If cache chips are faster than RAM, why don't we built RAM using the cache chips?
can anyone tell me why?
"Cache chips" haven't been used in a very long time. "Cache" is usually SRAM or a variant that is tightly integrated into the CPU. SRAM has each memory cell as a different transistor and is extremely bulky vs DRAM which use's large arrays of rows and columns. SRAM is much faster as it doesn't require a CAS / RAS strobe nor is it limited to one cell read per cycle. This also makes it prohibitively expensive, ridiculously expensive for any large quantity of it. Mere megabytes would be hundreds of USD, Gigabytes would be the cost of entire systems. Also physics plays a part, SRAM can be so fast that there isn't a single external bus fast enough for it. The "slot" you'd plug it into would immediately become the bottleneck. We learned that with the older external SRAM sticks and when CPU's had backside bus's for cache.