Why does DOS use 100% CPU under Virtual PC?

From time to time someone notices that when they run DOS under Virtual PC it uses 100% of the host CPU to do absolutely nothing, and wonder why this is the case. The answer is relatively simple - and it is: 'DOS is never actually doing nothing'.

Most modern operating systems are designed with the concept of being able to 'do nothing'. That is - if the user is not actively providing input, and no programs are demanding attention - most operating systems will tell the CPU to 'do nothing' (usually through a HLT command - or through some more advanced power state commands).

DOS does not do this.

So when DOS is sitting at the command prompt 'doing nothing' it is actually in a very tight loop of code which is responsible for blinking the cursor and checking for new user input. As we operate at a very low level - there is no way for us to tell if DOS is actually doing something productive with the CPU or just 'running around in circles'. To help deal with this - there is a utility called 'IDLE.COM' that is part of the Virtual Machine Additions for DOS - which is able to send 'HLT' commands when it detects that DOS is 'doing nothing'. However if you start any program (like EDIT) then your CPU usage will go up to 100% again.

So at the end of the day - if you are using DOS under Virtual PC you are pretty much going to have to learn to live with the fact that it will try to use as much CPU as possible at all times.