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10.2 Indexable Data Structures 295

whose precisions are 30,4,45, and 4, respectively. In C, a list is conveniently handled
by a struct. The list above can be represented by struct{ char name [ 30 ]; long ss;
char address [45]; long phone; } s;. The phone element of struct s is indicated
by the notation s.phone. The assembly-language implementation of lists is simple.
Assuming that the label L is used for the address of the first byte of the list, we can load
the jth byte of L2 (the address) into A with the sequence

LDX #L+34 ; Point X to L2 (5)
LDAA A, X ; jth byte of L2 into A

where we have also assumed that the value of j is initially in accumulator A. While the
segment (5) seems simple enough, remember that we have had to compute the proper
offset to add to L in order to point X to L2. For simple lists, such as this example, this
is not much of a problem, and the programmer may elect to do it "in his (or her) head."
But the assembler can help. For our example list, we can create labels for the offsets to
avoid remembering the sizes of each element.
NAME: EQU 0 ; Name of person (6)
SSN: EQU NAME+30 ; Social Security number
ADDRE SS:EQU SSN+4 ; Address of person
TN: EQU ADDRESS+45 ; Telephone number
NBYTES: EQU TN+4 ; Number of bytes in the list

In the following program segment, a telephone number is stored in accumulator D (high
16 bits) and index register X (low 16 bits). If the list L's telephone number matches this
D:X, put the Social Security number in D:X, otherwise go to label NoMatch:

CMPD L+TN ; check high 16-bits of telephone number (7)
BNE NoMatch
CPX L+TN+2 ; check low 16-bits of telephone number
BNE NoMatch
LDD L+SSN ; get high 16-bits of Social Security number
LDX L+SSN+2 ; get low 16-bits of Social Security number

Notice that with the EQU directives of (6) the program segment (7) becomes much more
self-documenting. What makes this technique work is that it is easy to associate labels
with attributes, particularly because the order of the list elements is usually unimportant.
Notice that the EQU statements (6) not only let the programmer use labels for offsets,
but also let the assembler calculate the offsets for the programmer. This same example
will be continued for the description of a table, which is a vector of lists. However, we
will first discuss an array, which is a bit simpler.
AR(0,0) , AR(0,1), AR(0,2),.. .
AR(1,0), AR(1,1), AR(1,2),.. .
AR(2,0) , AR(2,1), AR(2,2) r .. .

Figure 103. An Array