Tandy 1000 Basic Reference Manual page 360

Chapter 11 1 Technical Information
Accessing String Variables
If the parameter passed in a CALL statement is a string expres-
sion, the parameters offset points to the
string descriptor.
If the
argument passed in a USR function call is a string expression,
the
DX register points to the string descriptor.
The
string descriptor
is a 3-byte area of memory that points to
the text of the s t r i n g . The
string descriptor
contains the
following:
Byte 0 contains the length of the string (0 to 255).
Byte 1 contains the lower 8 bits of the string
starting address in BASIC's data segment.
Byte 2 contains the upper 8 bits of the string
starting address in BASIC's data segment.
The text of the string may be altered by the subroutine, but the
length of the string must not be changed. BASIC cannot cor-
rectly manipulate strings if their lengths are modified by exter-
nal routines.
Since the
string descriptor
points to an area of memory in your
BASIC program, you must be careful not to alter or destroy your
program. To avoid unpredictable results, add the concatenation
symbol
( + )
to the string. This forces the string to be copied into
string space, where the string may be modified without affecting
the program.
Example
20 A$
=
"MONTHLY SALES REPORT"
+
" "
File Control Block
A file control block is a storage area in BASIC's data segment
that contains information BASIC needs for all functions per-
formed on that file. When you execute the VARPTR function and
specify the buffer number, BASIC returns the address of the
BASIC file control block for that file. Note that this is the BASIC
file control block, not the DOS file control block. The address is
specified as an offset into BASIC data segment. In this section
we define the information in the file control block. Offsets are
relative to the value returned by VARPTR. Length is in bytes.
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