Low Level System Call Interface

INT 4 to INT 18 are software interrupt handlers. They are used by the user program to access kernel services (e.g. Read from a file, fork a child process etc).

The OS design is fixed such that only the user program can call the software interrupts. They should not be called from inside another interrupt handler or kernel module.

The system call interface is presented below.

System Call	System Call Number	Interrupt Routine Number	Argument 1	Argument 2	Argument 3	Return Value
Create	1	4	File Name (String)	Permission (Integer) *	-	0 - Success / File alredy exists
						-1 - No free inode table entry
Delete	4	4	File Name (String)	-	-	0 - Success/File not found
						-1 - Permission denied
						-2 - File is open
Seek	6	5	File Descriptor	Offset	-	0 - Success
						-1 - File Descriptor is invalid
						-2 - Offset moves File pointer outside file
Open	2	5	File Name	-	-	File Descriptor - Success
						-1 - File Not found or file is not data file or root file
						-2 - System has reached its limit of open files
						-3 - Process has reached its limit of resources
Close	3	5	File Descriptor	-	-	0 - Success
						-1 - File Descriptor is invalid
Read	7	6	File Descriptor (-1 for terminal read)	Word Address (Buffer)	-	0 - Success
						-1 - File Descriptor given is invalid
						-2 - File pointer has reached the end of file
Write	5	7	File Descriptor(-2 for terminal write)	Word to write	-	0 - Success
						-1 - File Descriptor given is invalid
						-2 - No disk space / File Full
						-3 - Permission denied
Fork	8	8	-	-	-	PID - Success (in parent process)
						0 - Success (in child process)
						-1 - Failure (in parent process), Number of processes has reached maximum limit
Exec	9	9	File Name	-	-	-1 - File not found or file is not executable
Exit	10	10	-	-	-	-
Getpid	11	11	-	-	-	current PID - Success
Getppid	12	11	-	-	-	parent PID - Success
Wait	13	11	PID	-	-	0 - Success
						-1 - Given PID is invalid or it is PID of invoking process
Signal	14	11	-	-	-	0 - Success
Semget	17	13	-	-	-	SEMID - Success
						-1 - Process has reached its limit of resources
						-2 - Number of semaphores has reached its maximum
Semrelease	18	13	SEMID	-	-	0 - Success
						-1 - Invalid SEMID
SemLock	19	14	SEMID	-	-	0 - Success or semaphore is already locked by the current process
						-1 - invalid SEMID
SemUnLock	20	14	SEMID	-	-	0 - Success
						-1 - Invalid SEMID
						-2 - Semaphore was not locked by the calling process
Shutdown	21	15	-	-	-	-1 - Permission denied
Newusr	22	16	User Name	Password	-	0 - Success
						-1 - User already exists
						-2 - Permission denied
						-3 - No. of users have reached the system limit.
Remusr	23	16	User Name	-	-	0 - Success
						-1 - User does not exist
						-2 - Permission denied
						-3 - Undeleted files exist for the user
Setpwd	24	16	User Name	New password	-	0 - Success
						-1 - Unauthorised attempt to change password
						-2 - The user does not exist.
Getuname	25	16	User ID	-	-	User Name - Success
						-1 - Invalid User ID
Getuid	26	16	User Name	-	-	User ID - Success
						-1 - Invalid username
Login	27	17	User Name	Password	-	0 - Success
						-1 - Invalid username or password
						-2 - Permission denied
Logout	28	12	-	-	-	-1 - permission denied
Test0	96	18	-	-	-	-
Test1	97	18	-	-	-	-
Test2	98	18	-	-	-	-
Test3	99	18	-	-	-	-
Test4 **	100	19	-	-	-	-
Test5 **	101	19	-	-	-	-
Test6 **	102	19	-	-	-	-
Test7 **	103	19	-	-	-	-