TMS-9900 ASSEMBLY LANGUAGE WITH TI EXTENDED BASIC

        TI Extended BASIC has the capability, when used with the Memory
        Expansion accessory, to load and execute TMS-9900 Assembly 
        Language subprograms.  Assembly Language routines may be entered 
        and assembled using the TI-99/4 UCSD PASCAL(tm) Development
        System.  This document provides an overview of the capabilities, 
        how they are accessed from TI Extended BASIC, and how assembly
        routines may be developed.

        Assembly Language routines called from Extended BASIC may be
        used to make algorithms execute more rapidly, or to provide 
        complex control of the video screen or the sound chip.  Assembly 
        routines are normally loaded in a relocatable form into a
        dedicated 8K byte block of the Memory Expansion.  Using 
        relocatable code allows routines to be written and used
        independently of the actual loaded address in memory.  Also, 
        combinations of several routines may be used from one BASIC 
        program without concern for the load address.  Assembly routines
        are called from the BASIC program by name.  The actual address of 
        the routine is resolved during execution.  One or more entry 
        names for a routine are defined when the routine is written.

        The 8K byte reserved block of the Memory Expansion may be used
        by Assembly Language.  In addition, a portion of the other 24K 
        bytes in the Memory Expansion is available depending on the size 
        of the BASIC program being run.  An assembly routine may or may 
        not return control to BASIC.  It may actually take control of 
        the TI-99/4 and act as an entire application.  In this instance, 
        the entire memory resources of the TI-99/4 are available for use
        by the assembly subprogram.

        A set of utility routines is provided for Assembly Language 
        subprograms to easily access unique hardware resources such as 
        the Video Display Processor, and to pick up the values of 
        arguments or return values to the BASIC program.  A set of 
        subroutines may be linked with an assembly routine to provide 
        access to peripherals.  With these routines, an assembly 
        subprogram may easily execute I/O to a diskette, RS-232 or other 
        peripheral.

        Three statements in the Extended BASIC language (CALL INIT, CALL 
        LOAD, and CALL LINK) are provided to support the Assembly
        Language capability.


        The BASIC subprogram CALL INIT is used to intitialize the RAM
        Expansion for assembly subprograms.  This subprogram ensures
        that the Memory Expansion has been attached.  It then loads a
        set of utilities from ROM memory in the Extended BASIC command


        TMS-9900 Assembly Language                               Page 2
         with TI Extended BASIC


        module.  into the Memory Expansion.  CALL INIT also causes any pre- 
        viously loaded assembly routines to be undefined.  Any
        subsequently loaded routines will remain defined until another 
        CALL INIT is executed or the Memory Expansion is turned off.

        The BASIC subprogram CALL LOAD is used to load an Assembly
        object file into the Memory Expansion.  Alternatively, 
        direct data may be specified in the form of addresses and data
        in the CALL LOAD statement.  An object file may contain 
        relocatable or absolute address code.  Direct data may only be
        absolute.  Because of the difficulty in defining the entry point
        name from direct data, it is not recommended for general use in
        an entire program.  However, if this is necessary, we 
        can provide technical assistance to help accomplish this. 
        Examples of CALL LOAD statements follow:

                1.  CALL LOAD("CS1")

                2.  CALL LOAD("DSK1.MYOBJECT", "DSK1.OBJECT2")

                3.  CALL LOAD(12000,04,01,02,0,255,255)

        The first two are examples of loading from a file while the
        third uses direct data.

        The BASIC subprogram CALL LINK is used to link to an Assembly
        Language subprogram.  This statement specifies the routine to be 
        called and any arguments to that routine.  For example

                CALL LINK("NSORT",A(,))

        could be written to sort array elements in row A(2,n) so that 
        elements in row A(1l,n) are in ascending order.  In this example, 
        NSORT is the name of the routine.  One argument is passed as two 
        dimensional array A.  Values in this array are changed and 
        returned to the BASIC program.

        Development of Assembly Language programs to run under Extended 
        BASIC may be done on the PASCAL Development System.  This system 
        includes a powerful text editor, macroassembler, and link
        editor.  A set of macros is provided during assembly to 
        facilitate accessing specific TI-99/4 hardware features such as
        the Video Display Processor, or the sound chip.  An I/0 utility 
        is available which may be linked with an assembly routine to
        provide access to peripherals such as disk drive, RS-232, etc. 
        Although the disk format of the PASCAL system is different from
        that used by Extended BASIC, a utility is provided to convert a 
        PASCAL object file to the format required by Extended BASIC.

        **UCSD PASCAL is a trademark of the Regents of the University of 
          California.