Dear Edwin,

Please refer to my comments below.

Dear Prof. Jason,

In the polled I/O, does the CPU continuously keep asking “are you ready?” to the I/O device for a period of time (like infinite loop until the device says “I’m ready!”) or does the CPU ask the question periodically, like for each x micro second, the CPU will ask the I/O device and the CPU keeps asking as long as the I/O is connected to the computer?

[Jason] The CPU checks the I/O status periodically, each X amout of time.

In addition, I am just curious about the VIP diagram in the Computer Arithmetic part. I have attached the screenshot with this email.

Why are the process of addition/subtraction, logical operation, unsigned multiplication, etc. done before the multiplexer? Does not this mean that it is not efficient since each operation is finished first, and later we choose which result we want to use (using the control bit for the mux gate)? Why do we not use multiplexer first to produce an “enable” bit for the operation block?

[Jason] As you see in the diagram, the VIP ALU can perform logical operations, ADD/SUB, rotation, etc… depending on the instruction. The multiplexer is used to select the relevant operation.

Thank you.

Regards,
Edwin Candinegara