software | Windows |
In computing, a code segment, also known as a text segment or simply
as text, is a phrase used to refer to a portion of memory or of an
object file that contains executable instructions.
source :
http://en.wikipedia.org/wiki/Code_segment
Note that code may always modify all segment registers except CS (the
code segment). This is because the current privilege level (CPL) of the
processor is stored in the lower 2 bits of the CS register. The only way
to raise the processor privilege level (and reload CS) is through the
lcall (far call) and int (interrupt) instructions. Similarly, the only way to
lower the privilege level (and reload CS) is through lret (far return) and
iret (interrupt return).
source :
http://en.wikipedia.org/wiki/X86_memory_segmentation
software | GNU/Linux |
(gdb) info registers
rax 0xfffffffffffffdfc -516
rbx 0x5dc 1500
rcx 0xffffffffffffffff -1
rdx 0x5dc 1500
rsi 0x1 1
rdi 0x7fff6f396d50 140735059422544
rbp 0xb4a160 0xb4a160
rsp 0x7fff6f396d00 0x7fff6f396d00
r8 0x0 0
r9 0xffffffff 4294967295
r10 0x8 8
r11 0x246 582
r12 0x7fff6f396d50 140735059422544
r13 0x7fff6f396d60 140735059422560
r14 0x0 0
r15 0x1 1
rip 0x7fc4561ec0c8 0x7fc4561ec0c8
eflags 0x246 [ PF ZF IF ]
cs 0x33 51
ss 0x2b 43
ds 0x0 0
es 0x0 0
fs 0x0 0
gs 0x0 0
(gdb)
cs 0x33 51
Register operands are always prefixes with `%'. The 80386 registers
consist of the 6 section registers `%cs' (code section), `%ds' (data
section), `%ss' (stack section), `%es', `%fs', and `%gs'.
source :
http://www.cs.utah.edu/dept/old/texinfo/as/as.html#SEC152
| Related Discussion |
CS Register Setting by VnutZ :: NR10 :: Show
The article correctly mentions the importance of setting up segment
registers, yet like most neglects to set up CS (which is 0×0000). This is
one nasty latent bug that shows itself as soon as you try doing indirect
jumps. So if you want to use something like threaded code in your first
stage bootloader set CS by "jmp 0×07c0:foo" first.
You’re right – it would have been "good practice" to set the CS register.
However, the CS register is already correctly set by the BIOS. If it
were not set … a computer would never boot up! CS (code segment)
and IP (instruction pointer) are both set to point directly at 0000:7C00
which is where the BIOS loads the bootsector into.
source:
http://www.omninerd.com/comments/10807
| Variation |
The way to execute user processes in kernel mode in AMD64 is almost
the same as it is in IA-32. To execute user processes in kernel mode,
the only thing KML does is launch user processes with the CS segment
register, which points to the kernel code segment instead of user code
segment.
In AMD64 CPUs, the privilege level of running programs is determined by
the privilege level of their code segment. This is almost the same as in
IA-32 CPUs; the only difference is the segmentation memory system is
degenerated in AMD64. Although segment registers still are used in 64
-bit mode of AMD64, the only segment that the segment registers can
use is the 16 EB flat segment. Thus, the role of the segment
descriptors is simply to specify privilege levels. Therefore, only four
segments—kernel code segment, kernel data segment, user code
segment—exist in 64-bit mode.
source and link(s) :
http://www.linuxjournal.com/article/8023?page=0,1
http://www.thefreedictionary.com/degenerated
valavil.in 