It's a question that comes up quite often, and much has been said about it already. But I figured I'd throw in my two cents.
The situation is this: you've got a computer with 4GB RAM, but Windows (or Linux, or MacOS, etc.) reports you have only 3.25GB (or some other number less than 4GB). Where did the rest of the RAM go? How can you use the full amount? The answer to that last question is unfortunately that you can't, not with a 32 bit version of Windows anyway.
Here's what's going on. To read and write information in memory a computer program must specify where in memory it wishes to do so. For this it uses an address (I'm leaving virtual memory out of this discussion because it's irrelevant and would only complicate matters, so in this article when I say address I mean a physical address, that refers to an actual byte on your real RAM chips). Ever since the introduction of the 386 these addresses have been 32 bits in size. This means that there are 2 to the power of 32, or 4294967296 possible addresses. Because the x86 architecture addresses individual bytes, this means a maximum of 4294967296 bytes, or 4GB, can be addressed. This is the physical address space.
Many devices like your video or sound card use memory mapping to allow software to communicate with those devices. What this means is that they use certain addresses don't actually refer to a byte in memory; instead writing to that address sends information to that device. To do this they reserve a portion of the physical address space. If your video card is using 512MB of that address space and you install 4GB of memory in the system, you would need 4.5GB of address space to address all that, which you don't have. So 512MB of physical RAM cannot be addressed (because those addresses are used by the video card) and thus cannot be used or even "seen" by the OS.
Note that it's not only the video card; most other devices do this as well. Using the System Information tool in Windows (Start, Run, msinfo32.exe) you can see all hardware devices that do this and what address ranges they use under the "Hardware Resources, Memory" option (for the oldies: notice how there are still some devices that map into the old "upper memory" region (the area between the first 640KB and 1MB of RAM)). The ranges are shown as hexadecimal numbers.
Most 32 bit processors support an extended addressing mode called PAE (Physical Address Extension) which allows it to use 36 bit addresses for a total of 64GB of address space. 64 bit CPUs can use 64 bit addresses which allow for (theoretically) more than 16 million GB RAM (16 ExaBytes). However, for compatibility reasons all hardware devices will map into the first 4GB reason of address space (otherwise they wouldn't be able to run a 32 bit OS). So there's still an overlap and in the default scenario you'd still lose part of your RAM (even if you have e.g. 8GB RAM, you'd lose part of the first 4GB and see only e.g. 7.2GB). But most modern motherboards are capable of remapping the upper portion of RAM into the range above 4GB so all of it can theoretically be used (unless you actually have 64GB of RAM on a system with PAE).
I say theoretically because you still need OS support for the 36 bit addressing (or a real 64 bit OS on 64 bit systems). Windows does support PAE (among other things it's needed for Data Execution Prevention in Windows XP SP2 and up) but unless you have Windows Server it doesn't actually allow you to use the address above the 4GB boundary so you still lose RAM even if your motherboard has this support. The 64 bit versions of Windows (even the client versions) allow you to use 18TB of RAM so naturally it can use the full 4GB if your motherboard can do the remapping.
So simply put, if you want to use 4GB RAM and not lose any of it, you need a 64 bit CPU and either Windows Server, a 64 bit version of Windows, or some other operating system that either supports PAE or 64 bit.
Or nearly, anyway.
And it's been quite a year for me. A year in which I found out I would go to Japan, went there, and passed the exams so I could stay. Eight months I spent outside the Netherlands, beating my previous record of six weeks by a large margin.
I left behind friends and family, but made a lot of new friends, visited some great places, and generally had a great time. And I got to see my family again in the past week, and tonight I'm having a new years party with a lot of my old friends which is bound to be lots of fun too.
And there was work, dominated first by the Tokyo University entrance exams, then by lectures and the Rinko, and a lot of reading throughout. Looking forward I hope to get a more solid grasp on my research topic soon, and finish the lectures so I can concentrate on research. Work also meant learning Japanese, which I'm still nowhere near as good at as I would like. I have my moments, in which I construct compex sentences that surprise even me, but most of the time I'm still struggling, mainly lacking the necessary vocabulary to express myself, and the experience in building Japanese sentences on the fly.
What will 2008 bring? I can't say. More good times in Japan, I'm sure. Good progress towards my PhD would be nice too. :)
As I post this, 2008 is only 15 minutes away in Japan. Here I have another eight hours to go. But no matter in which time zone you are: happy new year! あけましておめでとうございます！ (akemashite omedetou gozaimasu)
I'm going back to the Netherlands for two weeks starting today. I must say I'm looking forward to eating decent bread again. :P
The disadvantage: I had to get up really early, it's 6:45 in the morning as I'm writing this. But I'll survive. :)
It's time to once again break my usual habit of not rehashing other people's news. Microsoft has remained very tight-lipped about what Internet Explorer 8 will be like, but just recently they've started trickling out information.
Today they announced that IE8 renders the Acid2 CSS compliance test correctly. This, and other work being done on standards compliance in IE8, constitute a major leap forward for IE, and this means good news for web developers like myself (although it'll be a while until we can truly use this stuff, seeing as how even now IE6 still outnumbers IE7, so no telling when IE8 will become the majority).
I hope they'll release a beta soon. :)
Every semester, the Graduate School of Information and Communication Engineering at the University of Tokyo holds a special seminar (in Japanese: 輪講 rinko) where the various students give a presentation on a topic that will likely be related to their research (although people switching subject entirely afterwards is not unheard of :P ). Basically you do a survey of some recent papers in your field. Since I'm a new student, I had to give a presentation as well (in the future I will need to attend this seminar but I won't need to do another presentation myself).
This presentation is taken rather seriously. Although it is only 25 minutes long, everybody makes an aweful fuss about it. I don't think there was this much to do about even my final presentation for my Master's degree. :)
They even made me do two rehearsals beforehand with some members from my lab. Although I'm not particularly fond of doing that, one positive result was that this is the first presentation in years where I didn't go over time. I had 25 minutes, and I did it in 23.
Now I've passed this particular hurdle, I should really start looking into what exactly I want to do for my research (I have a general idea, but nothing fixed yet). Not that I won't still be busy in the mean time: next week I've got another paper to do for the Web Engineering lecture, and I also have a presentation coming up for Distributed Systems. Neither of which should be anywhere near as time-consuming as the Rinko stuff, though.
On top of which, I seem to have cought a cold somewhere. :( It doesn't look like it's going to be too bad though.
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