Lab 3 - CS 2011
Implementing a new trap for the LC-3

Objectives

What you should do...

• Sign the attendance sheet.

• Use LC3Edit to create a new file called trap28.asm. The code for the trap service routine will go into this file. The purpose of trap x28 is to display an asterisk (*) on the console when a key is pressed on the keyboard. (This is not an entirely useless routine...it could be called by a password-checking program. Each time a character of a password is typed in, it could be "echoed" as an *). Here are the steps for completing this routine (refer to Section 8.3.4 in your textbook if you need to):
  1. Start the program at location x0700.

  2. Load a register with the contents of the keyboard status register (address of KBSR is xFE00)

  3. Use a busy-wait loop to wait for a character

  4. When the character comes in, read it from the keyboard data register into R0 (address of KBDR is xFE02)

  5. Load a register with the contents of the display status register (address of DSR is xFE04)

  6. Use a busy-wait loop to detect when the display is ready to output a char

  7. When the display is ready, put the ASCII code for * (code x2A) into the display data register (address of DDR is xFE06)

  8. Return to the calling program
  The routine is not quite complete...you need to save and restore any registers used by the service routine (except R0, which will be used to return the typed character back to the calling program). Once you have added the code to save/restore the registers, put a .end at the end of the file, save it, and assemble it.

• Now write a short program that uses trap x28. Here's a program that calls the trap in a loop, displaying * for every character typed, until the user types a newline:

  ;; program to test trap x28
  ;; this program must be loaded with trap28.obj
  
           .orig x3000
  
  loop	 trap     x28           ;read a char and echo a *
                                  ;(char comes back in R0)
           ld       r1, NL        ;ASCII code for newline
           not      r1, r1
           add      r1, r1, #1    ;negate the newline...
           add      r0, r0, r1    ;...to subtract it from the char
           brnp     loop          ;not a newline, do another
  
           halt                   ;newline detected, so halt
  
  NL       .fill x0A              ;ASCII code for newline                   
           .end
  

  Enter (or copy/paste) this program into LC3Edit, save it as lab4.asm, and assemble it.

• Use the LC-3 simulator to load both programs into memory. There's one more thing to do before the program can be run...you must initialize location x0028 in the trap vector table with the starting address of the trap service routine. You can either do that manually in the simulator, or you can write a third assembly language module that consists of the following:

          .orig x0028
          .fill x0700
          .end
  

  If you write the third module, save it, assemble it, and load it into memory with the other two modules.

• Run the program. Does it work? (It should.) If not, use the debugger to figure out what's wrong.

• At the end of the lab period, turn in your files using web-based turnin.

See you next week!