Intel 8080 - B3_Page_31

Chapter 4. Assembler Directives
In the examples, the LXI instructions load the address of the desired data into the Band C registers. The LDAX
instructions then load the accumulator with one byte of data from the address specified in the Band C registers.
The assernbler neither knows nor cares that only one character from the three-character
field ALPHA has been
accessed. The program must account for the characters at
ALPHA+l and ALPHA+2, as in the following coding
sequence:
ALPHA: DB
'ABC'
;DEFINE ALPHA
LXI B,ALPHA ;LOAD ADDRESS OF ALPHA
LDAX
B ;FETCH 1 ST ALPHA CHAR
INX B
;SET B TO ALPHA+l
LDAX
B
;FETCH 2ND ALPHA CHAR
INX B ;SET B TO ALPHA+2
LDAX
B
;FETCH 3RD ALPHA CHAR
The coding above is acceptable for short data fields like ALPHA. For longer fields, you can conserve memory
by setting up an instruction sequence that is executed repeatedly until the source data is exhausted.
Add Symbols for Data Access
The following example was
presented earlier as an illu5tration of the OS directive:
Label Opcode Operand
Comment
TTYBUF:
OS 72
;RESERVE TTY BUFFER
To access data in this buffer using only expressions such as TTYBUF+l, TTYBUF+2, ... TTYBUF+72 can be
a laborious and confusing chore, especially when you want only selected fields from the buffer. You can simplify
this task
by subdividing the buffer with the EQU directive:
Label Opcode
Operand
Comment
TIYBUF:
OS 72
;RESERVE TTY BUFFER
10 EQU
TTYBUF ;RECORD IDENTIFIER
NAME EQU
TTYBUF+6 ;20-CHAR NAME FIELD
NUMBER
EQU
TTYBUF+26
;10-0-lAR EMPLOYEE NUMBER
DEPT
EQU
TTYBUF+36 ;5-CHAR DEPARTMENT NUMBER
SSNO
EQU
TTYBUF+41 ;SOCIAL SEC. NUMBER
DOH EQU
TTYBUF+50 ;DATE OF HIRE
DESC EQU
TTYBUF+56 ;)OB DESCRIPTION
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