GWBASIC User's Manual

GW-BASIC User's Guide
Note
● Values are returned to GW-BASIC by including in the argument list the name of the
variable that is to receive the result.
● If the argument is a string, the parameter offset points to three bytes called the string
descriptor. Byte 0 of the string descriptor contains the length of the string (0 to 255). Bytes
1 and 2, respectively, are the lower- and upper-eight bits of the string starting address in
the string space.
● If the argument is a string literal in the program, the string descriptor points to program
text. Be careful not to alter or destroy a program this way. To avoid unpredictable results,
add +"" to the string literal in the program, as in the following:
20 A$="BASIC"+""
This forces the string literal to be copied into the string space. Now the string may be
modified without affecting the program.
Strings may be altered by user routines, but their length must not be changed. GW-BASIC cannot
correctly erase strings if their lengths are modified by external routines.
For more information on the CALL statement and USR function, see Appendix D in the GW-
BASIC User's Guide.
Example 1:
100 DEF SEG=&H2000
110 ARK=0
120 CALL ARK(A, B$, C)
.
.
.
Line 100 sets the segment to hex 2000. ARK is set to zero so that the call to ARK executes the
subroutine at location 2000:0.
Example 2:
The following sequence of 8086 Assembly Language demonstrates access of the parameters
passed and stored in variable C:
PUSH BP
MOV BP, SP ; Gets current stack position in BP.
MOV BX, 8[BP] ; Gets address of B$ descriptor.
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