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A20 gate
On x86-based systems, a hardware component that forces the A20 address line on the bus to zero, regardless of the actual setting of the A20 address line on the processor. This component is in place to support legacy systems, but the QNX Neutrino OS doesn't require any such hardware. Note that some processors, such as the 386EX, have the A20 gate hardware built right into the processor itself -- our IPL will disable the A20 gate as soon as possible after startup.
Scheduling algorithm whereby a thread's priority is decayed by 1. See also FIFO, round robin, and sporadic.
Of or relating to atoms. :-)

In operating systems, this refers to the requirement that an operation, or sequence of operations, be considered indivisible. For example, a thread may need to move a file position to a given location and read data. These operations must be performed in an atomic manner; otherwise, another thread could preempt the original thread and move the file position to a different location, thus causing the original thread to read data from the second thread's position.

attributes structure
Structure containing information used on a per-resource basis (as opposed to the OCB, which is used on a per-open basis).

This structure is also known as a handle. The structure definition is fixed (iofunc_attr_t), but may be extended. See also mount structure.

A term indicating that a certain memory component (usually the device holding an image) isn't entirely addressable by the processor. In this case, a hardware component manifests a small portion (or "window") of the device onto the processor's address bus. Special commands have to be issued to the hardware to move the window to different locations in the device. See also linearly mapped.
base layer calls
Convenient set of library calls for writing resource managers. These calls all start with resmgr_*(). Note that while some base layer calls are unavoidable (e.g. resmgr_pathname_attach()), we recommend that you use the POSIX layer calls where possible.
BIOS/ROM Monitor extension signature
A certain sequence of bytes indicating to the BIOS or ROM Monitor that the device is to be considered an "extension" to the BIOS or ROM Monitor -- control is to be transferred to the device by the BIOS or ROM Monitor, with the expectation that the device will perform additional initializations.

On the x86 architecture, the two bytes 0x55 and 0xAA must be present (in that order) as the first two bytes in the device, with control being transferred to offset 0x0003.

The requirement that data be transferred such that individual structure components are transferred in their entirety -- no partial structure component transfers are allowed.

In a resource manager, directory data must be returned to a client as block-integral data. This means that only complete struct dirent structures can be returned -- it's inappropriate to return partial structures, assuming that the next _IO_READ request will "pick up" where the previous one left off.

An image can be either bootable or nonbootable. A bootable image is one that contains the startup code that the IPL can transfer control to.
The part of an OS image that runs the startup code and the Neutrino microkernel.
In sporadic scheduling, the amount of time a thread is permitted to execute at its normal priority before being dropped to its low priority.
A text file containing instructions for mkifs specifying the contents and other details of an image, or for mkefs specifying the contents and other details of an embedded filesystem image.
canonical mode
Also called edited mode or "cooked" mode. In this mode the character device library performs line-editing operations on each received character. Only when a line is "completely entered" -- typically when a carriage return (CR) is received -- will the line of data be made available to application processes. Contrast raw mode.
A kernel object used with message passing.

In QNX Neutrino, message passing is directed towards a connection (made to a channel); threads can receive messages from channels. A thread that wishes to receive messages creates a channel (using ChannelCreate()), and then receives messages from that channel (using MsgReceive()). Another thread that wishes to send a message to the first thread must make a connection to that channel by "attaching" to the channel (using ConnectAttach()) and then sending data (using MsgSend()).

Common Internet File System (aka SMB) -- a protocol that allows a client workstation to perform transparent file access over a network to a Windows 95/98/NT server. Client file access calls are converted to CIFS protocol requests and are sent to the server over the network. The server receives the request, performs the actual filesystem operation, and sends a response back to the client.
Card Information Structure -- a data block that maintains information about flash configuration. The CIS description includes the types of memory devices in the regions, the physical geometry of these devices, and the partitions located on the flash.
combine message
A resource manager message that consists of two or more messages. The messages are constructed as combine messages by the client's C library (e.g. stat(), readblock()), and then handled as individual messages by the resource manager.

The purpose of combine messages is to conserve network bandwidth and/or to provide support for atomic operations. See also connect message and I/O message.

connect message
In a resource manager, a message issued by the client to perform an operation based on a pathname (e.g. an io_open message). Depending on the type of connect message sent, a context block (see OCB) may be associated with the request and will be passed to subsequent I/O messages. See also combine message and I/O message.
A kernel object used with message passing.

Connections are created by client threads to "connect" to the channels made available by servers. Once connections are established, clients can MsgSendv() messages over them. If a number of threads in a process all attach to the same channel, then the one connection is shared among all the threads. Channels and connections are identified within a process by a small integer.

The key thing to note is that connections and file descriptors (FD) are one and the same object. See also channel and FD.

Information retained between invocations of functionality.

When using a resource manager, the client sets up an association or context within the resource manager by issuing an open() call and getting back a file descriptor. The resource manager is responsible for storing the information required by the context (see OCB). When the client issues further file-descriptor based messages, the resource manager uses the OCB to determine the context for interpretation of the client's messages.

cooked mode
See canonical mode.
core dump
A file describing the state of a process that terminated abnormally.
critical section
A code passage that must be executed "serially" (i.e. by only one thread at a time). The simplest from of critical section enforcement is via a mutex.
A condition in which one or more threads are unable to continue due to resource contention. A common form of deadlock can occur when one thread sends a message to another, while the other thread sends a message to the first. Both threads are now waiting for each other to reply to the message. Deadlock can be avoided by good design practices or massive kludges -- we recommend the good design approach.
device driver
A process that allows the OS and application programs to make use of the underlying hardware in a generic way (e.g. a disk drive, a network interface). Unlike OSs that require device drivers to be tightly bound into the OS itself, device drivers for QNX Neutrino are standard processes that can be started and stopped dynamically. As a result, adding device drivers doesn't affect any other part of the OS -- drivers can be developed and debugged like any other application. Also, device drivers are in their own protected address space, so a bug in a device driver won't cause the entire OS to shut down.
Domain Name Service -- an Internet protocol used to convert ASCII domain names into IP addresses. In QNX native networking, dns is one of Qnet's builtin resolvers.
dynamic bootfile
An OS image built on the fly. Contrast static bootfile.
dynamic linking
The process whereby you link your modules in such a way that the Process Manager will link them to the library modules before your program runs. The word "dynamic" here means that the association between your program and the library modules that it uses is done at load time, not at linktime. Contrast static linking. See also runtime loading.
One of two ways in which a PIC (Programmable Interrupt Controller) can be programmed to respond to interrupts. In edge-sensitive mode, the interrupt is "noticed" upon a transition to/from the rising/falling edge of a pulse. Contrast level-sensitive.
edited mode
See canonical mode.
End Of Interrupt -- a command that the OS sends to the PIC after processing all Interrupt Service Routines (ISR) for that particular interrupt source so that the PIC can reset the processor's In Service Register. See also PIC and ISR.
Erasable Programmable Read-Only Memory -- a memory technology that allows the device to be programmed (typically with higher-than-operating voltages, e.g. 12V), with the characteristic that any bit (or bits) may be individually programmed from a 1 state to a 0 state. To change a bit from a 0 state into a 1 state can only be accomplished by erasing the entire device, setting all of the bits to a 1 state. Erasing is accomplished by shining an ultraviolet light through the erase window of the device for a fixed period of time (typically 10-20 minutes). The device is further characterized by having a limited number of erase cycles (typically 10e5 - 10e6). Contrast flash and RAM.
A notification scheme used to inform a thread that a particular condition has occurred. Events can be signals or pulses in the general case; they can also be unblocking events or interrupt events in the case of kernel timeouts and interrupt service routines. An event is delivered by a thread, a timer, the kernel, or an interrupt service routine when appropriate to the requestor of the event.
File Descriptor -- a client must open a file descriptor to a resource manager via the open() function call. The file descriptor then serves as a handle for the client to use in subsequent messages. Note that a file descriptor is the exact same object as a connection ID (coid, returned by ConnectAttach()).
First In First Out -- a scheduling algorithm whereby a thread is able to consume CPU at its priority level without bounds. See also adaptive, round robin, and sporadic.
flash memory
A memory technology similar in characteristics to EPROM memory, with the exception that erasing is performed electrically instead of via ultraviolet light, and, depending upon the organization of the flash memory device, erasing may be accomplished in blocks (typically 64k bytes at a time) instead of the entire device. Contrast EPROM and RAM.
Fully Qualified NodeName -- a unique name that identifies a QNX Neutrino node on a network. The FQNN consists of the nodename plus the node domain tacked together.
garbage collection
Aka space reclamation, the process whereby a filesystem manager recovers the space occupied by deleted files and directories.
High Availability -- in telecommunications and other industries, HA describes a system's ability to remain up and running without interruption for extended periods of time.
A pointer that the resource manager base library binds to the pathname registered via resmgr_attach(). This handle is typically used to associate some kind of per-device information. Note that if you use the iofunc_*() POSIX layer calls, you must use a particular type of handle -- in this case called an attributes structure.
In the context of embedded QNX Neutrino systems, an "image" can mean either a structure that contains files (i.e. an OS image) or a structure that can be used in a read-only, read/write, or read/write/reclaim FFS-2-compatible filesystem (i.e. a flash filesystem image).
An event (usually caused by hardware) that interrupts whatever the processor was doing and asks it do something else. The hardware will generate an interrupt whenever it has reached some state where software intervention is required.
interrupt handler
See ISR.
interrupt latency
The amount of elapsed time between the generation of a hardware interrupt and the first instruction executed by the relevant interrupt service routine. Also designated as "Til". Contrast scheduling latency.
interrupt service routine
See ISR.
interrupt service thread
A thread that is responsible for performing thread-level servicing of an interrupt.

Since an ISR can call only a very limited number of functions, and since the amount of time spent in an ISR should be kept to a minimum, generally the bulk of the interrupt servicing work should be done by a thread. The thread attaches the interrupt (via InterruptAttach() or InterruptAttachEvent()) and then blocks (via InterruptWait()), waiting for the ISR to tell it to do something (by returning an event of type SIGEV_INTR). To aid in minimizing scheduling latency, the interrupt service thread should raise its priority appropriately.

I/O message
A message that relies on an existing binding between the client and the resource manager. For example, an _IO_READ message depends on the client's having previously established an association (or context) with the resource manager by issuing an open() and getting back a file descriptor. See also connect message, context, combine message, and message.
I/O privity
A particular privilege, that, if enabled for a given thread, allows the thread to perform I/O instructions (such as the x86 assembler in and out instructions). By default, I/O privity is disabled, because a program with it enabled can wreak havoc on a system. To enable I/O privity, the thread must be running as root, and call ThreadCtl().
Interprocess Communication -- the ability for two processes (or threads) to communicate. QNX Neutrino offers several forms of IPC, most notably native messaging (synchronous, client/server relationship), POSIX message queues and pipes (asynchronous), as well as signals.
Initial Program Loader -- the software component that either takes control at the processor's reset vector (e.g. location 0xFFFFFFF0 on the x86), or is a BIOS extension. This component is responsible for setting up the machine into a usable state, such that the startup program can then perform further initializations. The IPL is written in assembler and C. See also BIOS extension signature and startup code.
Interrupt Request -- a hardware request line asserted by a peripheral to indicate that it requires servicing by software. The IRQ is handled by the PIC, which then interrupts the processor, usually causing the processor to execute an Interrupt Service Routine (ISR).
Interrupt Service Routine -- a routine responsible for servicing hardware (e.g. reading and/or writing some device ports), for updating some data structures shared between the ISR and the thread(s) running in the application, and for signalling the thread that some kind of event has occurred.
See microkernel.
One of two ways in which a PIC (Programmable Interrupt Controller) can be programmed to respond to interrupts. If the PIC is operating in level-sensitive mode, the IRQ is considered active whenever the corresponding hardware line is active. Contrast edge-sensitive.
linearly mapped
A term indicating that a certain memory component is entirely addressable by the processor. Contrast bank-switched.
A parcel of bytes passed from one process to another. The OS attaches no special meaning to the content of a message -- the data in a message has meaning for the sender of the message and for its receiver, but for no one else.

Message passing not only allows processes to pass data to each other, but also provides a means of synchronizing the execution of several processes. As they send, receive, and reply to messages, processes undergo various "changes of state" that affect when, and for how long, they may run.

A part of the operating system that provides the minimal services used by a team of optional cooperating processes, which in turn provide the higher-level OS functionality. The microkernel itself lacks filesystems and many other services normally expected of an OS; those services are provided by optional processes.
mount structure
An optional, well-defined data structure (of type iofunc_mount_t) within an iofunc_*() structure, which contains information used on a per-mountpoint basis (generally used only for filesystem resource managers). See also attributes structure and OCB.
The location in the pathname space where a resource manager has "registered" itself. For example, the serial port resource manager registers mountpoints for each serial device (/dev/ser1, /dev/ser2, etc.), and a CD-ROM filesystem may register a single mountpoint of /cdrom.
Mutual exclusion lock, a simple synchronization service used to ensure exclusive access to data shared between threads. It is typically acquired (pthread_mutex_lock()) and released (pthread_mutex_unlock()) around the code that accesses the shared data (usually a critical section). See also critical section.
name resolution
In a QNX Neutrino network, the process by which the Qnet network manager converts an FQNN to a list of destination addresses that the transport layer knows how to get to.
name resolver
Program code that attempts to convert an FQNN to a destination address.
Node Discovery Protocol -- proprietary QNX Software Systems protocol for broadcasting name resolution requests on a QNX Neutrino LAN.
network directory
A directory in the pathname space that's implemented by the Qnet network manager.
Name of an OS developed by QNX Software Systems.
Network FileSystem -- a TCP/IP application that lets you graft remote filesystems (or portions of them) onto your local namespace. Directories on the remote systems appear as part of your local filesystem and all the utilities you use for listing and managing files (e.g. ls, cp, mv) operate on the remote files exactly as they do on your local files.
Nonmaskable Interrupt -- an interrupt that can't be masked by the processor. We don't recommend using an NMI!
Node Discovery Protocol
See NDP.
node domain
A character string that the Qnet network manager tacks onto the nodename to form an FQNN.
A unique name consisting of a character string that identifies a node on a network.
A nonbootable OS image is usually provided for larger embedded systems or for small embedded systems where a separate, configuration-dependent setup may be required. Think of it as a second "filesystem" that has some additional files on it. Since it's nonbootable, it typically won't contain the OS, startup file, etc. Contrast bootable.
Open Control Block (or Open Context Block) -- a block of data established by a resource manager during its handling of the client's open() function. This context block is bound by the resource manager to this particular request, and is then automatically passed to all subsequent I/O functions generated by the client on the file descriptor returned by the client's open().
package filesystem
A virtual filesystem manager that presents a customized view of a set of files and directories to a client. The "real" files are present on some medium; the package filesystem presents a virtual view of selected files to the client.
pathname prefix
See mountpoint.
pathname space mapping
The process whereby the Process Manager maintains an association between resource managers and entries in the pathname space.
When applied to storage media, the ability for the medium to retain information across a power-cycle. For example, a hard disk is a persistent storage medium, whereas a ramdisk is not, because the data is lost when power is lost.
Photon microGUI
The proprietary graphical user interface built by QNX Software Systems.
Programmable Interrupt Controller -- hardware component that handles IRQs. See also edge-sensitive, level-sensitive, and ISR.
Process ID. Also often pid (e.g. as an argument in a function call). See also Process ID.
An IEEE/ISO standard. The term is an acronym (of sorts) for Portable Operating System Interface -- the "X" alludes to "UNIX", on which the interface is based.
POSIX layer calls
Convenient set of library calls for writing resource managers. The POSIX layer calls can handle even more of the common-case messages and functions than the base layer calls. These calls are identified by the iofunc_*() prefix. In order to use these (and we strongly recommend that you do), you must also use the well-defined POSIX-layer attributes (iofunc_attr_t), OCB (iofunc_ocb_t), and (optionally) mount (iofunc_mount_t) structures.
The act of suspending the execution of one thread and starting (or resuming) another. The suspended thread is said to have been "preempted" by the new thread. Whenever a lower-priority thread is actively consuming the CPU, and a higher-priority thread becomes READY, the lower-priority thread is immediately preempted by the higher-priority thread.
prefix tree
The internal representation used by the Process Manager to store the pathname table.
priority inheritance
The characteristic of a thread that causes its priority to be raised or lowered to that of the thread that sent it a message. Also used with mutexes. Priority inheritance is a method used to prevent priority inversion.
priority inversion
A condition that can occur when a low-priority thread consumes CPU at a higher priority than it should. This can be caused by not supporting priority inheritance, such that when the lower-priority thread sends a message to a higher-priority thread, the higher-priority thread consumes CPU on behalf of the lower-priority thread. This is solved by having the higher-priority thread inherit the priority of the thread on whose behalf it's working.
A nonschedulable entity, which defines the address space and a few data areas. A process must have at least one thread running in it -- this thread is then called the first thread.
process group
A collection of processes that permits the signalling of related processes. Each process in the system is a member of a process group identified by a process group ID. A newly created process joins the process group of its creator.
process group ID
The unique identifier representing a process group during its lifetime. A process group ID is a positive integer. The system may reuse a process group ID after the process group dies.
process group leader
A process whose ID is the same as its process group ID.
process ID (PID)
The unique identifier representing a process. A PID is a positive integer. The system may reuse a process ID after the process dies, provided no existing process group has the same ID. Only the Process Manager can have a process ID of 1.
Pseudo-TTY -- a character-based device that has two "ends": a master end and a slave end. Data written to the master end shows up on the slave end, and vice versa. These devices are typically used to interface between a program that expects a character device and another program that wishes to use that device (e.g. the shell and the telnet daemon process, used for logging in to a system over the Internet).
In addition to the synchronous Send/Receive/Reply services, QNX Neutrino also supports fixed-size, nonblocking messages known as pulses. These carry a small payload (four bytes of data plus a single byte code). A pulse is also one form of event that can be returned from an ISR or a timer. See MsgDeliverEvent() for more information.
The native network manager in QNX Neutrino.
Quality of Service -- a policy (e.g. loadbalance) used to connect nodes in a network in order to ensure highly dependable transmission. QoS is an issue that often arises in high-availability (HA) networks as well as realtime control systems.
Random Access Memory -- a memory technology characterized by the ability to read and write any location in the device without limitation. Contrast flash and EPROM.
raw mode
In raw input mode, the character device library performs no editing on received characters. This reduces the processing done on each character to a minimum and provides the highest performance interface for reading data. Also, raw mode is used with devices that typically generate binary data -- you don't want any translations of the raw binary stream between the device and the application. Contrast canonical mode.
In sporadic scheduling, the period of time during which a thread is allowed to consume its execution budget.
reset vector
The address at which the processor begins executing instructions after the processor's reset line has been activated. On the x86, for example, this is the address 0xFFFFFFF0.
resource manager
A user-level server program that accepts messages from other programs and, optionally, communicates with hardware. QNX Neutrino resource managers are responsible for presenting an interface to various types of devices, whether actual (e.g. serial ports, parallel ports, network cards, disk drives) or virtual (e.g. /dev/null, a network filesystem, and pseudo-ttys).

In other operating systems, this functionality is traditionally associated with device drivers. But unlike device drivers, QNX Neutrino resource managers don't require any special arrangements with the kernel. In fact, a resource manager looks just like any other user-level program. See also device driver.

Rate Monotonic Analysis -- a set of methods used to specify, analyze, and predict the timing behavior of realtime systems.
round robin
Scheduling algorithm whereby a thread is given a certain period of time to run. Should the thread consume CPU for the entire period of its timeslice, the thread will be placed at the end of the ready queue for its priority, and the next available thread will be made READY. If a thread is the only thread READY at its priority level, it will be able to consume CPU again immediately. See also adaptive, FIFO, and sporadic.
runtime loading
The process whereby a program decides while it's actually running that it wishes to load a particular function from a library. Contrast static linking.
scheduling latency
The amount of time that elapses between the point when one thread makes another thread READY and when the other thread actually gets some CPU time. Note that this latency is almost always at the control of the system designer.

Also designated as "Tsl". Contrast interrupt latency.

A collection of process groups established for job control purposes. Each process group is a member of a session. A process belongs to the session that its process group belongs to. A newly created process joins the session of its creator. A process can alter its session membership via setsid(). A session can contain multiple process groups.
session leader
A process whose death causes all processes within its process group to receive a SIGHUP signal.
software interrupts
Similar to a hardware interrupt (see interrupt), except that the source of the interrupt is software.
Scheduling algorithm whereby a thread's priority can oscillate dynamically between a "foreground" or normal priority and a "background" or low priority. A thread is given an execution budget of time to be consumed within a certain replenishment period. See also adaptive, FIFO, and round robin.
startup code
The software component that gains control after the IPL code has performed the minimum necessary amount of initialization. After gathering information about the system, the startup code transfers control to the OS.
static bootfile
An image created at one time and then transmitted whenever a node boots. Contrast dynamic bootfile.
static linking
The process whereby you combine your modules with the modules from the library to form a single executable that's entirely self-contained. The word "static" implies that it's not going to change -- all the required modules are already combined into one.
system page area
An area in the kernel that is filled by the startup code and contains information about the system (number of bytes of memory, location of serial ports, etc.) This is also called the SYSPAGE area.
The schedulable entity under QNX Neutrino. A thread is a flow of execution; it exists within the context of a process.
A kernel object used in conjunction with time-based functions. A timer is created via timer_create() and armed via timer_settime(). A timer can then deliver an event, either periodically or on a one-shot basis.
A period of time assigned to a round-robin or adaptive scheduled thread. This period of time is small (on the order of tens of milliseconds); the actual value shouldn't be relied upon by any program (it's considered bad design).

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