Re: The best place to begin learning Assembler

On May 26, 2010, at 12:14 PM, Ron Hudson wrote:

> (I noticed AR and ALR seem to be the same instruction??)

AR and ALR are different.

The first is 2's complement whereas the second is 1's complement.

It is important to have both instructions available, particularly if  
one is going to be using double-word (meaning, 64-bit) integer  
operations which are simulated using two ALRs in sequence, branching  
around the second should the high-order word NOT need to be adjusted  
by an AL =F'1'.

This option is often important when taking an integer which is 64- 
bits in width, and needs to be converted into a floating-point number.

Remember, the TOD clock is a 64-bit integer, and taking the beginning  
TOD (STCK) and the ending TOD (STCK) values, and then forming the  
difference, hence the elapsed-time, requires these 64-bit operations,  
but it also usually gives a 32-bit result.