Page 321 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

298 Chapter 10 Elementary Data Structures

HUM: EQU 100 ; Number of lists in the table (9)
TABLE: DS NUM*NBYTES ; Allocation of table

creates a buffer for 100 of the lists defined by (6). The address of the first byte of the
buffer is TABLE. The following program segment searches such a table for a certain
telephone number, which is stored (in binary) in accumulator D (low 16-bits) and index
register X (high 16 bits). We will assume it finds a matching telephone number, which
Is left in accumulator D (low 16-bits) and index register X (high 16 bits) when we exit.

LOOP: CPX TN, Y ; check high 16-bits of telephone number of row Y
BNE NOMTC
CPD TN+2, Y ; check low 16-bits of telephone number of row Y
BEQ MTCH
NOMTC: LEAY NBYTES,Y ; skip to next list
CPY #TABLE+NUM*NBYTES ; at end of table?
BNE LOOP ; if not, loop
MTCH: LDD SSN, Y ; get high 16-bit Social Security number of row Y
LDX SSN+2 , Y ; get low 16-bit Social Security number of row Y

This discussion has examined indexable data structures. Each element of an
indexable data structure can be accessed, and, furthermore, some form of indexing can be
used for the access. The simple, but very useful, vector was easy to access because the
address of the ith element, assuming a zero origin, is obtained by adding imprecision) to
the address of the vector. A list is like a vector but has fewer restrictions, in that
elements can be of any precision. Arrays and tables are just mixtures of these two
structures. These indexable structures are used often in a microcomputer like the 6812,
because they are so easy to handle with its index addressing options and multiply
instructions,

10.3 Sequential Data Structures

We now consider sequential data structures. The ubiquitous string, which you met earlier,
and various deques, including the stack, are sequential structures. The key characteristic of
sequential structures is that there is a current location, or top or bottom, to the structure,
and access to the data in this structure is limited to this location.
Strings can be variable or constant. A buffer is used to hold a variable string,
which, in particular, can have a variable length. In a program with string manipulations,
the length of a particular string can change in the program (up to the size of its buffer) in
contrast to a vector of, say, 2-byte numbers that has a constant length throughout most
programs. In C, a global constant string is declared char s [ 11 ] = "High there";.
Most strings in C are terminated by a null character ($0); the number of bytes allocated
for a string generally must include this extra null character at the end. In assembly
language, constant strings can be created in memory with assembler directives like
s: DC.B "This is a string" (9)