Page 198 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

PROBLEMS 175

PSHA
PSHB
PSHX
CLR I, -SP Hole for NUM
BSR SEARCH
PULA Put NUM into A
LEAS 4, SP Balance stack
Figure 6.47. Program for Problem 13

14. Write a position-independent reentrant subroutine QUAD that evaluates the quadratic
2
function ax + bx + c, where signed 16-bit arguments a, b, c, and x are passed on the
stack, named PARA, PARE, PARC, and PARK, and the output is passed on the stack,
named RESULT. In order to demonstrate local variables, as part of your subroutine, store
2
ax in a 16-bit local variable on the stack. Write a calling sequence that pushes 1, 2, 3,
and 4; calls QUAD; pulls the result from the stack; and stores the result in global variable
ANSWER.

15. 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 on the stack and named Rl and R2, and the output is passed on the stack, named
RESULT. In order to demonstrate local variables, as part of your subroutine, store Rl
times R2 in a 32-bit local variable on the stack. Write a calling sequence that pushes 100
twice, calls PAR, pulls the result from the stack, and stores the result in global variable
ANSWER.

16. Do the same thing as in Problem 13, assuming that the input parameters are passed
after the call in the same order, while the parameter NUM is returned on the stack as
before. Do not use a BRA instruction before the parameter list, but follow the style of
Figure 6.27. Provide an example of a calling sequence.

17. Write a position-independent reentrant subroutine QUAD that evaluates the quadratic
2
function ax + bx + c, where signed 16-bit arguments a, b, c, and x are passed after the
call, named PARA, PARB, PARC, and PARX, and the output is passed after the call,
named RESULT. In order to demonstrate local variables, as part of your subroutine, store
2
ax in a 16-bit local variable on the stack. Do not use a BRA instruction before the
parameter list but follow the style of Figures 6.25 and 6.26. Write a calling sequence
that writes 1, 2, 3, and 4 into PARA, PARB, PARC, and PARX; calls QUAD; and moves
the result to global variable ANSWER.

18. Write a shortest position-independent reentrant subroutine PAR that computes the
parallel resistance of two resistors, where unsigned 16-bit arguments are passed after the
call and named Rl and R2 and the output is passed after the call, named RESULT. In
order to demonstrate local variables, as part of your subroutine, store Rl times R2 in a
32-bit local variable on the stack. Do not use a BRA instruction before the parameter list,
but follow the style of Figures 6.25 and 6.26. Write a calling sequence that writes 100
into Rl and R2, calls PAR, and moves the result to global variable ANSWER.