On Nov 3, 2011, at 12:13 PM, George R Goffe wrote:
grep 'BIOS-e820' /var/log/dmesg[ 0.000000] BIOS-e820: 0000000000000000 - 000000000009f000 (usable)
0009F000=636K; this is to the bottom of the UMB. ("640K should be enough for anyone")....
[ 0.000000] BIOS-e820: 000000000009f000 - 00000000000a0000 (reserved) [ 0.000000] BIOS-e820: 00000000000d2000 - 00000000000d4000 (reserved) [ 0.000000] BIOS-e820: 00000000000dc000 - 0000000000100000 (reserved)
UMB/BIOS (Real mode) which is still 'reserved' in this x86_64 day....
[ 0.000000] BIOS-e820: 0000000000100000 - 00000000bfed0000 (usable)
1MB and up to 3070MB, your 3GB of RAM.
[ 0.000000] BIOS-e820: 00000000bfed0000 - 00000000bfedf000 (ACPI data) [ 0.000000] BIOS-e820: 00000000bfedf000 - 00000000bff00000 (ACPI NVS) [ 0.000000] BIOS-e820: 00000000bff00000 - 00000000c0000000 (reserved) [ 0.000000] BIOS-e820: 00000000f0000000 - 00000000f4000000 (reserved) [ 0.000000] BIOS-e820: 00000000fec00000 - 00000000fec10000 (reserved) [ 0.000000] BIOS-e820: 00000000fed00000 - 00000000fed00400 (reserved) [ 0.000000] BIOS-e820: 00000000fed14000 - 00000000fed1a000 (reserved) [ 0.000000] BIOS-e820: 00000000fed1c000 - 00000000fed90000 (reserved) [ 0.000000] BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved) [ 0.000000] BIOS-e820: 00000000ff800000 - 0000000100000000 (reserved)
PCI and PCI express mapping areas. Your video (among other things) needs memory addresses; think of this as the 32-bit equivalent of the 20-bit upper memory blocks. In the case of 386 and up processors, hardware remapping of the reserved addresses was accomplished through virtual 8086 mode from 32-bit protect mode, and thus a 386 memory manager (QEMM, EMM386, etc) actually was a 32-bit 'kernel' that presented a single V86 'VM' and emulated real mode in that V86 VM, and set up the 386's MMU to map RAM into those holes. Microsoft built upon this foundation the House of Windows/386, which morphed into the House of Windows 3.x (enhanced), which morphed into the House of Win9x/ ME.....
Many 386 motherboards from that era had '1MB' of RAM, but only 640K was usable without UMB mapping (and address line A20 'rollover' into the HMA, thanks to the segmented x86 architecture, for boards with more than 1MB). The 32-bit boards have the same line, at 3GB, and for much the same reasons.
The BIOSs on Dells with 945 chipsets allow the ACPI business to move up, and thus frees up a few hundred MB of RAM address space.
The 965 chipset has hardware memory remapping, and can thus take the 1GB of RAM 'lost' by the PCI hole and put it above the 4GB line, thus making the system need 33 bit or better hardware addressing (just because the CPU has more than 32-bit addressing doesn't mean those lines are connected to anything, after all.....)
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