What units in a computer normally employ a register?

Answer 1

1. Arithmetic Logic Unit, usually contains registers for storing operands and results.
2. Control Unit, contains registers for the program counter, and current instruction, may contain other registers like a program status word (PSW) register, subroutine return address register, stack pointer register, branch condition code register, etc. depending on the architecture.
3. Memory Unit, may contain a memory address register (MAR) and a memory data register (MDR) on some architectures it may contain memory management registers to control memory mapping, memory protection, virtual memory, etc.
4. Input/Output Units, contains data input and data output registers, may also contain port configuration control registers, port status registers, direct memory access (DMA) registers, data channel management registers, etc.

In other words, all units in a computer normally contain and employ at least one register. Some of these registers are directly visible to the programmer, but others are hidden and managed entirely by the hardware (e.g. the program counter and current instruction registers in the Control Unit).