Hardware Support for Operating Systems Sunny Gleason Vivek Uppal COM S 414

Multitasking In a multitasking uniprocessor OS, the OS tries to give each process the illusion that it is running on its own CPU. In reality, the OS: –runs process A for a while –gets interrupted by a timer, [the timer may need to be reset by the CPU] –saves the state of process A to memory –restores the state of another process, B (loading the PC, resumes execution of B) –runs process B for a while … [repeat] What’s the state of a process? Think!

Protection However, in a multitasking system, we can’t let the processes do everything that the CPU can do Examples: they shouldn’t be able to: –Halt the CPU –Read/Write to arbitrary devices, or locations in main memory (For example, the system timer, or the saved state of another process) We need methods of protection!

User Mode and Kernel Mode Modern CPUs support multiple modes of operation At the very least, we need user mode and kernel mode User mode is the mode under which normal applications run Kernel mode (also called Supervisor mode) is the mode under which the OS code runs How do you go between them? We’ll see – “interrupts and returning from interrupts”

User Mode and Kernel Mode Modes are implemented as integer privilege levels: user == 3, kernel == 0 In the x86 architecture, the current mode is stored using 2 bits of the MSW (machine status word) The mode is typically implemented by tagging a block of instructions in memory with the mode Jumping to new locations in memory causes the mode to change to the tagged mode

User Mode and Kernel Mode In the Intel Pentium architecture, examples of privileged instructions are: –LMSW, SMSW (load/store MSW) –MOV DBn, MOV CRn (move to debug/control registers) –LSL (load stack limit, adjusting one type of memory available to a process) –HLT (halt the CPU) A general protection exception is caused if these instructions are reached by the CPU while in user mode

User Mode and Kernel Mode How is memory protected? Typically, we check to make sure that an address is within a given range [s1, s2], that is, s1 <= a <= s2 To do this quickly, we will need additional hardware support …

Memory Protection In a shared environment, processes should not be able to write to arbitrary locations in the physical memory Processes need to have their own memory to work with, or logical address space The CPU must check each memory access to make sure that it is within the address space of the process

Memory Protection A simple scheme: –The program, as compiled, uses memory addresses from a start address A to end address B –When a program is loaded, the O/S allocates physical memory to the process from base address A’ to limit address B’ –While the process is running, A’ and B’ are loaded into special hardware registers –Special hardware translates a program address a to a physical address p using p = a – A + A’ –If p > B’, or p < A’, the hardware triggers an access out-of-bounds exception

Exceptions, faults, traps, interrupts… CPU ordinarily executes instructions in order After every intruction (clock cycle), checks the IRQ (interrupt request signal) Interrupts occur when: –a device sends a signal on the IRQ line going into the CPU (external interrupt) –an INT n instruction is executed (software interrupt) Exceptions occur due to the CPU instruction that was just executed: often these are categorized into faults and traps

Interrupts Each interrupt has a numerical priority The interrupt vector – array of function pointers to ISRs (interrupt service routines) When an interrupt occurs: –Push ret_addr = PC+4 onto the stack –Jump to [ivector_base + priority * 4] –[the CPU mode may switch] –Run the interrupt service routine code –Inform the interrupt controller that the interrupt has been handled –Pop ret_addr from the stack, jump to ret_add –[the CPU mode returns to the previous mode]

Interrupts Since interrupt service routines may access critical data structures (I/O buffers, PCB’s, etc.) we typically don’t want to be interrupted during their execution When interrupt i is being serviced, interrupts of priority j >= i are masked, or disabled Priority is not enough to save you! The kernel must be written so that higher-priority interrupts do not clobber the data structures used by lower- priority interrupts that they may have interrupted

System Calls System calls are just a special interrupt! –The process pushes system call arguments into registers, or onto the stack –Under the covers, the first argument to a system call is an integer system call identifier, call it id –[implementation note: Linux keeps an array of pointers to the actual system call routines, let’s call it sys_call[] ] –The process executes a software interrupt instruction (in Linux, INT 0x80) –The ISR for interrupt 0x80 jumps to address [sys_call[id]] –System call code places return value into return register –Returns from the interrupt using an IRET instruction

Returning from Exceptions For faults, execution returns to the original instruction (we’ll learn about this later – a page fault may trigger an ISR that loads the page from disk into memory) For traps, execution returns to the instruction following the original (have you ever wondered how debugging assembly code works?)

Peripherals Printer Key Board Mouse Speaker Storage Devices CPU communicates with the peripheral devices through a peripheral interface.

Peripheral Interface Abstract model of the interface. Top most layer of the interface High level commands (Like postscript commands) Device Model Detailed device behavior (printer/monitor)

Peripheral Interface Cont... Protocol layer Recognition of Data and command bytes Registers for giving commands to the device Register to transfer the data to the device. Lowest level Voltages, currents, Cables.

Device Organization Application Program Device Controller Device Abstract I/O machine External hardware Disks, Tapes, C D Roms

Controller Interface CommandStatus Data 0 Data 1 Data n-1 Logic... busy done Error code... busy done 0 0 idle 0 1 finished 1 0 working 1 1 undefined

Controller Interface A controller is at the protocol layer of the device. The controller could have registers dedicated for the commands and status or it could have some memory location reserved for it. In many cases the same controller may be driving many devices.

Direct Memory Access (DMA) What is the easiest I/O interface ? One I/P instruction. One O/P instruction. Each instruction selects a device, and transfers a byte of data. Disadvantages:

DMA cont… Better solution: CPU selects the device, memory address, and bytes of data to be transferred After this initial set up the data transfer is taken control of by the device controller and I/O and computation at the CPU take place in parallel. This is Direct memory Access(DMA). Cycle Stealing:

Interrupts A scenario when the I/O has finished. What should the device do to let the process know that I/O has finished ? 2 options Set flag in status. Wait for CPU to check it (Polling) Send a signal to CPU saying the I/O finished (Interrupts)

Priorities Scenario Few resources Many contenders So we need arbitration Leads to priorities

Interrupt Priorities Power event 30 Inter processor signal 29 Clock tick 28 Performance monitoring 27 General device interrupts 3-26 Scheduler operations 2

Daisy chained I/O buses Device 1 Highest priority Device 2 Bus Arbiter Device n Lowest Priority Grant Release Request

Daisy Chain Bus

Daisy Chain Disadvantages Unfair One device goes down the whole down stream is down The bus may have to be turned off when adding a new device to it

Review Several devices are on the same I/O bus Use some mechanism like daisy chain to determine who gets to use the bus I/O devices do DMA for bulk transfers Devices make interrupts when they are done with the I/O. Device controllers are visible in memory at some predefined address.