CE Operating Systems Lecture 2 Low level hardware support for operating systems

Overview of lecture In this lecture we will be looking at low level hardware facilities that are needed to support operating systems In particular we will look at: 1. How computer system operation requires interrupts and how interrupts are handled 2. How CPU dual mode operation can control which programs can execute which instructions

3. The need to provide mechanisms to protect the CPU, memory and I/O from being used to corrupt the proper operation of the system 4. Direct Memory Access & the memory hierarchy

Computer-System operation is interrupt driven I/O devices and the CPU can execute concurrently. So we need a mechanism for the running program to begin I/O and for I/O devices to signal that it has completed whatever I/O has been requested Each type of I/O device has a piece of hardware called a device controller which controls the operation of the I/O devices.

Each device controller has a local buffer. CPU moves data from main memory to the local buffer and vice versa. Actual I/O occurs between the device and the local buffer of controller. Device controller informs CPU that it has finished its operation by causing an interrupt.

These are the devices that make up a typical system. Any of these devices can cause an electrical interrupt that grabs the attention of the CPU. Operating system is interrupt driven

I/O processing High level view of I/O interrupt processing

Interrupt Handling An interrupt is a signal that stops execution of currently executing program because some other code needs to use the CPU to deal with the request for service This interrupt may be signal from Hardware From I/O device – signaling I/O completion From any hardware signaling some fault or problem that needs dealing with e.g. power low on a laptop Running program itself (software interrupt) – will be discussed more later

Interrupt Handling (Cont.) The operating system saves the state of the CPU by saving various working registers and the program counter. The OS then determines which type of interrupt has occurred by either: Polling Using vectored interrupts

Interrupt Handling (Cont.) Polling involves checking device controller status registers to see if device needs service and if service required invoking appropriate code Vectored interrupt system - uses a table of addresses (called vectors) of interrupt service routines (ISRs) - interrupt passes to OS a number which is an index into the table - thus identifies which ISR needs to be executed

Interrupt Handling (Cont.) Interrupt Service Routine (ISR) - part of OS - carries out appropriate action for each type of interrupt when ISR has finished the OS either restores the state of CPU (restores saved register values of program that was interrupted into correct registers in CPU) or invokes scheduler to determine whether a different program should run next

Interrupt Handling (Cont.) Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt However, you can organise interrupts into priority levels, so that interrupts of a higher priority can interrupt interrupts of a lower priority level

Interrupt Handling (Cont.) A trap is a software-generated interrupt caused either by 1. an instruction executed as part of the running program – it is the means by which the running program can signal the operating system that it needs the operating system to do something for it - how system calls (see later) are ultimately implemented OR 2. a software error (e.g. attempt to divide by zero)

CPU Dual-Mode Operation - the need for it Why does user program need to ask OS to do things for it? User programs do not run in isolation but run on system with other programs System resources need to be shared between these programs and this requires operating system to ensure that one program cannot cause other programs to execute incorrectly. Programs must not interfere with each other

Thus a normal user program must not be allowed to use instructions that could corrupt the proper execution of other programs.

CPU Dual-Mode Operation - what it is To prevent user programs from executing instructions that might corrupt another user’s programs dual-mode operation was introduced. CPU needs at least 2 modes of operation: 1. User mode – when executing user programs - CPU only permits execution of subset of its instruction set. 2. Supervisor mode (also called monitor or system mode) – when executing operating system - can execute all instructions.

CPU Dual-Mode - how it works

Mode bit added to computer hardware to indicate the current mode: supervisor (0) or user (1). When an interrupt or fault occurs hardware switches to supervisor mode - when OS restarts user program it switches it to user mode instructions that can only be used in supervisor mode are called Privileged instructions.

Only OS runs in supervisor mode Must ensure that a user program never gains control of the computer in supervisor mode At system start only OS is running - in supervisor mode just before running a user program OS switches CPU to user mode user program then runs - in user mode Of course changing mode bit needs to be a privileged instruction

Only OS runs in supervisor mode CPU goes into supervisor mode only when an interrupt occurs When interrupt occurs, user program is halted temporarily and control of CPU is passed to ISR for the interrupt – but ISR is part of OS Thus only OS runs in supervisor mode

Dual-mode operation implies need for memory protection BUT what if user program stores the address of part of its own code in an interrupt vector - it can gain control of CPU in supervisor mode. Thus system memory needs some form of protection

Memory Protection Must provide memory protection for the interrupt vector and the interrupt service routines - but also user programs and data One simple mechanism to provide 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.

Attempt to access memory outside range causes an error interrupt to OS to deal with problem

Example Memory Protection

Memory protection using base/limit registers

When executing in supervisor mode, the operating system has unrestricted access to all of memory – memory of OS itself and each users’ memory. The load instructions for the base and limit registers need to be privileged instructions.

CPU Protection What if a user program goes into an infinite loop? We need something that will enable OS to gain control of CPU so it can stop running program and start other programs. Timer – interrupts computer after specified time has elapsed to ensure operating system can maintain control. Timer is decremented every clock tick. When timer reaches 0, an interrupt occurs.

Timer commonly used to implement time sharing. Timer also used to compute the current time. Loading the timer needs to be a privileged instruction.

I/O structure a) synchronous I/O b) asynchronous I/O

I/O Structure Synchronous I/O - after I/O starts, control returns to user program only when I/O completed. CPU waits by executing an instruction that makes it go idle until next interrupt or goes into a busy loop repeatedly polling device to see if I/O completed. at most one I/O request is outstanding at a time; no simultaneous I/O processing.

Asynchronous I/O - after I/O starts, control returns to user program without waiting for I/O to complete. This needs a device-status table to contain entries for each I/O device indicating its type, address, and state Multiple requests for particular I/O can then be queued (linked list) on the device OS indexes into device table to determine device status

Device status table

I/O Protection To prevent one user program from interfering with the output or input of data that belongs to another user program all I/O instructions are privileged instructions.

System calls Given that I/O instructions are privileged, how does the user program perform I/O? System call – this is the method used by a running program to request action by the operating system. Usually takes the form of a trap (software interrupt) – we met these earlier

The trap (software interrupt) will provide an interrupt vector to identify the interrupt service routine (ISR) required, the mode bit will then be set to supervisor mode and ISR begins execution. The running program passes information to OS about the exact service it requires via parameters to system call OS verifies that this information (parameters) are correct and legal, executes the request, and returns control to the instruction following the system call.

System call sequence

Direct Memory Access (DMA)

Direct Memory Access is 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 - uses cycle stealing Only one interrupt is generated per block of data, rather than the one interrupt per byte.

Memory Structure Main memory – only large data area that the CPU can access directly - but volatile not large enough to hold all data/programs Secondary memory – extension of main memory that provides large nonvolatile storage capacity

Memory Hierarchy Storage systems organized in hierarchy: higher levels give more speed, but at greater cost and with greater volatility

Storage-Device Hierarchy

Caching principle Caching principle – maintaining a copy of some of the information from a slower storage medium on a faster medium; information held in cache is that currently being used. main memory can be viewed as a fast cache for secondary memory problem - to provide mapping between copy and original information and maintain consistency between them both

References Operating System Concepts. Chapter 1.