Page 323 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

300 Chapter 10 Elementary Data Structures

and are displayed differently on different terminals. These are usually input from the
terminal keyboard by holding down a "control" key and pressing one of the other keys.
These characters will not be needed in this discussion.) Implicit in the subroutine DUTCH
is a segment of code that will wait until the previous character is displayed on the
terminal before c is displayed. In assembly language, we might use two subroutines
equivalent to these procedures; to input a character, execute subroutine INCH , which
leaves the character in accumulator A, and to output a character, put it in accumulator A,
and execute subroutine OUTCH. The exact code used in INCH and OUTCH are not important
at this point.
In C, if ptr points to the beginning of a null-terminated string s, a statement
while (*ptr) outch( *ptr++); outputs the string s, if ptr points to the beginning
of a buffer b, a statement do *ptr++ = c = inch(); while (c != '\r' ) inputs
characters to the buffer b, until a carriage return is received (the carriage return is
written at the end of the string that is in b). In assembly language, we can input or
output strings of ASCII characters almost as easily. For example, we could display the
constant string (9) on the terminal beginning at the current cursor position with the
segment
LDAB #16 ; Number of characters in STRING (10)
LDX #STRING
LOOP: LDAA 1,X+ ; Next character of STRING; into A
JSR OUTCH
DBNE B,LOOP

Program segment (10) assumes that the programmer will count the number of characters
in the string. This can be avoided by adding
LENGTH: EQU *-STRING

after the definition of STRING and replacing LDAB #16 with LDAB #LENGTH in the
sequence (10). We can also input a string of characters from the terminal, terminated by
an ASCII carriage return ($OD), with the program segment below. The string is stored in
a buffer labeled BUFFER established with the directive
BUFFER: DS 100
The string is entered with the program segment (11)

LDX #BUFFER ; X -> buffer to hold the string (11)
AGAIN: JSR INCH
STAA 1, X+ ; Place character in buffer
CMPA #$ OD ; is the character input a carriage return?
BNE AGAIN
OUT: RTS

Another type of sequential data structure is the deque, which we now discuss. A
special case of the deque is the stack, which we studied extensively in Chapters 3 and 8.
Our stacks have also been indexable, because the S register can be used as an index
register as well as a stack pointer. Nevertheless, when these stack pointers are used only
with push and pull instructions, they become true sequential structures.
A deque is a generalization of a stack. It is a data structure that contains elements
of the same precision. There is a top element and a bottom element, and only the top and