Date: Tue, 17 Oct 2000 17:46:14 -0500 (CDT) From: Mike Eldridge <diz@cafes.net> To: salman latif <saquresh@yahoo.com> Cc: questions@FreeBSD.ORG Subject: Re: what r virtual devices? Message-ID: <Pine.LNX.4.10.10010171736280.27042-100000@mail.cafes.net> In-Reply-To: <20001017223049.77005.qmail@web9608.mail.yahoo.com>
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On Tue, 17 Oct 2000, salman latif wrote: > dear sir: > > 1- i need to know that what r virtual devices? what is > the purpose to creat a virtual machine/device? Well, virtual means not really, so a virtual device or machine would mean a device or machine that's not really there. :) Basically, a virtual machine is something like a java bytecode interpreter that does an 'in-between' task. Ie, java isn't directly executed, it is interpreted by a java virtual machine, which in turn executes the neccessary instructions. A virtual device is a device that is emulated in some way, shape or form. When you hear virtual, think emulation. > 2- what is the main difference b/w virtual memory, > buffer, cache memory & registers? Is this your homework? Again, virtual memory isn't real memory, it's some other form of space that is used as memory, such as additional hard disk space. Hard disk space would of course be a much slower form of memory than real memory, but since hard disks are very inexpensive compared to memory, they work well when additional memory is needed. Registers and cache are two forms of 'memory.' They keep state. Registers are located IN the processor. They are limited in number, but since they are IN the processor, they are VERY fast. i386 processors have 32-bit registers (meaning they can store 4 bytes). The new IA64 processor (codenamed Merced, recodenamed Itanium) has 64-bit registers. Alphas also have 64-bit registers. Cache is fast memory that is located on the board close to the processor. Cache can come in several levels. L1 cache is usually very small amount of memory (64K, 128K) that is on the processor die. It runs at the clock speed of the processor. L2 cache used to be on the motherboard, running at fractions of the speed of the processor. Some L2 cache is on the processor or on the processor board, such as Pentium IIs, other cache is on the motherboard somewhere. L2 cache can be anywhere from 128K (crappy) up to several MBs for the Xeon line. You usually can't find a motherboard or processor with less than 512K cache nowadays. L3 cache is a new concept, brought about by the release of the K6-3 that had faster on-die L2 cache, making the L2 cache built into the board become L3 cache. A buffer is anything that holds data for a short period of time. Mike ----------------------------------------------------- Save the whales. Feed the hungry. Free the mallocs. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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