Detecting Memory (x86): Difference between revisions
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* On modern systems there can also be faulty RAM that INT 0x15, eax=0xE820 says not to use, that can be anywhere (except in the first 1 MB, probably).
* There can also be large arbitrary memory holes. (e.g. a NUMA system with RAM up to 0x1FFFFFFF, a hole from 0x20000000 to 0x3FFFFFFF, then more RAM from 0x40000000 to 0x5FFFFFFF.)
* It's possible for physical addresses to be truncated in various ways. For example, older chipsets often have less address lines than supported by the processor and typically on motherboard using such chipsets, the extra address lines are simply not connected. For example, Intel desktop chipsets prior to the 955X only have 32 address lines, even though they are usually used with a processor that supports at least PAE. The extra address lines (A32-A35) are simply not connected on the motherboard, and if the processor attempts to access memory using physical addresses exceeding 32-bit, the physical address is truncated by virtue of the extra address lines not being connected on the motherboard.
* There are memory mapped devices (PCI video cards, HPET, PCI express configuration space, APICs, etc) with addresses that must be avoided.
* There are also (typically older) motherboards where you can write a value to "nothing" and read the same value back due to bus capacitance; there are motherboards where you write a value to cache and read the same value back from cache even though no RAM is at that address.
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