Instruction Register? Whats it's purpose/how is it connected? (And what happens after)
Since there are 16 bytes in the RAM you need 4 bits of address information to uniquely specify one of the RAM locations. Therefore, the MAR is 4 bits wide. The MAR doesn't store anything in RAM, it just provides an address. Even though the RAM has just 4 address lines it has 8 data lines, because each "address" in the RAM holds an entire byte. So, the MAR selects one of the 16 bytes in the RAM and the contents of that location appears on the data output bus from the RAM. It's this data that goes into the IR. There's no need for the 4 address bits to go into the IR since the processor already knows what value it stuck into the MAR. The 8 bits from the RAM, having been loaded into the IR, cause the processor to do something that may or may not cause something to happen in the accumulator.