12/07/2006

Apparently, it is still not clear to some of you the difference between
code you write to push or pop elements on and off a stack, and these
operations when they are performed as part of the instruction cycle in carrying
out the work of that one instruction.

If you understand this distinction, please delete and move on.  If you
are still having trouble, perhaps this will help.  Also, please see your
favorite TA or me.

Let's start with an instruction you are well familiar with: LD R1, A.

The LC-3 executes this ONE SINGLE instruction by carrrying out the six
stages of the instruction cycle:  It uses the PC to know where to FETCH
the instruction and fetches it.  It DECODES it so it knows what the
instruction is asking the LC-3 to do in carrying it out.  It computes
the address of the memory location containing the data to be loaded.

STOP!!!  Is it enough to just write a load instruction, or do you also have
to write an ADD instruction to get the computer to add the sign-extended
offset to incremented PC in order to know where in memory to go to find the
data to be loaded?

I hope everyone knows that you do not write an ADD instruction.  I hope 
everyone knows that as the LC-3 carries out the instruction cycle for the
LD instruction, one of the steps in the instruction cycle will be to perform
this addition.

Now then, let's look at a more complicated instruction: RTI.  Like EVERY
instruction in the LC-3, the LC-3 executes this ONE SINGLE instruction by
carrying out the six stages of the instruction cycle: It uses the PC to know
where to FETCH the instruction and fetches it.  It DECODES it so it knows what
the instruction is asking the LC-3 to do in carrying it out. Then it pops the
top two entries on the system stack and loads them into the PSR and PC.

Stop!!! Is it enough to just write RTI or do you also have to write the code
to do the two pops.  Recall above that the LC-3 carried out an add of the
sign-extended offset and the incremented PC to get the memory location
containing the data when executing the LD R1, A.  In the same way, the LC-3
carries out the two pops in carrying out the work of the RTI.

The bottom line: You all know about finite state machines.  We studied them in
Chapter 3.  The LC-3 (and all computers) are controlled by a finite state
machine that carries out each instruction by going from the initial state to
the next state to the next state UNTIL the SINGLE instruction is completely
executed.  Then the LC-3 returns to the initial state to work on the next
instruction.

If the LC-3 is carrying out a LD, one of those states contains the addition
of sign-extended offset and incremented PC.  If the LC-3 is carrying out an
RTI, two of those states contain "pops."

Hope this helps.
Yale Patt