Tandy 1000 Basic Reference Manual page 355

Chapter 11 I Technical Information
Poking a Subroutine into Memory
You can code short subroutines in machine language and use the
POKE
statement to put the code into memory.
To
do so, follow
these steps:
1. Code the machine language instructions for your subroutine.
2. Put the assembly instruction code for each byte of the ma-
chine language code into DATA statements, preceded by the
&H
symbols t o denote that they are hexadecimal values.
Execute a loop that reads the DATA statements and POKES
them into an area of memory.
3.
For example, the instruction code for the statement
P U S H BP
is 55. The DATA statement for that instruction is
D A T A & H 5 5
After the loop is complete, the subroutine is in memory. Whether
you are using the
USR function or the CALL statement to call
the subroutine, you must set the value of the subroutine entry
point as the location specified in the first POKE statement.
CALL Statement
When the CALL statement is executed, the following occurs:
1. For each parameter in the parameter list, the two-byte offset
of the parameter's location within the data segment (DS) is
pushed onto the stack. If the parameter is a string variable,
the offset points to the
string descriptor.
See the section "Ac-
cessing String Parameters" in this appendix.
The BASIC return address code segment (CS) and offset (IP)
are pushed onto the stack.
Control is transferred to the subroutine by an 8086 long call
to the segment address given in the last DEF SEG state-
ment and the offset given in
uariabk.
2.
3.
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