or
156. 6:24on the stack as shown here
157. 6:26okay
158. 6:29all right
159. 6:31so next would be a types of system calls
160. 6:33so basically we have six major
161. 6:35categories outlined on this fireprint
162. 6:36presentation and we are going to start
163. 6:38with process control
164. 6:40so under process control okay so this
165. 6:43process controls allows the following
166. 6:45activities or access so create process
167. 6:48terminate process and the board okay
168. 6:51load execute and so on
169. 6:53so that's one okay so as adam said there
170. 6:56are six major categories of the system
171. 6:58calls so first is
172. 7:00process control
173. 7:02second is file management so it allows
174. 7:04us to create file delete file close or
175. 7:07open file
176. 7:08read write reposition
177. 7:10get and set file attributes okay that is
178. 7:13under file management second third
179. 7:17you've got device management
180. 7:19so under device management system calls
181. 7:21so the users can request device release
182. 7:24device read write reposition and so on
183. 7:28okay
184. 7:29next is
185. 7:32information maintenance all right so
186. 7:35under information maintenance you've got
187. 7:36get time date set time or date okay so
188. 7:40take note that we can do
189. 7:42changing of time and date on our
190. 7:44computer systems and we're just calling
191. 7:46the information maintenance system calls
192. 7:49all right
193. 7:50communication system calls create delete
194. 7:52communication connections
195. 7:54okay so send receive messages if message
196. 7:57passing model the host name or process
197. 8:00name
198. 8:01or from client to server and vice versa
199. 8:04all right so if the applications is
200. 8:07capable of communicating with the
201. 8:09outside world all right so that means
202. 8:13we are using this communication system
203. 8:15calls
204. 8:16all right
205. 8:17and the last one is protection so
206. 8:20control access to resources
207. 8:22okay authorization
208. 8:24get and set permissions allow internet
209. 8:26user access okay so basically under
210. 8:28protection system calls it the uh it
211. 8:32tackles more on the authorization
212. 8:35okay
213. 8:36and authentication
214. 8:38also all right so
215. 8:41these were the six categories of system
216. 8:44calls mentioned earlier process file
217. 8:47device information communication and
218. 8:49protection
219. 8:50okay and this table here summarizes okay
220. 8:54it shows the comparison
221. 8:56okay or similarities of the areas of
222. 8:59windows and unix system calls
223. 9:01all right so in windows when we create
224. 9:04process in the unix we call it fork okay
225. 9:07creating a process means fork in unix
226. 9:09exit process is also exit okay
227. 9:14waiting for single object is wait here
228. 9:17okay
229. 9:19so
230. 9:20these are examples of windows and unix
231. 9:22system calls
232. 9:24all right
233. 9:26next
234. 9:27standard c library example
235. 9:30so take note that
236. 9:32when we are doing our first program okay
237. 9:35hello world
238. 9:36in
239. 9:37c or c plus plus okay so we often use
240. 9:40this
241. 9:42sharp include stdio so this article
242. 9:44library or directives all right
243. 9:48so
244. 9:49standard library calls
245. 9:51may also generate system calls as shown
246. 9:53here
247. 9:54all right so as shown in c or any
248. 9:57programming language we are using a
249. 9:59system called stair
250. 10:01so c program invoking printf library
251. 10:04call
252. 10:05which calls write system call so that
253. 10:07means printf we want to print something
254. 10:09on the screen so we are calling
255. 10:12a write system call there
256. 10:15all right
257. 10:18okay
258. 10:18so next would be
259. 10:20arduino so iot is getting popularity and
260. 10:25basically this arduino is single tasking
261. 10:29there is no operating systems on it all
262. 10:32right so programs
263. 10:34okay named sketch is loaded via usb
264. 10:38into a flash memory
265. 10:40so single memory space
266. 10:41bootloader loads program and then
267. 10:44program exit shell reloaded okay
268. 10:48so
269. 10:49this is how it looked like basically at
270. 10:51system startup you have the bootloader
271. 10:53and then you've got a free memory there
272. 10:55okay so when you run the program okay so
273. 10:58we copy the user program or the sketch
274. 11:01on the free memory
275. 11:02and then that's it
276. 11:04again in arduino
277. 11:06no operating systems on it
278. 11:08all right
279. 11:10all right so next is freebsd so freebsd
280. 11:13is a unix variant just like linux
281. 11:16okay
282. 11:17so it is multitasking
283. 11:19user can login invoke user's choice of
284. 11:22shell okay shell is the cli of a unix
285. 11:26based systems all right so shell
286. 11:28executes fork
287. 11:30fork meaning we are going to create a
288. 11:32process from the table presented earlier
289. 11:34okay
290. 11:36so processes or process exits when
291. 11:39code equals zero that means no error our
292. 11:42code is greater than zero that means
293. 11:44there is an error code
294. 11:47all right
295. 11:49next
296. 11:50system services
297. 11:52so system programs provide os
298. 11:55functionality
299. 11:57through separate applications which are
300. 11:59not part of the kernel or the command
301. 12:01interpreters
302. 12:02okay so system programs provide a
303. 12:04convenient environment
304. 12:06for program development and execution so
305. 12:09they can be divided into this
306. 12:11all right so most users
307. 12:14view
308. 12:15of the operating system or operation of
309. 12:17the system is defined by the system
310. 12:18programs
311. 12:19not the actual system calls
312. 12:23okay
313. 12:25next system services continuation we've
314. 12:28got file management so this basically
315. 12:32detailed okay this file manipulation
316. 12:34status informations and so on on the
317. 12:36next slide here so they are also known
318. 12:39as system utilities or system
319. 12:40applications
320. 12:42so most system
321. 12:43also ship with useful applications such
322. 12:46as calculators all right or simple
323. 12:48editors like notepad
324. 12:50so some debate arises
325. 12:52to a border between system and
326. 12:54non-system applications all right
327. 12:57so
328. 12:59system programs may be divided into this
329. 13:01categories okay so you've got file
330. 13:04management
331. 13:05programs to create
332. 13:07delete
333. 13:08copy rename
334. 13:09print list and generally manipulate file
335. 13:12and directories
336. 13:14right you also have the status
337. 13:16information
338. 13:17utilities to check on the date time
339. 13:21number of users
340. 13:23process is running like what i've shown
341. 13:25earlier
342. 13:26using the task manager
343. 13:28data logging etc so system registers are
344. 13:31used to store and recall configuration
345. 13:34information
346. 13:36for particular applications
347. 13:39all right
348. 13:40so next is
349. 13:42file modification
350. 13:44for example text editors and other tools
351. 13:46which can change file content so we call
352. 13:49it file modification
353. 13:51all right programming language support
354. 13:54example compilers linkers debuggers
355. 13:57providers assemblers
356. 13:59so library achieve or archive management
357. 14:03okay
358. 14:04interpreters for common languages and
359. 14:06support from it
360. 14:09okay
361. 14:10so next is communications
362. 14:13programs for providing connectivity
363. 14:15between
364. 14:17processes or
365. 14:18again when you process this our program
366. 14:20and execution or running program
367. 14:22and the users so including mail web
368. 14:25browsers remote logins file transfers
369. 14:29okay and remote command execution so
370. 14:31that's
371. 14:32under communications
372. 14:35all right
373. 14:37next background services
374. 14:40so system demons are commonly started
375. 14:43when the system is booted
376. 14:45and run
377. 14:46for as long as the system is running so
378. 14:49handling necessary services
379. 14:51examples include network demons
380. 14:54print servers
381. 14:56process schedulers and system error
382. 14:58monitoring services
383. 15:01all right
384. 15:03and the last one is application programs
385. 15:05so don't pertain to system
386. 15:07okay so once the application programs
387. 15:09these are run by the users
388. 15:12not typically considered part of the
389. 15:14operating systems
390. 15:16okay
391. 15:18all right so next is linkers and loaders
392. 15:22all right
393. 15:23so source code compiled into an object
394. 15:26files designed to be loaded into the
395. 15:28physical or any physical memory location
396. 15:30so we look at a relocatable
397. 15:33object file
398. 15:34it's just like when we created our first
399. 15:36program in c plus plus okay so basically
400. 15:39what we see on the screen is our source
401. 15:40code all right and that source code is
402. 15:43usually in that cpp format
403. 15:46okay
404. 15:48so
405. 15:49this source code when we compile it well
406. 15:52that generates an object file
407. 15:54okay
408. 15:56so linker
409. 16:00linker combines this into a single
410. 16:02binary executable file
411. 16:05also brings in libraries so program
412. 16:07resides on the secondary storage as
413. 16:09binary executable must be brought into
414. 16:12the memory by a loader to be executed
415. 16:16okay
416. 16:17so relocation assigns final address
417. 16:20to program parts and adjusts code and
418. 16:23data in program to match those addresses
419. 16:28so modern general purpose systems don't
420. 16:30link libraries into executables rather
421. 16:33dynamically link libraries or dll so
422. 16:36that is what we use in windows are
423. 16:38loaded as needed
424. 16:39shared by all those that use the same
425. 16:41version
426. 16:42of that same library
427. 16:44okay
428. 16:45so object executable files have standard
429. 16:48formats so operating system knows how to
430. 16:51load and start them
431. 16:53okay
432. 16:54so
433. 16:55if you will observe in
434. 16:58c plus plus not during our early days in
435. 16:59programming okay so once we created a
436. 17:02program let's say that cpp okay so we
437. 17:05usually compile it okay so with a
438. 17:08successful compiling okay meaning no
439. 17:11errors
440. 17:12so that generates an object file
441. 17:15okay that object file when we run that
442. 17:18program
443. 17:19okay
444. 17:20and
445. 17:21that object file
446. 17:23is executed
447. 17:24so we can see an output on the screen
448. 17:27all right
449. 17:28so
450. 17:29that's what we do
451. 17:31during our early days in programming so
452. 17:33you that uses linkers and loaders okay
453. 17:36so compilers are used for high-level
454. 17:38language programming
455. 17:40and at low level
456. 17:42instead of compilers we use assembler
457. 17:45okay so compiler high level programming
458. 17:48assemblers low level programming
459. 17:51okay
460. 17:52so what is the role of the linker in the
461. 17:54loader so for example this is what i'm
462. 17:56i'm talking earlier
463. 17:57okay so for example you have the source
464. 17:59program
465. 18:00main.c or main.cpp okay so when we
466. 18:03compiled it
467. 18:04it creates an object file
468. 18:06okay
469. 18:08so next is
470. 18:12when
471. 18:13we load it
472. 18:15okay so the linker
473. 18:16with other object files associated with
474. 18:18that
475. 18:19will generate an executable file okay so
476. 18:22that's the linker there so compiler
477. 18:24linker
478. 18:25so compiler from source to object file
479. 18:29from object file to executable file
480. 18:31linker
481. 18:32and if there are other object files
482. 18:34there so that would be linked up also
483. 18:37okay and an executable file to be loaded
484. 18:40onto the ram for execution
485. 18:43and calling other dynamically linked
486. 18:45libraries or dll so that is where loader
487. 18:47comes in
488. 18:49okay so basically we don't care about
489. 18:51this during our early days in
490. 18:52programming all right so what we want to
491. 18:55see there is just our code our program
492. 18:57is successfully running
493. 19:00all right
494. 19:10[Music]
495. 19:18you