Uses of pages near zero

Posted Aug 18, 2009 20:55 UTC (Tue) by jreiser (subscriber, #11027)
In reply to: Null pointers, one month later by patrick_g
Parent article: Null pointers, one month later

Is it just for Wine or [are] there other softwares using the map at adress zero ?

"All memory is equal, but the memory near address zero is more equal than others." On x86 (protected mode, both 32-bit and 64-bit) and PowerPC (both 32-bit and 64-bit) the hardware itself supports the low 64KiB or 32KiB better than any other region. Some forms of every branch instruction can access low memory always, in addition to the usual region near the program counter. On the PowerPC this is explicit: the AA bit (Absolute Addressing: the bit with positional value (1<<1)) in the instruction. On x86 it is implicit: the 0x66 prefix byte, which performs target_address &= 0xffff; just before branching, and the 0x67 prefix byte, which makes the 0xe9 (and 0xe8) opcodes take a 16-bit displacement instead of a 32-bit displacement. On PowerPC the benefit is a larger set of target addresses, including some targets that are universally accessible regardless of the current value of the program counter. On x86, another benefit also is smaller size: 2, 3, or 4 bytes for a branch instead of 5, or only 5 bytes for some universally-accessible targets on x86_64. Also, do not overlook the advantage of using just 16 bits for storing pointers to an important collection.

Most traditional static compilers such as gcc never use these features. However, there are other compilers, program processors, and runtime re-writers which take advantage of the hardware to offer otherwise-impossible features.