Page 135 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

112 Chapter 4 Assembly Language Programming

19. Write an assembly-language program that adds N 2-byte signed numbers stored in
the vector Z, N < 10. Your program should sign-extend each number as it is being added
into the current sum so that the result SUM is a 3-byte signed number. Your program
should have the assembler directives
ORG $800
N DS.B 1
Z DS.W 10
SUM DS 3
Can overflow occur in your program?

2 0. Write an assembly-language program to find the smallest nonzero positive number
NUM in the array Z of N 2-byte signed numbers. If there are no nonzero positive
numbers, the result should be put equal to zero. Your program should have the following
assembler directives:
ORG
N DS 1
Z DS.W 100
NUM DS 2
21. Write an assembly-language program that finds the sum SUM of two N-byte
numbers NUM1 and NUM2 and, when the SWI is encountered, has the condition code bits
Z, N, V and C set properly. Your program should have the directives

N DS 1
NUM1 DS 20
NUM2 DS 20
SUM DS 20

ORG $800 Begin program
LEAX N,VECTOR Point X to VECTOR
LDAB N Length of VECTOR into B
LDDP LD A f X+ A vector element into A
BLO LI if negative.
COM A replace with two's-complement
Ll ANDA #~1 Make contents of A even
STAA -1, X Put number back
DECB Counter is in accumulator B
* BCS \0
SWI End of program
VECTOR DS N
N EQU 5 Number of elements in VECTOR

Figure 4.24. Program with Errors