The memory address register in SPAM-1 is a pair of 74HCT377 8 bit flipflops that together comprise a 16 bit memory address register (MAR). The MAR can be used in register addressing mode to reference any RAM location for a read or write operation.
The MAR registers play a dual and as well as their special purpose for RAM addressing the two 8 bit MAR registers can be used as general purpose registers in ALU operations in the same manner as the register file. The MAR register can provide an value onto either or both of the ALU input busses.
The MAR operates is constructed as a quadruple port register. Each 8 bit MAR register can be:
- written to by the output of the ALU
- provide a value onto ALU input bus A
- provide a value onto ALU input bus B
- provide a value onto the RAM memory address bus.
In each case where the MAR flipflops integrate with a bus there is a 74HCT245 tristate buffer chip.