Thu, 21 Nov 2013, 00:33

Late in the lecture today, I wrote on the board the code that the
operating system would have to write in order to set the I/E bit
(bit 14) of the KBSR.  I just got email from a student:


 
> Hello Dr. Patt,I was studying today's lecture and i am confused 
> with one thing.In the piece of code that you wrote for addressing 
> bit 14 of KBSR, you wrote as following:


 
> 	LDI R4,KBSR
> 	LD  R3,A
> 	ADD R4,R4,R3
> 	STI R4,KBSR
> KBSR  .FILL XFE00
> A     .FILL X4000


 
> My question is, shouldn't we use "AND R4,R4,R3" instead of "ADD R4,R4,R3"?
> 
> Thanks.
> <<name withheld to protect the student who wants to substitute AND for ADD>>


Two things are important here.

Thing 1.  No, I meant ADD, not AND.  Recall what I was trying to do.

KBSR contained  00xxxxxxxxxxxxxx
   A contained  0100000000000000

If I AND them as you suggest, I get 0000000000000000 in KBSR.  But  
if I ADD them, and then store the result in KBSR, I get
in KBSR         01xxxxxxxxxxxxxx  so it works!

And, then I asked, "But does it really work."  

And several students were quick to point out that if bit 14 is already a 1,
then adding x4000 does not set bit 14; it clears bit 14 and sets bit 15.
Not what I wanted to do!

Which brings us to thing 2.  What should I do?  And several students pointed
out the correct answer which is to first clear bit 14 and then add x4000.

So, the real operating system code to set the I/E bit of the KSBR is:

  	LDI R4,KBSR
  	LD  R3,A
	NOT R5,R3     ;R5=1011111111111111, a mask for clearing bit 14
	AND R4,R4,R5  ; this clears bit 14
  	ADD R4,R4,R3  ; this sets bit 14
  	STI R4,KBSR
  KBSR  .FILL XFE00
  A     .FILL X4000


Good luck with the rest of the course.  See you in class on Monday.

Yale Patt