Spring 2001 CSCI 255 Homework 7 Solution

Problem 1 and only

Write in LC-2 assembler, using the device registers introduced in chapter 8, a program that reads characters from the terminals and echoes input lower-case letters back to the monitor. All other charcters are ignored.

This answer was provided by Bryan Powell and slighted edited by me.

START LDI   R1,KBSR	; Test the status register
      BRzp  START
      LDI   R0,KBDR	; Read the character into R0
      LD    R1,aNEG
      ADD   R1,R1,R0
      BRn   START	; Don't print if < 'a'
      LD    R1,zNEG
      ADD   R1,R1,R0
      BRp   START	; Don't print if > 'z'
ECHO  LDI   R1,CRTSR	; Print it here
      BRzp  ECHO
      STI   R0,CRTDR
      BR    START
KBSR  .FILL xF400
KBDR  .FILL xF401
CRTSR .FILL xF3FC
CRTDR .FILL xF3FF
aNEG  .FILL #-97	; -'a'
zNEG  .FILL #-122	; -'z'
      .END