What does .byte mean in this asm line?

Question

I am revisiting code that I wrote a while ago that does some math on large numbers. When I wrote the code, the lab had a few x86s that were split between 32 and 64 bits. My work was on UltraSPARCs, and I vaguely remember pulling this line of code from an Intel manual to be sure that the code was being used on a 64-bit CPU.

unsigned long x[4];
x[0] = 0;
x[1] = 0;
x[2] = 0;
x[3] = 0;
asm volatile(".byte 15;.byte 162" : "=a"(x[0]),"=b"(x[1]),"=c"(x[3]),"=d"(x[2]) : "0"(0) );

If x[0] was 0, all was well and the program started chugging away.

Can anyone explain to me what this line of code actually does?

Answer 1

The bytes .byte 15 and .byte 162 represent the CPUID instruction.
When it executes you get results in EAX, EBX, ECX, and EDX.

These results will be stored in the array elements:

x[0] <- EAX
x[1] <- EBX
x[2] <- EDX
x[3] <- ECX