Exam 1

Microprocessors Lab
Spring 1998

1. For each of the M68000 instructions given below
   
   1. indicate the addressing mode used in the source operand
   2. give the instruction's RTL equivalent
   1. ADD.B D3,D5
      1. register direct
      2. D5(0:7) <-- D5(0:7) + D3(0:7)
   2. AND.B $1004,D1
      1. memory direct or absolute
      2. D1(0:7) <-- D1(0:7) ^ M[$1004]
   3. SUB.B #$12,(A4)
      1. immediate
      2. M[A4](0:7) <-- M[A4](0:7) - $12
   4. ADD.W (A5)+,D4
      1. address register indirect with postincrement
      2. M[A5](0:15) <-- M[A5](0:15) + D4(0:15)
         A5 <-- A5 + 2
   
   
2. Write a M68000 program that uses the TUTOR functions to prompt the user to type characters by printing string to the screen and then display any character typed if it is a capital letter (A-Z) and its ASCII value is even.  The program should exit to TUTOR if the ESC key is pressed or after twenty characters have been displayed.

    
                    org      $1000
         
         charin     equ      247
         charout    equ      248
         stringout  equ      243
         tutorexit  equ      228
         escape     equ      $1B
         
   
                    lea      prompt,a5
                    lea      endprompt,a6
                    move.w   #tutorexit
                    trap     #14
         
                    move.b   #20,d3
   
         read       move.w   #charin,d7
                    trap     #14
   
                    cmp.b    #escape,d0
                    beq      exit
                    cmp.b    #'A',d0
                    blt      read
                    cmp.b    #'Z',d0
                    bgt      read
   
                    move     d0,d1
                    and.b    #$01,d1
                    bne      read
   
                    move.w   #charout,d7
                    trap     #14
   
                    sub.b    #1,d3
                    bne      read
   
         exit       move.w   #tutorexit,d7
                    trap     #14
         
         
         prompt     dc.b     'Please begin typing characters',$0D,$0A
         endprompt  dc.b     0
                    end      $1000
        

   
   
3. Complete the following M68000 program below so that is sums the 16-bit numbers starting at the label vector and places the result in the longword labeled result. The number of values in the vector is equated to the label VEC_LENGTH.

                     org      $1000
   
         VEC_LENGTH  equ      10
         tutorexit   equ      228
   
         main        lea      vector,A1
   
         
                     move.l   #0,d1
                     move.l   #0,result
                     move.b   #VEC_LENGTH,d2
   
         sum         move.w   (A1)+,d1
                     add.l    d1,result
                     sub.b    #1,d2
                     bne      sum
         
          
         exit        move.w   #tutorexit,D7
                     trap     #14
   
         * examine the next two lines of errors
         vector      dc.w     45,6356,10,134,9012
                     dc.w     345,1753,926,213,10032
   
         result      ds.l     1
                     end      $1000
         

4. Thoroughly compare the M68000 microprocessor to the M68HC11 microcontroller for each of the given criteria.
   1. Programmer's Model
   2. Word Size
   3. Instruction Set
   4. Addressing Modes
   5. Board space
   6. Control or Data Procesing Orientation
   7. Peripheral Architecture
   
   
5. Answer the following questions about the MC68HC11 program segment given below.

                     org      $100
   
         main        ldaa     #5
                     ldab     #4
                     ldx      #10
         loop        jsr      addit
                     dex
                     bne      loop
   
         more_code   ...
   
         addit       pshb
                     aba
                     rts
         

   1. What is wrong with the subroutine addit? Explain the program's behavior if it is executed as written. Write a correct version of addit.

      Since the subroutine pushes the contents of ACCB onto the stack, but does not restore them, the stack pointer will incorrectly point to the return address of the instruction following the subroutine call. In this case, the correct address would be $010A. Without the pull, the processor would return to the instruction at address $0A04, which is obviously incorrect. A corrected version of the subroutine is given by removing the pshb instruction. Alternatively, a pulb could be added before the rts instruction.

      	    addit     aba
      	              rts
      	    

   2. What program structure (while...do, do...until, etc.) is used in the main loop?

      do...until

   3. Assuming that the subroutine addit is written correctly, what value would ACCA contain after execution of the main loop?

      45