Hardware protection and interrupts

Computer-System Operation
I/O devices and the CPU can execute concurrently.
Each device controller is in charge of a particular device type.
Each device controller has a local buffer.
CPU moves data from/to main memory to/from the local buffers.
I/O is from the device to local buffer of controller.
Device controller informs CPU that it has finished its operation by causing an interrupt.
Common Functions of Interrupts
Interrupt transfers control to the interrupt service routine, generally, through the interrupt vector, which contains the addresses of all the service routines.
Interrupt architecture must save the address of the interrupted instruction.
Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt.
A trap is a software-generated interrupt caused either by an error or a user request.
An operating system is interrupt driven.
Interrupt Handling
The operating system preserves the state of the CPU by storing registers and the program counter.
Determines which type of interrupt has occurred:
polling
vectored interrupt system
Separate segments of code determine what action should be taken for each type of interrupt. I/O Structure
I/O Interrrupts
After I/O starts, control returns to user program only upon I/O completion.
wait instruction idles the CPU until the next interrupt.
wait loop (contention for memory access).
at most one I/O request is outstanding at a time; no simultaneous I/O processing.
After I/O starts, control returns to user program without waiting for I/O completion.
System call – request to the operating system to allow user to wait for I/O completion.
Device-status table contains entry for each I/O device indicating its type, address, and state (not functioning, idle or busy)
Multiple requests for the same device are maintained in a wait queue.
Operating system indexes into I/O device table to determine device status and to modify table entry to include interrupt.
DMA Structure
Used for high-speed I/O devices able to transmit information at close to memory speeds.
Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention.
Only one interrupt is generated per block, rather than the one interrupt per byte.
Basic Operation
User program (or OS) requests data transfer
The OS finds a buffer (empty buffer for i/p or a full buffer for o/p) from the pool of buffers to transfer.
The Device Driver (part of OS) sets the DMA controller registers to use appropriate source and destination addresses and transfer length.
The DMA controller is instructed to start transfer operation
While transfer is going on the CPU is free to perform other tasks
DMA controller steals cycles from CPU
DMA controller interrupts CPU when the work is done. Storage Structure
Main memory – only large storage media that the CPU can access directly.
Secondary storage – extension of main memory that provides large nonvolatile storage capacity.
Magnetic disks – rigid metal or glass platters covered with magnetic recording material.
Disk surface is logically divided into tracks, which are subdivided into sectors.
The disk controllerdetermines the logical interaction between the device and the computer.
Storage Hierarchy
Storage systems organized in hierarchy:
speed
cost
volatility
Caching – copying information into faster storage system; main memory can be viewed as a fast cache for secondary storage.
Storage-Device Hierarchy:
Hardware Protection
Dual-Mode Operation
Sharing system resources requires operating system to ensure that an incorrect program cannot cause other programs to execute incorrectly.
Provide hardware support to differentiate between at least two modes of operations.
User mode – execution done on behalf of a user.
Monitor mode(also supervisor mode or system mode) – execution done on behalf of operating system.
Mode bit added to computer hardware to indicate the current mode: monitor (0) or user (1).
When an interrupt or fault occurs hardware switches to monitor mode
Privileged instructions can be issued only in monitor mode.
I/O Protection
All I/O instructions are privileged instructions.
Must ensure that a user program could never gain control of the computer in monitor mode (i.e., a user program that, as part of its execution, stores a new address in the interrupt vector).
Memory Protection
Must provide memory protection at least for the interrupt vector and the interrupt service routines.
In order to have memory protection, add 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.
Protection Hardware:
When executing in monitor mode, the operating system has unrestricted access to both monitor and users’ memory.
The load instructions for the baseand limitregisters are privileged instructions.
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.
Timer also used to compute the current time.
Load-timer is a privileged instruction.
General-System Architecture
Given that I/O instructions are privileged, how does the user program perform I/O?
System call – the method used by a process to request action by the operating system.
Usually takes the form of a trap to a specific location in the interrupt vector.
Control passes through the interrupt vector to a service routine in the OS, and the mode bit is set to monitor mode.
The monitor verifies that the parameters are correct and legal, executes the request, and returns control to the instruction following the system call.