为什么x86 Linux程序起始地址是从0x08048000开始的？

1. The load address is arbitrary, but was standardized back with SYSV for x86. It's different for every architecture. What goes above and below is also arbitrary, and is often taken up by linked in libraries and mmap() regions.

2. The answer is really: a bunch of things. There is no magical meaning to the load address of the executable and pretty much anything can be mapped to the lower addresses. Common examples including: C library (such as the C library), the dynamic loader ld.so and the kernel VDSO (kernel mapped dynamic code library that provides some of the interface to the kernel in x86 Linux). But you can pretty much map anything you desire there, using the mmap() system call.

For example on my specific machine the map is as follows (acquired but "cat /proc/self/maps"):

gby@watson:~$ cat /proc/self/maps
001c0000-00317000 r-xp 00000000 08:01 245836     /lib/libc-2.12.1.so
00317000-00318000 ---p 00157000 08:01 245836     /lib/libc-2.12.1.so
00318000-0031a000 r--p 00157000 08:01 245836     /lib/libc-2.12.1.so
0031a000-0031b000 rw-p 00159000 08:01 245836     /lib/libc-2.12.1.so
0031b000-0031e000 rw-p 00000000 00:00 0
00376000-00377000 r-xp 00000000 00:00 0          [vdso]
00852000-0086e000 r-xp 00000000 08:01 245783     /lib/ld-2.12.1.so
0086e000-0086f000 r--p 0001b000 08:01 245783     /lib/ld-2.12.1.so
0086f000-00870000 rw-p 0001c000 08:01 245783     /lib/ld-2.12.1.so
08048000-08051000 r-xp 00000000 08:01 2244617    /bin/cat
08051000-08052000 r--p 00008000 08:01 2244617    /bin/cat
08052000-08053000 rw-p 00009000 08:01 2244617    /bin/cat
09ab5000-09ad6000 rw-p 00000000 00:00 0          [heap]
b7502000-b7702000 r--p 00000000 08:01 4456455    /usr/lib/locale/locale-archive
b7702000-b7703000 rw-p 00000000 00:00 0
b771b000-b771c000 r--p 002a1000 08:01 4456455    /usr/lib/locale/locale-archive
b771c000-b771e000 rw-p 00000000 00:00 0
bfbd9000-bfbfa000 rw-p 00000000 00:00 0          [stack]

3. We can use the address space below 0x08048000, use mmap system call.