A microprocessor uses the instruction format shown for instructions that use a direct address. The amount of memory that can be accessed using direct address mode is most nearly:

A.  256 words
B.  512 words
C.  1,024 words
D.  2,048 words

By analyzing the instruction format displayed in this problem, we can see that our direct address is 9 bits wide. The amount of memory that may be accessed using the direct address mode is limited by the number of unique direct addresses we can point to. Since binary numbers are limited to 0's and 1's, we have 2 options per digit to make a unique number. Therefore:

$2^n=total unique direct addresses, where n=number of bits$

Plugging in n=9:

$2^n=2^9=512$

Since there are 512 unique direct addressed we can point to, the amount of memory that can be accessed using direct address is limited to B, 512 words.