(c) Copyright 1989 Commodore-Amiga, Inc. All rights reserved. The information contained herein is subject to change without notice, and is provided "as is" without warranty of any kind, either express or implied. The entire risk as to the use of this information is assumed by the user. MIDI Output in Assembler by Darius Taghavy, CATS The Amiga's serial.device provides a software interface to the serial port which is suitable for a wide variety of purposes. However, high-performance applications such as MIDI drivers may require direct access to the Amiga hardware which controls the serial port. The program listed below shows how to send MIDI data over the serial port using direct access to the Amiga's Paula chip I/O registers. It is important to note that MIDI applications which use direct access to the serial port I/O registers must first gain exclusive access to the serial hardware by opening the misc.resource and allocating MR_SERIALBITS and MR_SERIALPORT. By doing this, the application becomes the "owner" of the serial port which prevents other tasks which may be running from interfering with serial I/O. The example program demonstrates direct access to the internal serial port through the Paula chip I/O registers and provides a very useful MIDI function: it turns off all notes. If you have ever used MIDI software, then you may be familiar with the stuck-note phenomenon. A stuck note is a note that sustains forever. Stuck notes happen because the duration of a note is determined by the elapsed time between a NOTE ON and a NOTE OFF event. If a NOTE OFF event ever gets lost you end up with a stuck note. The MIDI standard does provide an ALL NOTES OFF command, however, not all tone generators respond to it properly, and not all sequencers provide a way to send the command. The example program, named Panic, solves this problem by sending a NOTE ON command with a velocity of zero for all 128 notes on all 16 channels. In other words, each possible note is turned of individually forcing the sustaining culprit in your MIDI system to shut up. The Panic program is written in assembler to keep the code small and to save execution time. The program also employs the idea of a running status to save execution time by reducing the number of bytes it has to send. In accordance with the official MIDI specification, as long as data of the same status is being sent (i.e., NOTE ON) we can just keep sending the associated data bytes which, in this case, is 2 bytes for pitch and velocity, respectively. There is no need to re-transmit the status byte with every message. In the example code, this is implemented by cycling through the (inner) note_loop for 128 times, once for each MIDI note number. Since the MIDI channel information is imbedded in the lower nybble of the status byte, a new status byte must be sent when switching to a different channel. This is implemented as the (outer) channel_loop which loops 16 times, once for each MIDI channel. Using a running status cuts I/O activity by a third. Panic demonstrates assembler independent code by using the SECTION CODE and SECTION DATA directives. All Amiga assemblers will understand this. Panic also uses the statement, XDEF _panic, to make it callable from another module (say, a C main program). Manx users, please note that this is much better than using the Manx directive public since it is portable between assemblers (public is not). Use XREF for referencing external labels and XDEF for making labels available to external modules. Panic can be assembled and linked as is. No special assembler or linker flags are needed. The resulting executable is a mere 164 bytes long and only takes approximately 1.3 seconds to execute (using MANX 3.6a as and ln). The length of the transmitted MIDI data itself is 4112 bytes: (2 note/velocity bytes * 128 notes) + (1 status byte * 16 channels) = 4112 bytes. The MIDI transfer rate is defined as 31250 baud or about 3125 MIDI bytes/sec. This figure takes into account start and stop bits ( a MIDI byte is 10 bits ). Hence, time t for transmitting these events in an ideal world (ideal UART and crystal) is: total number of bytes 4112 bytes t = ---------------------------- = --------------------- = 1.31584 seconds. MIDI transfer rate 3125 bytes/sec Note that the Panic program assumes the serial baud rate is already set to 31250 baud and that the misc.resource has been allocated. As such there are two uses for this program. First, Panic could be used as a utility in conjunction with a sequencer that does not have a true all-notes-off feature or that lacks even the generic MIDI command \ffALL NOTES OFF\ff. In fact, I originally wrote Panic because no Amiga sequencer at the time offered this important feature. When Panic is used this way, it makes sense to not set the serial baud rate (your sequencer already took care of it) and to not allocate the serial port (the sequencer probably already has exclusive access to it). A second use for Panic would be to link it with your own code (say, a sequencer) which properly allocates the serial port and sets the baud rate. I encourage developers to include Panic in their code. In either case, it is important to make sure that other tasks have been locked out from using the serial port before hitting the serial port hardware registers directly. The program is listed on the next page. ;---------------------------------------------------------------- ; PANIC (c)1990 by Darius Taghavy ;---------------------------------------------------------------- ;sends note #0-127 with zero (0) velocity on all 16 MIDI channels ;MANX: as panic.asm; ln panic panic.o ;---------------------------------------------------------------- ; this code assumes SERPER register to be set to MIDI baud rate ; it also assumes that we have exclusive access to the serial port ; hardware and related resources SECTION CODE XDEF _panic ;make available as 'C' function SECTION DATA CUSTOM_CHIPS equ $dff000 ;custom chips base address TBE equ 5 ;Transmitt Buffer Empty Bit ;Paula serial port register offsets from custom chip base address SERDAT equ $030 ;data output(WRITE ONLY) SERDATR equ $018 ;data/status(READ ONLY) SECTION CODE _panic move.l d2,-(a7) ;save d2 lea CUSTOM_CHIPS,a0 ;load custom chip base move.w #15,d1 ;16 MIDI channels channel_loop move.w #127,d2 ;128 MIDI notes ;-------------status byte move.b d1,d0 ;get MIDI channel andi.b #$f,d0 ;nibble or.b #$90,d0 ;Note On Status andi.w #$ff,d0 ;clear high byte ori.w #$100,d0 ;set stop bit 1$ btst.b #TBE,SERDATR(a0) ;wait for free serialport beq 1$ move.w d0,SERDAT(a0) ;send byte ;-------------note number note_loop move.b d2,d0 ;get note number andi.w #$ff,d0 ;clear high byte ori.w #$100,d0 ;set stop bit 2$ btst.b #TBE,SERDATR(a0) ;wait for free serialport beq 2$ move.w d0,SERDAT(a0) ;send pitch byte ;------------velocity = 0 3$ btst.b #TBE,SERDATR(a0) ;wait for free serialport beq 3$ move.w #$100,SERDAT(a0) ;send zero velocity dbra d2,note_loop ;loop 128 times dbra d1,channel_loop ;loop 16 times move.l (a7)+,d2 ;restore d2 rts END