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Latch & ACCU

Latch​

The latch is an 8-bit register used to supply the second operand (B) of each ALU operation. It can be set from the bus or the shifter output, and can be written to the bus.

Implementation in Hardware​

The latch can be set from both, bus and shifter output. To implement this selection, we use two 4-bit mux (74-157 latchMuxLow and latchMuxHigh). The latch itself is an 8-bit register (74-377 latchRegister). Writing the latch to the bus is done through a bus transceiver (74-245 latchTransceiver).

Schematic​

ALU latch schematic

ACCU​

The accumulator is an 8-bit register used on an architectural level as operand in most arithmetic operations (e.g., right shifting is only possible on the accu register on an architectural level). It cannot be set from the bus directly, but only from the shifter output. Like the latch, it can be written to the bus.

Implementation in Hardware​

The accu itself is an 8-bit register (74-377 accuRegister). As the accu can only be set from the shifter output, no mux is required. However, to write the accu to the bus, we again use a bus-transceiver (74-245 accuTransceiver).

Schematic​

ALU accu schematic