16 bits into the segment register…(GS)

software | Windows |

These instructions read a full pointer from memory and store it in the selected segment register:register
pair. The full pointer loads 16 bits into the segment register SS, DS, ES, FS, or GS

software |GNU/Linux|

(gdb) info  registers
rax            0xfffffffffffffdfc	-516
rbx            0x5dc	1500
rcx            0xffffffffffffffff	-1
rdx            0x5dc	1500
rsi            0x1	1
rdi            0x7fff599ac280	140734696702592
rbp            0x1f08af0	0x1f08af0
rsp            0x7fff599ac230	0x7fff599ac230
r8             0x0	0
r9             0xffffffff	4294967295
r10            0x8	8
r11            0x246	582
r12            0x7fff599ac280	140734696702592
r13            0x7fff599ac290	140734696702608
r14            0x0	0
r15            0x1	1
rip            0x7f0129e710c8	0x7f0129e710c8
eflags         0x246	[ PF ZF IF ]
cs             0x33	51
ss             0x2b	43
ds             0x0	0
es             0x0	0
fs             0x0	0
gs             0x0	0
Instead of FS segment descriptor on x86 versions of the Windows NT family, GS segment descriptor is used
to point to two operating system defined structures: Thread Information Block (NT_TIB) in user mode
and Processor Control Region (KPCR) in kernel mode. Thus, for example, in user mode GS:0 is the address
of the first member of the Thread Information Block. Maintaining this convention made the x86-64 port
easier, but required AMD to retain the function of the FS and GS segments in long mode — even though
segmented addressing per se is not really used by any modern operating system.

| Related Discussion |

leilei wrote:
I am writting a program for target board which have a 486 cpu, 512K ram(0x0 to 0x7ffff), 512k flash
(0x80000 to 0xFFFFF).My program will be burned into flash. My program is to initialize the GDT, IDT, TSS,
move them to memory. Now I can enter protected model and mov GDT, IDT correctly.But when I am about to
mov TSS, some exception came out, and the CPU reset automatically. The code casue the problem is like
mov cx, gdt_idx
mov gs, cx

when cpu run to the instuction 'mov gs, cx', CPU will reset. i can assure the value in cx is correctly.
can any one give me some tips about how can this be happend?

This seems to have nothing to do with TSS, yet. The CPU is not happy with the selector attempted
to load GS with. Please check that the number in CX is a valid GDT selector within the table range.

It also seems that there is no handler available for the exception generated by the segment loading.


Tauno Voipio
tauno voipio (at) iki fi

| Variation |

I think eax has a typical closer connection to 32 bit software architecture of an operating system and rax is like for 64 bit OS.