Llamas and alpacas have been used by humans for transportation and fleece production for a few thousand years. Both species are primarily found in Peru and Bolivia and are part of the camel family, Camelidae. Alpacas and llamas are two of the four lamoid species—the other two species, vicuña and guanaco, are their wild cousins. Interestingly, all four species are able to interbreed and create fertile offspring. While often conflated, alpacas and llamas differ in key ways. The most-distinguishing physical differences between alpacas and llamas are their size, their hair, and their face shapes. Also, they differ in disposition, which affects the way humans have used them over the years.
The most-noticeable difference between the two animals is their sizes. Alpacas are smaller, around 90 cm (35 inches) high at the shoulder and between 55 and 65 kg (121 to 143 pounds). Llamas are the biggest lamoid at about 120 cm (47 inches) at the shoulder and about 113 kg (250 pounds). So llamas are going to be a lot bigger than their cousins. Their faces are also dissimilar: alpacas have small, blunt faces with short ears, while llamas have more-elongated faces with banana-sized ears.
To summarize, you can sell or give away a Mac with the version of Mac OS X that came pre-installed on it. If you bought 10.6 Snow Leopard on its own, you can sell that on its own. But if you purchased your license to Lion or Mountain Lion through the Mac App Store, it is not transferable and must be removed before you sell or give the Mac away.
Another key difference is their hair. Alpacas have shaggy hair that is used for fleece production. Their hair color can vary greatly, from whites and light yellows to browns and blacks. Llamas’ hair is coarser, and their wool is considered inferior, but llama breeders are working to create a llama breed with finer, softer hair.
Humans usually use llamas as pack animals, since they can carry a generous amount of weight. The average llama can carry a load of 45 to 60 kg (99 to 132 pounds) for up to 30 km (18.6 miles) each day. Yet llamas get a bad reputation, since, when overloaded or maltreated, they react by spitting, kicking, lying down, or refusing to move. Usually, however, they are gentle creatures. Alpacas, on the other hand, are a bit more timid and like to stay with their herd. Llamas can be used as guard animals for livestock like alpacas and sheep.
In computing, preemption is the act of temporarily interrupting an executingtask, with the intention of resuming it at a later time. This interrupt is done by an external scheduler with no assistance or cooperation from the task. This preemptive scheduler usually run in the most privileged protection ring, meaning that interruption and resuming are considered highly secure actions. Such a change in the currently executing task of a processor is known as context switching.
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User mode and kernel mode[edit]
In any given system design, some operations performed by the system may not be preemptable. This usually applies to kernel functions and service interrupts which, if not permitted to run to completion, would tend to produce race conditions resulting in deadlock. Barring the scheduler from preempting tasks while they are processing kernel functions simplifies the kernel design at the expense of system responsiveness. The distinction between user mode and kernel mode, which determines privilege level within the system, may also be used to distinguish whether a task is currently preemptable.
Most modern systems have preemptive kernels, designed to permit tasks to be preempted even when in kernel mode, however OS must be clearly specified (and tested) to be providing preemption features.[1] Example OS include (but are not limited):
- SunOS 5.0[2]
- RTLinux (now Wind River Linux)
Preemptive multitasking[edit]
The term preemptive multitasking is used to distinguish a multitasking operating system, which permits preemption of tasks, from a cooperative multitasking system wherein processes or tasks must be explicitly programmed to yield when they do not need system resources.
In simple terms: Preemptive multitasking involves the use of an interrupt mechanism which suspends the currently executing process and invokes a scheduler to determine which process should execute next. Therefore, all processes will get some amount of CPU time at any given time.
In preemptive multitasking, the operating system kernel can also initiate a context switch to satisfy the scheduling policy's priority constraint, thus preempting the active task. In general, preemption means 'prior seizure of'. When the high priority task at that instance seizes the currently running task, it is known as preemptive scheduling.
The term 'preemptive multitasking' is sometimes mistakenly used when the intended meaning is more specific, referring instead to the class of scheduling policies known as time-shared scheduling, or time-sharing.
Preemptive multitasking allows the computer system to more reliably guarantee each process a regular 'slice' of operating time. It also allows the system to rapidly deal with important external events like incoming data, which might require the immediate attention of one or another process.
At any specific time, processes can be grouped into two categories: those that are waiting for input or output (called 'I/O bound'), and those that are fully utilizing the CPU ('CPU bound'). In early systems, processes would often 'poll', or 'busy-wait' while waiting for requested input (such as disk, keyboard or network input). During this time, the process was not performing useful work, but still maintained complete control of the CPU. With the advent of interrupts and preemptive multitasking, these I/O bound processes could be 'blocked', or put on hold, pending the arrival of the necessary data, allowing other processes to utilize the CPU. As the arrival of the requested data would generate an interrupt, blocked processes could be guaranteed a timely return to execution.
Although multitasking techniques were originally developed to allow multiple users to share a single machine, it soon became apparent that multitasking was useful regardless of the number of users. Many operating systems, from mainframes down to single-user personal computers and no-user control systems (like those in robotic spacecraft), have recognized the usefulness of multitasking support for a variety of reasons. Multitasking makes it possible for a single user to run multiple applications at the same time, or to run 'background' processes while retaining control of the computer.
Time slice[edit]
The period of time for which a process is allowed to run in a preemptive multitasking system is generally called the time slice or quantum. The scheduler is run once every time slice to choose the next process to run. The length of each time slice can be critical to balancing system performance vs process responsiveness - if the time slice is too short then the scheduler will consume too much processing time, but if the time slice is too long, processes will take longer to respond to input.
An interrupt is scheduled to allow the operating systemkernel to switch between processes when their time slices expire, effectively allowing the processor's time to be shared between a number of tasks, giving the illusion that it is dealing with these tasks in parallel (simultaneously). The operating system which controls such a design is called a multi-tasking system.
System support[edit]
Today, nearly all operating systems support preemptive multitasking, including the current versions of Windows, macOS, Linux (including Android) and iOS.
Some of the earliest operating systems available to home users featuring preemptive multitasking were Sinclair QDOS (1984[3]) and Amiga OS (1985). These both ran on Motorola 68000-family microprocessors without memory management. Amiga OS used dynamic loading of relocatable code blocks ('hunks' in Amiga jargon) to multitask preemptively all processes in the same flat address space.
Early PC operating systems such as MS-DOS and PC DOS, did not support multitasking at all, however alternative operating systems such as MP/M-86 (1981) and Concurrent CP/M-86 did support preemptive multitasking. Other Unix-like systems including MINIX and Coherent provided preemptive multitasking on 1980s-era personal computers.
Later DOS versions natively supporting preemptive multitasking/multithreading include Concurrent DOS, Multiuser DOS, Novell DOS (later called Caldera OpenDOS and DR-DOS 7.02 and higher). Since Concurrent DOS 386, they could also run multiple DOS programs concurrently in virtual DOS machines.
Alpaca's Redemption Mac Os Download
The earliest version of Windows to support a limited form of preemptive multitasking was Windows/386 2.0, which used the Intel 80386's Virtual 8086 mode to run DOS applications in virtual 8086 machines, commonly known as 'DOS boxes', which could be preempted. In Windows 95, 98 and Me, 32-bit applications were made preemptive by running each one in a separate address space, but 16-bit applications remained cooperative for backward compatibility.[4] In Windows 3.1x (protected mode), the kernel and virtual device drivers ran preemptively, but all 16-bit applications were non-preemptive and shared the same address space.
Preemptive multitasking has always been supported by Windows NT (all versions), OS/2 (native applications), Unix and Unix-like systems (such as Linux, BSD and macOS), VMS, OS/360, and many other operating systems designed for use in the academic and medium-to-large business markets.
Although there were plans to upgrade the cooperative multitasking found in the classic Mac OS to a preemptive model (and a preemptive API did exist in Mac OS 9, although in a limited sense[5]), these were abandoned in favor of Mac OS X (now called macOS) that, as a hybrid of the old Mac System style and NeXTSTEP, is an operating system based on the Mach kernel and derived in part from BSD, which had always provided Unix-like preemptive multitasking.
See also[edit]
References[edit]
- ^Erciyes, K. (2019), 'Real-Time Operating Systems', Distributed Real-Time Systems, Cham: Springer International Publishing, pp. 65–88, doi:10.1007/978-3-030-22570-4_4, ISBN978-3-030-22569-8, retrieved 2021-03-08
- ^Khanna, S.; Sebree, M.; Zolnovsky, J. 'Realtime scheduling in SunOS 5.0'. Proceedings of the USENIX Winter Conference, 1992: 375–390.
- ^QL History FAQ: Firmware
- ^'How 16-Bit and 32-Bit Programs Multitask in Windows 95 (Q117567)'. Archived from the original on 2008-01-17. Retrieved 2008-01-17.
- ^'Re: newbie question: What is a Blue Task'. Archived from the original on 2007-10-13. Retrieved 2007-03-29.