On Sat, 1 Jan 2022 at 02:12, Travis Siegel <tsie...@softcon.com> wrote: > > supposedly, XP could handle up to 4GB > of ram, but when I installed 4GB in my machine, XP only saw 3.5GB.
What Jon Brase said, broadly. Remember the original PC's 640 kB limit? The 8088 and 8086 could address 1 MB of RAM, but DOS on the PC could only use the first 2/3 of it. The top 1/3 of the address space was reserved for ROMs (not just the PC BIOS, but also the video card's BIOS, possibly the ROM of a bootable network card or SCSI card and so on) and for I/O space. If you added an hardware LIM-spec EMS card, then the page frame went in that top 384 kB too. https://en.wikipedia.org/wiki/Expanded_memory#:~:text=Expanded%20memory%20is%20an%20umbrella,to%20as%20%22LIM%20EMS%22. Well, 32-bit PCs have an analogous issue. 32-bit Intel x86 chips can address a total of 4 GB of address space. This space is not just RAM, but also ROM, I/O space and so on. While a 1980s video card might only have 32 kB of RAM, mapped into the space above 640 kB, a 21st century video card may have several gigabytes of its own RAM. Too much to map into a 32-bit memory space, or there'd be no room left for RAM! So a relatively small window is mapped into the memory map -- maybe 256MB or so -- and that's used to write data into the video card's memory. Also mapped in there is the ROM of any bootable hardware, and I/O space, and so on. So just like 384 kB of the 1MB of address space in the PC was reserved for ROM, video memory and I/O, and so couldn't be used for RAM, in a 32-bit PC, a certain amount of its 4GB of address space is reserved for ROM, video memory and I/O and can't be used for RAM. And just like EMS on an 8088/8086 PC, in x86-32, there is an additional mechanism for accessing more by paging little chunks of it into the limited available space: it's called PAE. https://en.wikipedia.org/wiki/Physical_Address_Extension Mac OS X can use it, Linux can use it, Windows Server can use it, but in order to boost sales of 64-bit Windows and Windows Server, and accelerate the transition away from 32-bit OSes, Microsoft made sure that the ability is turned off in 32-bit Windows. It's there in the hardware, but 32-bit XP/Vista/7/8.x/10 can't use it. -- Liam Proven ~ Profile: https://about.me/liamproven Email: lpro...@cix.co.uk ~ gMail/gTalk/FB: lpro...@gmail.com Twitter/LinkedIn: lproven ~ Skype: liamproven UK: (+44) 7939-087884 ~ Czech [+ WhatsApp/Telegram/Signal]: (+420) 702-829-053 _______________________________________________ Freedos-user mailing list Freedos-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freedos-user
