Page 337 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

314 Chapter 10 Elementary Data Structures

1 9. Write a shortest subroutine GETS that puts three characters into B (first character)
and Y (second and third character) pointed to by X, and moving X past them and a next
space or carriage return. Write a shortest subroutine CHKEND that reads three characters
using subroutine GETS, and checks for the characters "END". Assume the calling
subroutine has not pushed anything on stack, so if "END" is read, return by pulling two
bytes from the stack and executing rts, so as to terminate the calling subroutine,
otherwise return by just executing rts as usual to return to the calling subroutine.
These subroutines are to be used in problem 21.
2 0. Write a shortest subroutine FIND which searches a binary tree, pointed to by X for
three letters, B:Y as returned by GETS of problem 19. The binary tree nodes contain a
3-byte character string, a one-byte value, and two two-byte addresses, the first of which
points to a left son, and the other points to a right son. If the three letters are found,
return with X pointing to the beginning of the node, else if the letters are lower in the
dictionary than the last node searached in the tree, X points to the left son field of the last
node, but if higher, X points to the right son field of the last node. This subroutine is to
be used in problem 21.

21. Write a 6812 assembler program using linked lists, able to assemble programs
having the following specifications.

(a) The operations will be encoded in the left two bits of a one-byte opcode: LDA is
00, ADD (for ADDA) is 01, STA is 10, and SWI is 11. There will also be
directives DCB and END.

(b) At most six lables can be used, each of which is exactly three uppercase letters
long. Operands will be encoded in the right six bits of a one-byte opcode.
(c) Only direct addressing can be used with instructions, which will be coded in the
right 6 bits of the instruction, and only hexadecimal numbers, beginning with $,
can be used with the DCB directive.
(d) The input line will have a fixed format: label (3 characters), space, instruction
mnemonic (three characters), space, address (3 characters or 2-digit hexadecimal
number prefixed with a $) ending in a carriage return. There are no comments, and,
if a label is missing, it is replaced by 3 spaces.

(e) The program will have from 1 to 10 lines, ending in an END directive.

(f) The source code has no errors (i.e., your assembler does not have to check
errors). The origin is always zero.

Your assembler will have the source code prestored as a character array TEXT, 10
rows by 12 columns, and will generate an object code string OBJ up to 10 bytes
long. No listing will be generated. The assembler should be able to at least
assemble the following two programs, shown on the left and shown on the right.