If A-select = 0, the A-MUX multiplexor will select the register as output. And if A-select = 1, the A-MUX multiplexor will select the MBR as output.
By selecting the input from the register, the CPU will perform
operations on register data.
By selecting the input from the MBR, the CPU will perform operations on memory data.
Note: by looking at the connections, you must conclude that the MBR can be updated with data from:
There are 32 bits total and one of such "packed bits" is called a micro instruction because it controls the CPU micro architecture.
The micro instruction is stored in a (32 bit) register called the micro instruction register (MIR).
This outputs of the MIR (which is the micro instruction) is tied to the various part of the CPU which each bit/bits controls. The connections are shown in the following figure:
The instructions (hardcoded in the program) that you can select are:
(The MIR's display is found in the upper-right corner - 2 rows of probes)
The Datapath is completely under the control of the bit setting in the Micro-Instruction !!!)
(Key 1 is the clock for the A- and B-latches)
(Key 2 is the clock for the registers)
in this exact sequence