CS270 Recitation 7
More LC-3 Programming


  1. To extend your knowledge of LC-3 programming with a more complex algorithm.
  2. To solidify your knowledge of how to use the LC-3 assembler and simulator to debug assembly code.

The Assignment

  1. Make a subdirectory called R7 for the recitation; all files should reside in this subdirectory. Copy the file from R7.asm to the R7 directory, a listing of the code is shown below.
        ; Recitation 7
        ; Author: <name>
        ; Date:   <date>
        ; Email:  <email>
        ; Class:  CS270
        ; Description: Mirrors least significant byte to most significant
        ; Begin reserved section: do not change ANYTHING in reserved section!
                        .ORIG x3000
                        JSR mirror           ; call function
        ; Parameter and return value
        Param           .BLKW 1              ; space to specify parameter
        Result          .BLKW 1              ; space to store result
        ; Constants
        One             .FILL #1             ; the number 1       
        Eight           .FILL #8             ; the number 8
        Mask            .FILL x00ff          ; mask top bits
        ; End reserved section: do not change ANYTHING in reserved section!
        mirror                               ; Mirrors bits 7:0 to 15:8
                                             ; ~20 lines of assembly code
                        LD R0,Param          ; load pattern
                                             ; your code here
                        ST R1,Result         ; store result
  2. Use the LC-3 assembler to transform your assembly code into object code that can run on the LC-3 simulator:
    ~cs270/lc3tools/lc3as R7.asm
  3. Load the LC-3 simulator. The TA will help you step through an invocation of one of the LC-3 subroutines:
    ~cs270/lc3tools/lc3sim-tk &
  4. Implement the mirror subroutine, using the following algorithm. It’s a poor algorithm. Resist the urge to improve it.
  5. Test the mirror subroutine in the simulator using Param = 0x1234. The answer in Result should be 0x3434.