Tue, 12 Oct 2021, 19:11 Re: Condition codes

A student writes:


> Hey Dr.Patt,
> Hope you are doing well. Why do instructions other than Branch set
> condition codes? Like I am just confused what they would use them for and
> how they even set them.
> Thanks,
> <<name withheld to protect the student confused about condition codes>>


In carrying out the work of a program, it is often useful to execute one
instruction next if some condition is satisfied, and a different instruction
next if that condition is not satisfied.  In the example we spent most of
yesterday doing in class, we wanted to jump out of a loop if the character
we were examining was the End_of_Text (EOT) character, and execute the loop
again if the character we were examining was not EOT.  Also, in the example
yesterday, we wanted to execute one instruction to increment R2 (a counter)
if the character we were examining was the same as the character someone
typed, and not increment the counter if the character was different from
what someone typed.

The point is that in executing the instructions of a program, it is often
the case that we create a value and then do two different things depending on
that value we have created.  Our decision as to which of the two paths we
take is based on whether the value we create is negative, zero, or positive. 
In most ISAs, this important concept is handled by condition codes.  That is,
we create a value and at the end of the instruction that creates that value,
we set the condition codes depending on whether that value is negative, zero,
or positive.  We then follow that instruction with a branch instruction which
examines one or more of the condition codes and changes the PC depending on
which instruction we want to execute next.

Consider again the two examples mentioned above.  With respect to checking
if we have just fetched the End_of_Text character, the ADD instruction will
produce 0 if the computer has fetched EOT.  Since during the fetch phase of
the ADD instruction, we have incremented the PC, if we check the Z bit and
if it is 0, we know we have not fetched EOT, and so we do not change the PC
and the next instruction executed is the start of our work with the character
we have fetched.  On the other hand, if Z = 1, we know we have fetched EOT
so we do not want to do that work again, but rather we want to change the PC
so we can go on to the next part of the program's execution.

Does that help?

Good luck on the exam.

YNP