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Memory Management, Filing Systems, and More Take a quick look at how memory management takes place inside Windows, how filing systems work, and some information on disk compression and dial-up networking. Be aware that a good portion of this information relates to Windows 3.1 and previous. I present this information as a complete backdrop on the information you should know to fix any Microsoft-based system. Memory Management We already know that Windows uses special drivers to manage memory. In Windows 3.x, HIMEM.SYS and EMM386.EXE allow you to manage your upper/high memory settings within DOS, and Windows itself was responsible for Virtual Memory management. Windows 9x is a little different. Before we go to far, I want to explain EXACTLY what virtual memory is. Under DOS, we didn't have the concept of Virtual Memory. Windows 3.x introduced it as a way to expand the available amount of memory in a computer to run more applications at once. It uses the hard drive as a memory page file or swap file. When your memory fills up beyond a certain point, RAM instructions are sent to a file on the hard drive and stored there much the same way they are in regular RAM. The difference is that RAM is about 300 times faster than your hard drive. So although virtual memory gave you the ability to run larger programs and more of them at the same time, it slowed down the running of these programs by making them dependant on hard drive speed. Here's a quick look at what you'll need to know about Virtual Memory Management. First off, Windows 9x uses Real and Protected Mode, which are modes used to load drivers. 16 bit legacy (older) drivers are loaded in real mode. 32 bit virtual device drivers are loaded in protected mode. All 32 bit device drivers have a file extension of .VXD or .386. If all the drivers loaded are 32 bit, Windows 9x does NOT need a CONFIG.SYS file. Secondly, all virtual memory management is handled automatically within Windows. There is no need to set up swap files or make settings for swap file sizes. Windows 9x manages virtual memory automatically for you, creating it's own dynamic swap file within the Windows directory. Lastly, Windows 9x doesn't use SMARTDRIVE.EXE or SHARE.EXE as Windows 3.x did. Windows 3.x required these files to free up conventional and upper memory space, but these files are built into Windows 9x. What these 3 things mean is that virtual memory management is completely handled by Windows 9x, and it can automatically adjust for any changes that have to be made. There are ways to modify how Windows 9x handles virtual memory. If you click on Start -> Settings -> Control Panels and double-click the System Icon. Click to the Performance tab, and select Virtual Memory. It should be set to "Let Windows Automatically Manage My Virtual Memory", but there are options to make changes there. We won't go into these changes, as they aren't necessary for the test. (Under Windows 2000 you can find the Page File management utility in the same spot, but messing with these files is EXTREMELY risky. Windows 2000 assumes you know more about how page file settings work, and you can seriously hamper Window's ability to manage this page file if you screw around with the settings improperly.) Filing Systems We've looked at filing systems briefly before, but I want to touch on them again in a little more detail. Filing systems are the utilities that state how files are written to a hard drive, and tell the computer how and where to look to find individual files. There are 5 filing systems that Windows has used since it's inception. They are; FAT - File Allocation Table - A 16 bit file allocation
scheme under Windows 3.x and 95. You see references to bit lengths on the filing system names. It's all well and good to know what the bit lengths are, but it's even better to understand what they mean. A lot of this ignores the idea of Filing System overhead and makes some pretty base assumptions, but here's my best shot. Let's say you have a 4-bit filing system. That means that the computer can allocate addresses on the hard drive up to 8 characters long. Because these addresses are kept in hexidecimal, we know that the total number of addresses is limited to 16x8, or 128 individual addresses. That isn't very much, as it would limit the total number of files on the hard drive to 128. An 8-bit filing system would be able to store 2048 files; still not a lot. A 16-bit filing system would have addresses 32,768 characters long, meaning it could hold 524,288 files. Now we're getting somewhere. A 32 bit operating system would have addresses 2,147,483,648 long, and could hold 34,359,738,368 files. WOW! A 64 bit operating system could handle addresses 18,446,744,073,709,552,000 long and could hold 295,147,905,179,352,832,000 files. You can see why filing systems with more bit addressing are better. The other side of this is cluster size. Each cluster corresponds to an address in the filing system. If a 100MB disk is formatted with a 4-bit operating system, it is split into 2048 sections each 48,828 bytes in size. If a file takes up 2 Kilobytes, it uses the whole 48.8K section of hard drive because only 1 file can exist per memory address. That means that there would be 46.8K of wasted space on the drive. Let's look at the same example with a 16 bit operating system. The same drive would be split into 524,288 clusters each consisting of 190 bytes. (In theory at least. The operating systems themselves automatically default to 4K cluster size as the smallest unit.) Assuming 4K cluster size, the 2K file now only wastes 2K of disk space. That's a big improvement from the 46K wasted in the 4-bit filing system. As filing systems improved, they made larger and larger drives possible. a 64-Bit filing system like NTFS could support a 1,180,591,620 Terabyte drive at 4K cluster size. Although I wish I had a hard drive that size, the 64 bit filing system won't be used to capacity for a LONG LONG LONG time. Disk Compression Another trick to improve hard drive usage is the concept of data compression. Let's say you had a 1GB drive and had 998MB of files on it. You're pretty much screwed, right? Disk compression allows you to set up a system where your 1GB drive acts like a 1.5GB drive or bigger. By using mathematical algorithms data that would normally take up 1GB of space is squeezed into a small space, making the computer (and you) think you have more space than you really have. Although this may seem like a blessing, there are problems with this system. First of all, data compression utilities like DriveSpace compress all the files into one big file, called a Compressed Volume File. (CVF) To use a file that's compressed, it must be extracted and uncompressed from the CVF. This is a slow process if you compress the wrong files. Secondly, because the drive is compressed to hold more data, you could technically compress more data than you could uncompress. Imagine compressing data so that 1.2GB was on that 1GB drive. If for some reason you needed to uncompress the drive, you couldn't. The compressed file would over-run the drive. Lastly, because the CVF is one huge, complicated, huge, algorithm-driven, did I say huge? file, it is highly susceptible to corruption. If one part of the CVF corrupts, you lost ALL the data in the CVF. (Unless you have big money to spend sending disk drives to rescue facilities.) That simple reason is why all files required to run the operating system (including the swap and page files) are NEVER compressed. Networking In Windows 9x One of the questions you probably will see is a question on whether a computer is networked or not. It used to be simple. You looked for a modem or a network card. Unfortunately, wireless networking and USB/Firewire have made this statement un-true. Therefore, the easiest way to tell if a computer is networked isn't so easy after all. There are a couple ways that computers in the Windows world are networked. They are; Dial-Up Networking - DUN uses PPP to transfer TCP/IP, NetBEUI or IPX/SPX over POTS. (God computer techies love their acronyms.) Essentially, a Dial-Up Networking system is the way a modem hooked up to a computer uses a telephone line (Plain Old Telephone System, or POTS) to communicate with another modem hooked up to another computer. This is called a Point-to-Point Protocol (PPP) connection. TCP/IP, IPX, and NetBEUI are the languages the computers talk to each other in. (Also remember that Serial Line Internet Protocol, or SLIP, was used in Windows 3.x as the connection protocol.) Direct Cable Connection - You can use the Direct Cable Connection wizard inside Windows to connect two computers up using Serial, Parallel, USB, or Firewire connections. This is called a Peer-to-Peer network environment, because every computer in the network is equal. Network Interface Cards - The standard method of communication for Windows 9x computers is a NIC. These cards are designed to use special cables to communicate directly with each other using one of the mentioned protocols. ©2002 COMPUTER INFO All rights reserved.
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