Kernel Drivers

I have read discussion from there http://board.flatassembler.net/topic.php?t=10328 and base of it i wrote code of 'driver.sys' and 'program.exe'.

1. In 'driver.sys' change IA32_SYSENTER_EIP of MSR to main proc - NEW_IA32_SYSENTER_EIP (in DriverEntry function)
2. In 'program.exe' i use "sysenter" instruction like this:

Code:

proc switch_mode
                 mov        ebx, paramlist   ; point to list of parameters
                 pushfd
                 pop ecx
                 mov edi,   esp
                 mov edx,   TEST_SIGNAL   ; *** need explain
                 sysenter
                 nop
                 ret
endp

And now after execute function "switch_mode" program going to
point NEW_IA32_SYSENTER_EIP.
There is the proplem:
Sometimes value of edx register is changed (<> TEST_SIGNAL ), bad sometimes everything is ok and then programm execute correctly.

Whats is the reason that EDX is changed among "sysenter" execute and NEW_IA32_SYSENTER_EIP ?

Or may be i wrong understand meaning of EDX register in this code ?

Thanks for any help and sorry for my english.