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Operating System Structure
• A key concept of operating systems is
multiprogramming.
– Goal of multiprogramming is to efficiently utilize all of the
computing resources.
– When a job issues an I/O request (e.g., open a file, read
data from a file), it cannot continue until the request is
fulfilled.
– The CPU then becomes idle (the job is blocked on the
request).
Operating System Structure
• Basic idea of multiprogramming:
– Keep multiple jobs in memory.
– When one job blocks on I/O (or other events), the
operating system:
• Starts the I/O operation.
• Switches to another job that is ready to execute.
• Now the CPU and I/O device are executing in parallel.
– When the I/O device has completed request, it generates
an interrupt to inform the CPU.
– Virtually all general purpose computers support
multiprogramming.
• First developed for batch systems in the 60s.
– Go to the computer center and give them your program
(stored on punch cards).
– The computer operator “batched” several jobs together and
loaded them into the computer.
– Come back at 5:00 to get the results of your program.
• Batch systems are non-interactive.
Time-Sharing Systems (Multitasking)
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Logical extension of multiprogramming termed
multitasking.
Quite often sitting at terminal using a “command line”
interface to interact with computer.
 Types in commands from keyboard.
 A system program called a shell reads command from
the command line and makes OS system calls to carry
out commands.
OS switches between user’s programs very quickly,
generally in round-robin fashion.
User 2
User 3
Shared
Computer
User 4
User1
User 2
User 3
Shared
Computer
User 4
User1
User 2
User 3
Shared
Computer
User 4
User1
User 2
User 3
Shared
Computer
User 4
User1
User 2
User 3
Shared
Computer
User 4
User1
Time-Sharing Systems (Multitasking)
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Switching between users is very fast.
Goal is to give the illusion that each user has own
machine.
 Think about using a text editor, or getting a directory
listing.
Response time is a priority.
Protection of System Resources
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I/O Devices
Memory
CPU
Files
Operating System
Protection of System Resources
• Based on dual-mode execution:
– kernel mode and user mode.
– Privileged instructions can be issued only in kernel mode.
– Mode bit in Process Status Word (PSW), checked on every
instruction.
User process
executing
Continue Execution
System Call
Mode
Bit=1
Trap, mode bit = 0
Execute System Call
Return. Mode
bit = 1
Protection of I/O Devices
• All I/O instructions are privileged instructions.
• Only accessed through system calls.
Memory Protection
• Must provide memory protection for the interrupt
vector, interrupt service routines, and other
applications address space.
• Two registers that determine the range of legal
addresses a program may access:
– Base register – holds the smallest legal physical
memory address.
– Limit register – contains the size of the range
• Memory outside the defined range is
protected.
Use of A Base and Limit Register
Hardware Address Protection
limit
CPU (and OS) Protection
• Keep user from monopolizing CPU.
• Ensure OS regains control of CPU.
CPU Protection
• Timer – interrupts computer after specified period to
ensure operating system maintains control.
– Timer is decremented every clock tick.
– When timer reaches the value 0, an interrupt occurs.
• Timer commonly used to implement time sharing.
Privileged Instructions
• Load base and limit registers?
Privileged Instructions
• Load base and limit registers?
• Set the system timer?
Privileged Instructions
• Load base and limit registers?
• Set the system timer?
• Read the system clock?
Privileged Instructions
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Set the system timer?
Read the system clock?
Load base and limit registers?
Open a file?
Privileged Instructions
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Load base and limit registers?
Set the system timer?
Read the system clock?
Open a file?
Compile a program and create executable?
Privileged Instructions
•
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Load base and limit registers?
Set the system timer?
Read the system clock?
Open a file?
Compile a program and create executable?
Enable/disable interrupts?