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        Using ROM Subroutines on the HX-20 with MBASIC
                        by Justin Bell


   Here is one way to set registers to execute ROM subroutines:

   1.  Load BASIC and set aside the memory necessary for your 
       program.  The following command sets aside 1000 bytes:

       MEMSET 3624 <RETURN>

   2.  Use the BASIC command MON to enter the monitor section.

   3.  Use the monitor's S command to enter codes.

   4.  Type a period to stop entering codes.

   5.  Use the monitor's G command to execute the routines.

   The following example executes ROM subroutine FF64 to produce
   a tone for a certain duration:

   86  0A   LDAA   #NOTE   ;load the note (note 10 in this                                  ;example) into the A register

   C6  10   LDAB   #TIME   ;load the time (1.6 seconds here)
                           ;into the B register

   BD  FF64 JSR    $FF64   ;jump to the subroutine

   39       RTS            ;return to the origin - this may
                           ;result in a trap if you did not
                           ;call from a subroutine 

   This is an example program:

   10 TITLE "ROMSUBS"
   20 MEMSET 3624
   110 POKE 2624,&H86
   120 POKE 2625,&H0A
   130 POKE 2626,&HC6
   135 POKE 2627,&H10
   140 POKE 2628,&HBD
   150 POKE 2629,&HFF
   160 POKE 2630,&H64
   170 POKE 2631,&H39
   180 POKE 2632,&H00
   200 EXEC 2624

   After the BASIC program has run, memory looks like this:

   0A40: 86 0A C6 10 BD
   0A45: FF 64 39 00 20
   0A4A: 00 00 00 00 00


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