Assembly Language for Beginners

6.2. THREAD LOCAL STORAGE

mov DWORD PTR _rand_state[ecx], eax

and eax, 32767 ; 00007fffH

ret 0

_my_rand ENDP

_TEXT ENDS

rand_stateis now in theTLSsegment, and each thread has its own version of this variable.

Hereishowit’saccessed: loadtheaddressoftheTIBfromFS:2Ch,thenaddanadditionalindex(ifneeded),

then calculate the address of theTLSsegment.

Then it’s possible to access therand_statevariable through the ECX register, which points to an unique

area in each thread.

TheFS:selectorisfamiliartoeveryreverseengineer, itisspeciallyusedtoalwayspointtoTIB, soitwould

be fast to load the thread-specific data.

TheGS:selector is used in Win64 and the address of theTLSis 0x58:

Listing 6.16: Optimizing MSVC 2013 x64

_TLS SEGMENT

rand_state DD 01H DUP (?)

_TLS ENDS

_DATA SEGMENT

$SG85451 DB '%d', 0aH, 00H

_DATA ENDS

_TEXT SEGMENT

init$ = 8

my_srand PROC

mov edx, DWORD PTR _tls_index

mov rax, QWORD PTR gs:88 ; 58h

mov r8d, OFFSET FLAT:rand_state

mov rax, QWORD PTR [rax+rdx*8]

mov DWORD PTR [r8+rax], ecx

ret 0

my_srand ENDP

my_rand PROC

mov rax, QWORD PTR gs:88 ; 58h

mov ecx, DWORD PTR _tls_index

mov edx, OFFSET FLAT:rand_state

mov rcx, QWORD PTR [rax+rcx*8]

imul eax, DWORD PTR [rcx+rdx], 1664525 ; 0019660dH

add eax, 1013904223 ; 3c6ef35fH

mov DWORD PTR [rcx+rdx], eax

and eax, 32767 ; 00007fffH

ret 0

my_rand ENDP

_TEXT ENDS

InitializedTLSdata

Let’s say, we want to set some fixed value torand_state, so in case the programmer forgets to, the

rand_statevariable would be initialized to some constant anyway (line 9):

1 #include <stdint.h>
2 #include <windows.h>
3 #include <winnt.h>
4
5 // from the Numerical Recipes book:
6 #define RNG_a 1664525
7 #define RNG_c 1013904223
8