> I've never heard any claims or evidence that 32/64 bit ODs have any > impact on the harddisk access times. Someone please correct me if I'm > wrong but this is not the reason people use 64 bit OSes. None at all on a typical system. Disk transfers are 16bit for ATA anyway (32bit PIO as a special case we don't normally do but some day I should add for the drives that do it), and DMA is used for all real disks today. > As far as I am aware there are two reasons. > > 1. For technical reasons, a 64 bit build is normally a bit faster than a > 32 bit build, just due to compiler options. For normal desktop usages > its unlikely you would notice a difference, but it can have one if you > do heavy number crunching. Its a bit more complicated. Generally speaking on systems user 64bit binaries are slower than 32bit as they are bigger. PC (x86/x86-64) is a bit different as x86-64 is very compact and also because it doubles the number of usable registers in the instruction set which is a huge win. So typically its about 10-20% faster. Desktop performance is usually bounded by video performance (lots of opportunity for the X folks to improve performance left) and most of the time the box is idle anyway so being able to display an icon 20% faster isn't a big visible hit. > > 2. 64 bit OSes, have a larger memory address they can access. With 32bit > OSes the largest amount of memory a single process can use is 4G I > think. With 64bit this problem goes away. You may wonder why any single > process may need that much ram... Its true not many will but some tasks, > like large databases or image processing can get that high. Even more importantly - the kernel can address more than 4GB at a time without doing expensive memory management unit updates. That makes a very big difference above 4GB and quite a big one above 1GB RAM (the kernel normally has 1GB of RAM and 3GB available for the user application - more requires switching between the user and physical memory view which is not cheap)
