ZiLOG Z80 Handbook page 134

The pseudo-ops DEFB and DEFW define constants and variables
in the program. The argument for the DEFB is a numeric or sym-
bolic expression that can be resolved in eight bits. The argument
for DEFW must be resolved in sixteen bits. Both pseudo-ops are
necessary because without them, the assembler could not generate
tables of data, constants, or locations for variables. The following
source lines generate a table of ten bytes, each byte representing
data from 1 to 10.
0100 01
TABLE
DEFB
1
0101 02
DEFB 2
0102 03
DEFB 3
0103 0405
DEFW 0405H
0105 06
DEFB 6
0106 0708
DEFW 0708H
0108 09
DEFB 1001B
0109 OA
DEFB AH
010A
DEFS is a pseudo-op that reserves a number of bytes. In many
cases, it is necessary to set aside a block of memory without actually
filling it with meaningful data, as in allocation of I/O buffers and
working storage areas. The effect of DEFS is to increment the as-
sembler location counter by the argument, which represents the
number of bytes to be reserved. When the assembled object module
is loaded by the loader program after assembly, the block of storage
allocated by the DEFS will not be affected and will retain the mean-
ingless data in the memory area before the load. An alternative way
to reserve storage is to use an ORG pseudo-op. Both of the state-
ments below reserve 22H bytes starting at location 1234H.
1234H
1234H
1256H
BUFFER
NEXTI
EQU
DEFS
LDD
c
$
22H
1234H BUFFER
EQU
$
1256H ORG
$ + 22H
1256H NEXTI
LDD
c
The last pseudo - op discussed here, TXT, is similar to the DEFB
and DEFW in that it generates data for use by the program. The
data in this case is ASCII text data. ASCII representation is used
for most I/O devices and is shown in Appendix E. Alphabetic,
numeric, and special characters must be encoded in ASCII format
before being transferred to the I/O device for printing , display, or
punching. The TXT pseudo- op generates one ASCII character for
each text character in the argument string. The argument string is
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