A/B Registers

The A and B registers are 8-bit registers that provide the inputs to the ALU. The A register acts as an accumulator and general-purpose register that is accessible from instructions available to the user.

The B register is only used as an operand register for the ALU operations. It is not accessible as a general-purpose register and there are no instructions to directly read or write its contents. The microcode may also use this register as temporarly storage for non-ALU instructions.

The outputs of each register are sent to the ALU module using the Module Interconnects on the right-hand side of the board. The A/B Module and ALU Module must be placed in adjacent slots of a backplane board.

The B register contains additional circuitry to invert its outputs to the data bus and to the ALU. When the B Invert (BI) signal is asserted, a set of XOR gates invert the output pins of B. For example if B contains the value 0111 1000, the value sent to the bus and ALU will be 1000 0111. The B0..B7 LEDs will show the value of B after the inversion, not the value stored in the register. The use of the BI signal is detailed in the ALU documentation.

The A and B registers are each implemented using a 74LS377 register to store the register value and a 74LS245 8-bit bus transceiver to output the register value to the data bus.