Page 133 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

110 Chapter 4 Assembly Language Programming

ABSENTRY ENT ABSENTRY ENT
ORG $800 ORG $800
ENT: LDD K,PCR ENT: LDD <K,PCR
ADDD #3 ADDD #3
STD K,PCR STD <K,PCR
SWI SWI
K: DS 1 K: DS 1
a) Illegal Forward Reference b) Legal Forward Reference

Figure 4.22. Program with Relative Address Reference

9 . Figure 4.22 shows two programs that attempt to add 3 to variable K, However,
Figure 4.22a may not assemble because of an illegal forward reference; or, if it does
assemble, it produces less efficient code than you can produce by hand. Explain why this
problem might occur. Figure 4.22b illustrates a solution to the problem, by using the
"<" operator. Explain why this will assemble to produce efficient code.

10. Figure 4.23 shows three programs that attempt to add 3 to the variable K. However,
Figure 4.23a may not assemble because of an illegal forward reference; or, if it does
assemble, it produces less efficient code than you can produce by hand. Explain why this
problem might occur. Figure 4.23b illustrates a solution to the problem by changing the
forward to a legal backward reference. Explain why this assembles correctly. Figure 4.23c
illustrates another solution to the problem, by using the "<" operator. Explain why this
will assemble directly. Finally, explain why programmers generally put their data in
front of (at lower addresses than) the program(s) that use the data,
11. Write a shortest assembly-language (source code) program that calls subroutine
GET, which inputs a character, returning it in accumulator A, and stores these characters
in the vector K as in Figure 4.8. Consecutively input characters are put in consecutive
bytes until a carriage return is input; then a null (0) byte is written in K.

12. Write a shortest assembly-language (source code) subroutine that moves characters
as in Figure 4.9 but converts lower case letters to upper case letters as it moves them,

ORG $800 ORG 0 ORG $800
ENT: LDD K K: DS 1 ENT: LDD <K
ADDD #3 * ADDD #3
STD K ENT: LDD K STD <K
SWI ADDD #3 SWI
ORG 0 STD K ORG 0
K: DS 1 SWI K: DS 1
a) Illegal Forward Reference b) Legal Backward Reference c) Legal Forward Reference

Figure 4.23. Program with Direct or Page Zero Reference