获取程序运行时间
RDTSC汇编指令用于程序的精确定时。原理是CPU从上电开始，其内部一个64位计数器就会记录下CPU所经过的周期数，RDTSC指令可以读取该计数器到寄存器EDX:EAX中。理论上计数精度可以达到纳秒级别。周期数/CPU主频=CPU通电以来的执行秒数。使用两次RDTSC，把两次的结果相减，得到“间隔周期数”，间隔周期数/CPU主频=CPU执行这两条指令间的秒数。
下面是RDTSC的一个简单应用实例：
static __inline __int64 read_tsc()
{
__int64 time_collected;
unsigned int ts1, ts2;
__asm{
rdtsc
mov ts1, eax
mov ts2, edx
}
time_collected = ((__int64) ts2 << 32) | ((__int64) ts1);
return time_collected;
}
void RDTSC_test()
{
double run_time;
unsigned int start_time, end_time, i, j;
start_time = (unsigned int)read_tsc();
for(i = 0; i < 100000; i++)
{
j = i * i;
}
end_time = (unsigned int)read_tsc();
run_time = (end_time - start_time) * 1.0 / 1000000;
printf("Run time: %f M cycle./n", run_time);
}
以下内如引用intel指令手册。
//引用开始
RDTSC—Read Time-Stamp Counter
Opcode Instruction Description
0F 31 RDTSC Read time-stamp counter into EDX:EAX
Description
Loads the current value of the processor’s time-stamp counter into the EDX:EAX registers.
The
time-stamp counter is contained in a 64-bit MSR. The high-order 32 bits of the MSR are
loaded
into the EDX register, and the low-order 32 bits are loaded into the EAX register. The
processor
increments the time-stamp counter MSR every clock cycle and resets it to 0 whenever the
processor is reset.
The time stamp disable (TSD) flag in register CR4 restricts the use of the RDTSC
instruction.
When the TSD flag is clear, the RDTSC instruction can be executed at any privilege level;
when
the flag is set, the instruction can only be executed at privilege level 0. The time-stamp
counter
can also be read with the RDMSR instruction, when executing at privilege level 0.
The RDTSC instruction is not a serializing instruction. Thus, it does not necessarily wait
until
all previous instructions have been executed before reading the counter. Similarly,
subsequent
instructions may begin execution before the read operation is performed.
This instruction was introduced into the IA-32 Architecture in the Pentium processor.
Operation
IF (CR4.TSD ← 0) OR ((CR4.TSD ← 1) AND (CPL=0))
THEN
EDX:EAX ← TimeStampCounter;
ELSE (* CR4 is 1 and CPL is 1, 2, or 3 *)
#GP(0)
FI;
Flags Affected
None.
Protected Mode Exceptions
#GP(0) If the TSD flag in register CR4 is set and the CPL is greater than 0.
Real-Address Mode Exceptions
#GP If the TSD flag in register CR4 is set.
Virtual-8086 Mode Exceptions
#GP(0) If the TSD flag in register CR4 is set.