An end to high memory?
An end to high memory?
Posted Feb 28, 2020 11:48 UTC (Fri) by ecm (subscriber, #129897)In reply to: An end to high memory? by farnz
Parent article: An end to high memory?
Posted Feb 28, 2020 14:18 UTC (Fri)
by farnz (subscriber, #17727)
[Link] (1 responses)
In the original PC, they didn't set a 640 KiB limit (that comes in with the EGA card. Original IBM PCs with MDA displays have a 704 KiB limit, CGA raises that to 736 KiB, and a theoretical video adapter using I/O ports instead of a memory mapped buffer could raise it to 768 KiB.
Honestly, it looks like IBM never really thought about the real mode address limits; EGA lowered it to 640 KiB, but that comes in with the 80286 in the PC/AT, which in theory could be run in protected mode and thus not have issues around the 1 MiB limit. OS/2 1.x could thus have ensured we never needed to know about HMA, UMA, XMS etc, had IBM's vision been successful, and hence the "640 KiB" limit of their 8088 products would never have mattered. It's just that IBM failed in delivering its vision, and thus we kept treating x86 systems as ways to run DOS for far longer than intended.
Posted Mar 6, 2020 11:32 UTC (Fri)
by khim (subscriber, #9252)
[Link]
Indeed IBM was never concerned about memory limits and haven't planned for that infamous 640KiB barrier to ever exist. You opponent says IBM didn't have to reserve ⅜ of the (20-bit) address space for the UMA, 256 KiB or even only 128 KiB would have been possible too… but IBM haven't reserved ⅜ of the address space for that! Look on the System Memory Map in the manual. It places "128KB RESERVED GRAPHIC/DISPLAY BUFFER" at 256K position! XT acknowledged the fact that you may actually add more RAM, but even AT manual says it's not a standard use but option which requires an addon card! In fact that's how IBM Itself perceived it till PS/2: that's why they were so happy to allow IBM PC DOS 4 to become so much larger than IBM PC DOS 3. The idea was: "hey, 512K is standard now, we could offer option card we talked around long ago and people would get bigger DOS, yet more space for programs, too. Only by that time “an option” have already become de-facto “standard” and many packages needed 640KiB with DOS 3… P.S. And after all that IBM decided that it's doesn't matter how much BIOS takes since everyone would soon use OS/2 anyway… and introduced ABIOS… but that's another story…
An end to high memory?
An end to high memory?