Lecture Objectives

Lecture 18 Objectives

At the end of today's class you should

KNOW:

what a TRAP is
how the TRAP mechanism works on the LC-3
which routine (the user program or the trap service routine) is responsible for saving which state information

BE ABLE TO:

find the code for any of the LC-3 TRAP service routines from within the LC-3 simulator
write a program that uses TRAPs and performs appropriate state-saving

Sample Exam Question: Find the bug in the following program, which is supposed to read in a character, and output the next consecutive character (i.e. if the user inputs 'A', the program displays 'B'):

     .orig x3000
     in               ; TRAP x23 - char comes back in r0
     add r7, r0, #1   ; increment character
     lea r0, message  ; r0 gets address of message
     puts             ; TRAP x22 - message is displayed
     add r0, r7, #0   ; move the incremented char back to r0 for output
     out              ; TRAP x21 - display char in r0
     halt             ; TRAP x25

message   .stringz "Here is the incremented character:  "
     .end