Page 334 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

PROBLEMS 311

PROBLEMS

1. What are the limitations on the precision and length of the vector Z that are
accessed by program segments (2) and (3)? How would you change these segments for a
vector Z with length 500?

2 . Write a shortest subroutine READY that returns in index register Y (high 16 bits)
and accumulators A and B (low 8 bits), the ith element of a zero-origin vector. For
instance, V may be allocated as in V DS M*N, where V has precision M bytes (M & 4)
and cardinality N elements. To read V, the parameters can be passed on the stack with the
calling sequence shown below. If M is 1, pass the result in ACCB; if 2, pass it in
ACCI>, if 3, pass it in Y (msb) and ACCB (Isb); and if 4, pass it in Y (msb) and ACCD
(Isb).

MOW #V,2,-SP
MOVE #M rl,-SP
MOVB #i,l,-SP
JSR READV
LEAS 4,SP

3 . Write a shortest subroutine WRITEV that writes the M bytes (M s 4) into the ith
element of a one-origin vector. For instance, the data may be in index register Y (high 16
bits) and accumulators A and B (low 8 bits). V may be allocated as in V DS M*N r
where V has precision M bytes (M s 4) and cardinality N elements. To write the data into
V, the parameters can be passed on the stack with the calling sequence as shown below.
Pass returned results as in Problem 2.

MOVW #V, 2, - SP ; Vector base address
MOVB #M, I f -SP ; Vector precision
MOVB #i, 1, -SP ; Desired element number
JSR WRITEV ;
LEAS 4, SP ; Balance stack

4 . Consider the zero-origin vector of 32 bytes in locations 0 through 31. Assume that
the bits in this vector are labeled 0 to 255 beginning with the first byte in the vector and
going right to left within each particular byte. Write a subroutine SETBIT that will set
the ith bit in this vector assuming that the value of i is passed on the stack with the
calling sequence

MOVB #5 8,1, - SP ; Value of i into parameter
BSR SETBIT
LEAS 1, SP ; Balance stack

5 . Write a subroutine STRBIT that will store the binary-valued variable BIT
in me ith bit of the zero-origin vector in Problem 4. The value of BIT and INDEX can
be passed on the stack with the calling sequence