Architectural Features for OS
Features that directly support OS needs include:
- Protected Instructions
- Direct user access to some hardware resources is not permitted.
- I/O devices such as discs, printers, etc. can be accessed directly.
- Instructions that alter the state of memory management (e.g., page table pointers, TLB load) must be under control.
- special mode bits are set
- halt instruction
2. OS Protection
- Hardware supports a scheme that allows us to tell apart
the trusted programmer (operating system designer) from
untrusted programmers (regular users).
- Most architectures support at least two modes of
operation: kernel mode and user mode
- The OS executes in kernel mode, user programs execute
in user mode
- Mode is indicated by a status bit in a protected processor
register
3. Memory Protection- Need to protect a user program from accessing the data in
other user programs
- Need to protect the OS from user programs
- Simplest scheme is base and limit registers:
4. I/O Control 
5. Synchronization- Interrupts cause potential problems because an interrupt
can occur at any time -- causing code to execute that
interferes with code that was interrupted
- OS must be able to synchronize concurrent processes
- This requires guaranteeing that certain instruction
sequences (read-modify- write) execute atomically
- One way to guarantee this is to turn off interrupts before the
sequence, execute it, and re-enable interrupts; CPU must
have a way to disable interrupts
– When would this not be sufficient
- Another is to have special instructions that can perform a
read/modify/write in a single bus transaction, or can
atomically test and conditionally set a bit, based on its
previous value
6. Timer Operation
- How does the OS prevent against runaway user programs
(infinite loops)?
- A timer can be set to generate an interrupt in a given time.
- Before it transfers to a user program, the OS loads the timer
with a time to interrupt.
- When the time arrives, the executing program is interrupted
and the OS regains control.
- This ensures that the OS can get the CPU back even if a user
program erroneously or purposely continues to execute
past some allotted time.
- The timer is privileged: only the OS can load it.
- Direct user access to some hardware resources is not permitted.
- I/O devices such as discs, printers, etc. can be accessed directly.
- Instructions that alter the state of memory management (e.g., page table pointers, TLB load) must be under control.
- special mode bits are set
- halt instruction
2. OS Protection
- Hardware supports a scheme that allows us to tell apart the trusted programmer (operating system designer) from untrusted programmers (regular users).
- Most architectures support at least two modes of operation: kernel mode and user mode
- The OS executes in kernel mode, user programs execute in user mode
- Mode is indicated by a status bit in a protected processor register
3. Memory Protection
- Need to protect a user program from accessing the data in other user programs
- Need to protect the OS from user programs
- Simplest scheme is base and limit registers:
- Interrupts cause potential problems because an interrupt can occur at any time -- causing code to execute that interferes with code that was interrupted
- OS must be able to synchronize concurrent processes
- This requires guaranteeing that certain instruction sequences (read-modify- write) execute atomically
- One way to guarantee this is to turn off interrupts before the sequence, execute it, and re-enable interrupts; CPU must have a way to disable interrupts – When would this not be sufficient
- Another is to have special instructions that can perform a read/modify/write in a single bus transaction, or can atomically test and conditionally set a bit, based on its previous value
6. Timer Operation
- How does the OS prevent against runaway user programs (infinite loops)?
- A timer can be set to generate an interrupt in a given time.
- Before it transfers to a user program, the OS loads the timer with a time to interrupt.
- When the time arrives, the executing program is interrupted and the OS regains control.
- This ensures that the OS can get the CPU back even if a user program erroneously or purposely continues to execute past some allotted time.
- The timer is privileged: only the OS can load it.