How to comprehend this code snippet: __asm__ __volatile__("rdtsc" : "=A"(t))?

Hi, list Here is the code snippet: #define ipipe_read_tsc(t) \ __asm__ __volatile__("rdtsc" : "=A"(t)) I found that the rdtsc (Read Time-Stamp Counter) instruction is used to determine how many CPU ticks took place since the processor was reset. But what does "=A"(t) mean? Thank you for your attention to this matter.