Memory Address Register (MAR)

The memory address register stores the address of the data the RAM module should be pointing at.

Mode of Operation

The memory address register is very similar in its operation principle to any of the data registers.

It uses the same 74LS173 4-bit D-type registers, however since it’s output is not connected to the databus but instead directly to the address pins of the RAM module, no buffers are used at the output.

Instead a 74LS157 quadruple 2-to-1 multiplexer is used to select either the register contents or a DIP switch array to set the address outputs. The DIP switches can be used in program mode to set the address manually while in run mode the address can be loaded as the lower 4 bits of the data bus which represent the instruction data (ID) nibble of an instruction (see the ISA section for more detail).

To select the current mode of operation (run / program), the SPDT switch Program / Run can be used.

Schematic