Lecture 19 Objectives

KNOW:

• what is meant by the "external reference" problem
• what is meant by the "relocation" problem
• how assemblers and linkers (more sophisticated than those supplied with the LC-3) resolve the external reference and relocation problems

BE ABLE TO:

• identify and fix assembly and link errors in separately-assembled modules

Sample Exam Question:

(Problem 9.16 from the textbook) The two code sequences a and b are assembled separately. There are two errors. Identify the bugs and describe why each bug will cause an error, and identify whether the errors will be detected at assembly time or at link time.

a.         .ORIG x3200
     SQRT  ADD   R0, R0, #0
           ; code to perform square root function
           ; and return the result in R0
           RET
           .END


b.         .EXTERNAL SQRT
           .ORIG x3000
           LD    R0, VALUE
           JSR   SQRT
           ST    R0, DEST
           HALT
    VALUE  .FILL x30000
    DEST   .FILL x0025
           .END