A computer system has 13 bit instruction , so 213 encodings are possible .
The memory contains 128 words , hence, 7 bits are needed to address each word in memory .
32 one address instructions has 32 * 27 possible encodings = 4096 encodings .
Now, 8192 - 4096 = 4096 possible encodings are remaining , which have no address part, because they are 0 - address instructions. ( only 1 - address and 0 - address instructions are used )
Hence, zero address instructions supported by system are 4096 .