Page 199 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

176 Chapter 6 Assembly Language Subroutines

19. One reason for not passing output parameters after the call is that a subroutine that
calls another subroutine and has some parameters passed back to it after the call will not
always be reentrant. Explain why this is so. Are there similar restrictions on input
parameters?

2 0. Give an example of passing output parameters after the call where the program can
still be stored in ROM.

21. Write a subroutine to search an N-byte vector Z until a byte is found that has the
same bits in positions 0,3,5, and 7 as the word MATCH. The address of Z is passed as
the first entry AZ in a table, the value of MATCH is passed below it in the table, the
value of N is passed below it in the table, and the address of the first byte found, ADDR,
is passed below it in a table. If no byte is found in Z with a match, $FFFF should be
placed in AZ. The address of the table is in X when the subroutine is called.

2 2, Write a position-independent reentrant subroutine QUAD that evaluates the quadratic
2
function ax 4- bx + c, where signed 16-bit arguments a, b, c, and x are passed in a table,
named PARA, PARB, PARC, and PARK; and the output is passed in the table, named
RESULT. The address of the table is in X when the subroutine is called. In order to
2
demonstrate local variables, as part of your subroutine, store ax in a 16-bit local
variable on the stack. Write a calling sequence that writes 1, 2, 3, and 4 into PARA,
PARB, PARC, and PARK; calls QUAD; and moves the result to global variable ANSWER.

2 3. Write a shortest position-independent reentrant subroutine PAR that computes the
parallel resistance of two resistors Rl and R2, where unsigned 16-bit arguments are
passed in a table and in elements Rl and R2 and the output is passed in the same table in
an element named RESULT. In order to demonstrate local variables, as part of your
subroutine, store Rl times R2 in a 16-bit local variable on the stack. The address of the
table is in X when the subroutine is called. Write a calling sequence that writes 100 into
Rl and R2, calls PAR, and moves the result to global variable ANSWER.

2 4. Repeat Problem 14, saving and restoring all the registers that were used.

2 5. Repeat Problem 15, saving and restoring all the registers that were used.

2 6. Repeat Problem 22, saving and restoring all the registers that were used.

2 7 . Repeat Problem 23, saving and restoring all the registers that were used.

2 8. How would the calling sequence of Figure 6.40 be modified if the subroutine SUB
in Figure 6.41 replaced the LBRA instructions with
SUB DC.W SUBO-SUB
DC.W SUB1-SUB
DC.W SUB2-SUB

What are the advantages of doing this, if any?