The Midi Connection, Part II

By Kirk Austin, San Anselmo, CA.

As you may recall from last issue all we need to get MIDI up and running on the Macintosh now that we have the necessary hardware are the driver routines for the serial ports. Fear not fellow coders, the coveted routines have arrived.

The normal device driver model for the Macintosh has too much overhead associated with it to make it approriate for use with MIDI which transfers data at a rate of 31.25K bits per second. Usually, real time applications need all the extra time they can get. This is particularly true of sequencer type applications that do MIDI "multi-track recording" while simultaneously maintaining a graphics display of some sort. routines that directly access the 8530 SCC chip have been utilized in order to minimize the time taken up by the serial I/O. I have tried to make things as easy as possible by providing "building block" style routines that follow the guidelines of the Lisa Pascal interface. This is the reason for the LINK and UNLNK instructions which aren't really necessary otherwise. You can basically treat RxMIDI as a Pascal function that accepts no arguments and returns a word of data as a result that is either a valid MIDI byte or else a flag indicating that no MIDI data is available. TxMIDI can be treated as a Pascal procedure that accepts a word of data containing the MIDI byte to be transmitted as an argument. Both of these routines are stack based.

Since these routines include interrupt handlers with pointers placed in low memory the routines cannot be in a relocatable block. One easy way of insuring this is to place them in your first code segment. If your code only consists of one segment you don't have anything to worry about, but if there is more than one segment the memory manager may move things around on you when you don't expect it. If your interrupt handlers are in one of these relocatable segments the pointers to them can be invalidated which would cause the program to crash.

Okay, let's get down to it. First the SCC chip has to be initialized by calling either SCCinitA or SCCinitB (this should be done when your application initializes quickdraw and the various managers). The initialization ends up being quite a bit of code actually, and if not done properly will mess things up but good! The 8530 can only be accessed at a maximum rate of every 2.2 microseconds. I know, this sounds unbelievable for a sophisticated piece of modern hardware, but it's true. This is the reason for all of the MOVE.L (SP),(SP) instructions. They don't accomplish anything, but they take a little more than a couple of microseconds to execute which is just the amount of delay that we need (silly huh?). As I mentioned in the article on the hardware interface the SCC chip can accept three different external clock frequencys to produce the desired baud rate. The appropriate divisor must be selected in the initialize routine. By the way, these routines are written so you can use either the modem port or the printer port, but if you want to use both simultaneously and keep the ports completely independant you will have to duplicate everything for each port. If you are writing an application of that complexity I think you can handle rewriting these routines.

The transmit and receive routines each maintain circular queues of outgoing and incoming data respectively. These queues can be of any length, but I have arbitrarily set them at $100 bytes each. To change the size of the queues just change the values in the equate table. There is no error detection code for a queue overrun condition so you will have to make sure that your application runs fast enough to avoid this. If an overrun condition occurs you will lose data. If you want to dump huge files all at once (if you are writing a patch librarian for example) just make the queue big enough to handle the entire file and you can do it without worrying about overruns.

The TxMIDI routine is the most complicated one, so let's have a look at it. When this routine receives a byte to transmit (in the lower byte of the word left on the stack by the calling routine) it first must check the queue to see if it is empty or not. If the queue is not empty the byte is simply added to the queue. If the queue is empty the routine checks the SCC chip to see if its transmit buffer is empty. If the transmit buffer is not empty the byte is just added to the queue. But, if the transmit buffer is empty the routine must write the byte to the SCC chip to transmit it.

The RxMIDI routine is more straightforward. It checks to see if there is any data in the queue, and if there is it returns it on the stack (the space for the result must be allocated by the calling routine). If there is no data available the routine returns $FFFF as its result.

The interrupt handlers do exactly what you might expect them to. When a byte is received by the SCC chip a receive interrupt is generated which calls the RxIntHand routine. This routine simply takes the byte from the SCC register and places it in the receive queue. The TxIntHand routine is called when the SCC chip's transmit buffer is empty. It takes a byte from the transmit queue if one is available and writes it to the SCC register. If no data is available it clears the interrupt and returns. One thing about the interrupt handlers that was not obvious to me when I was first coding them was the fact that register A5 must be saved at the beginning of the routines and its value loaded from the system variable CurrentA5 in order to insure that it is pointing to the application's globals area.

Only one thing left to do, and that is reset the 8530 before you quit the application or try to print (if you are using the printer port for MIDI too). Just call SCCResetA or SCCResetB and your application will exit gracefully.

; MIDI PORT ROUTINES
; copyright Kirk Austin 1985

; Transmit and Receive queue length equates

TxQSize EQU $100
RxQSize EQU $100

; Serial Chip Addresses, offsets, and system equates

sccRBaseEQU $9FFFF8
sccWBaseEQU $BFFFF9
Lvl2DT  EQU $1B2
aData EQU 6
aCtl    EQU 2
bData EQU 4
bCtl    EQU 0
TBEEQU  2
CurrentA5 EQU  $904

; This is an example of how to use the routines.
; If this were placed in your event loop your application would
; receive  MIDI data and echo it back out.
;
;MIDIThru
;CLR    -(SP)    ; clear space for result
;BSR    RxMIDI ; fetch data
;MOVE   (SP)+,D0
;CMPI   #$FFFF,D0; any bytes available?
;BEQ    NoMIDI ; if not, exit
;MOVE   D0,-(SP) ; if so, transmit them
;BSR    TxMIDI
;BRA    MIDIThru ; check for more
;NoMIDI
;
; -------------------------------------------------------------------------

; This section contains the necessary routines for MIDI
;
; These are the initialization routines which should be called
; when you  initialize quickdraw and the various managers. 
; Call SCCInitA to use the  modem port, and SCCInitB to use 
; the printer port.

SCCInitA
 MOVE   #aCtl,CtlOffset(A5) ; set up globals for Chn A
 MOVE   #aData,DataOffset(A5)
 MOVE.B #%10000000,ChnReset(A5)
 MOVE   #24,RxIntOffset(A5)
 MOVE   #16,TxIntOffset(A5)
 MOVE   #28,SpecRecCond(A5)
 BRA    SCCInit

SCCInitB
 MOVE   #bCtl,CtlOffset(A5) ; set up globals for Chn B
 MOVE   #bData,DataOffset(A5)
 MOVE.B #%01000000,ChnReset(A5)
 MOVE   #8,RxIntOffset(A5)
 MOVE   #0,TxIntOffset(A5)
 MOVE   #12,SpecRecCond(A5)

SCCInit
 MOVE   SR,-(SP) ; Save interrupts
 MOVEM.LD0/A0-A1,-(SP)  ; Save registers
 ORI    #$0300,SR; Disable interrupts
 
 MOVE.L #sccRBase,A1 ; Get base Read address
 ADD    CtlOffset(A5),A1  ; Add offset for control
 MOVE.B (A1),D0  ; Dummy read
 MOVE.L (SP),(SP); Delay
 MOVE.L #sccWBase,A0 ; Get base Write address
 ADD    CtlOffset(A5),A0  ; Add offset for control
 MOVE.B #9,(A0)  ; pointer for SCC reg 9
 MOVE.L (SP),(SP); Delay
 MOVE.B ChnReset(A5),(A0) ; Reset channel
 MOVE.L (SP),(SP); Delay
 MOVE.B #4,(A0)  ; pointer for SCC reg 4
 MOVE.L (SP),(SP); Delay
; This is where you determine the external clock rate

; %01000100 = 500K
; %10000100 = 1 Meg
; %11000100 = 2 Meg

 MOVE.B #%01000100,(A0) ; 16x clock, 1 stop bit
 MOVE.L (SP),(SP); Delay
 MOVE.B #1,(A0)  ; pointer for SCC reg 1
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00000000,(A0) ; No W/Req
 MOVE.L (SP),(SP); Delay
 MOVE.B #3,(A0)  ; pointer for SCC reg 3
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00000000,(A0) ; Turn off Rx
 MOVE.L (SP),(SP); Delay
 MOVE.B #5,(A0)  ; pointer for SCC reg 5
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00000000,(A0) ; Turn off Tx
 MOVE.L (SP),(SP); Delay
 MOVE.B #11,(A0) ; pointer for SCC reg 11
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00101000,(A0) ; Make TRxC clock sourc
 MOVE.L (SP),(SP); Delay
 MOVE.B #14,(A0) ; pointer for SCC reg 14
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00000000,(A0) ; Disable BRGen
 MOVE.L (SP),(SP); Delay
 MOVE.B #3,(A0)  ; pointer for SCC reg 3
 MOVE.L (SP),(SP); Delay
 MOVE.B #%11000001,(A0) ; Enable Rx
 MOVE.L (SP),(SP); Delay
 MOVE.B #5,(A0)  ; pointer for SCC reg 5
 MOVE.L (SP),(SP); Delay
 MOVE.B #%01101010,(A0) ; Enable Tx and drivers
 MOVE.L (SP),(SP); Delay
 MOVE.B #15,(A0) ; pointer for SCC reg 15
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00001000,(A0) ; Enable DCD int for 
 ; mouse
 MOVE.L (SP),(SP); Delay
 MOVE.B #0,(A0)  ; pointer for SCC reg 0
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00010000,(A0) ; Reset EXT/STATUS
 MOVE.L (SP),(SP); Delay
 MOVE.B #0,(A0)  ; pointer for SCC reg 0
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00010000,(A0) ; Reset EXT/STATUS 
 MOVE.L (SP),(SP); Delay
 MOVE.B #1,(A0)  ; pointer for SCC reg 1
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00010011,(A0) ; Enable interrupts
 MOVE.L (SP),(SP); Delay
 MOVE.B #9,(A0)  ; pointer for SCC reg 9
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00001010,(A0) ; Set master int enable
 MOVE.L (SP),(SP); Delay
 
 MOVE.L #Lvl2DT,A0 ; get dispatch table 
 ; pointer
 MOVE   RxIntOffset(A5),D0; get offset to Rx vector
 LEA    RxIntHand,A1 ; set Rx vector
 MOVE.L A1,0(A0,D0)
 MOVE   TxIntOffset(A5),D0; get offset to Tx vector
 LEA    TxIntHand,A1 ; set Tx vector
 MOVE.L A1,0(A0,D0)
 MOVE   SpecRecCond(A5),D0; get offset to 
 ; Special vector
 LEA    Stub,A1
 MOVE.L A1,0(A0,D0)
 
 CLR    RxByteIn(A5) ; init flags & pointers
 CLR    RxByteOut(A5)
 MOVE.B #$FF,RxQEmpty(A5)
 CLR    TxByteIn(A5)
 CLR    TxByteOut(A5)
 MOVE.B #$FF,TxQEmpty(A5)

 MOVEM.L(SP)+,D0/A0-A1  ; Restore registers
 MOVE   (SP)+,SR ; Restore interrupts
 RTS    ; and return

; This is the routine to transmit a MIDI byte of data.  To use this
; place the byte to be transmitted as the lower 8 bits of a word
; routine on the stack, then BSR to TxMIDI.

TxMIDI
 LINK   A6,#0    ; set frame pointer
 MOVE   SR,-(SP) ; Save interrupts
 MOVEM.LD0/A0-A2,-(SP)  ; Save registers
 ORI    #$0300,SR; Disable interrupts

 TST.B  TxQEmpty(A5) ; is TxQueue empty?
 BNE    TxQE; if so branch
 MOVE   TxByteIn(A5),D0 ; if not add byte to queue
 LEA    TxQueue(A5),A2  ; point to queue
 MOVE.B 9(A6),0(A2,D0)  ; place byte in queue
 ADDQ   #1,D0    ; update TxByteIn
 CMP    #TxQSize,D0
 BNE    @1
 MOVE   #0,D0
@1   MOVE D0,TxByteIn(A5)
 BRA    TxExit   ; and exit
 
TxQE  
 MOVE.L #sccRbase,A0 ; get SCC Read Address
 MOVE.L #sccWbase,A1 ; get SCC Write address
 MOVE   CtlOffset(A5),D0  ; get index for Ctl
 BTST.B #TBE,0(A0,D0); transmit buffer empty?
 BNE    FirstByte; if so branch
 MOVE   TxByteIn(A5),D0 ; if not add to queue
 LEA    TxQueue(A5),A2  ; point to queue
 MOVE.B 9(A6),0(A2,D0)  ; place byte in queue
 ADDQ   #1,D0    ; update index
 CMP    #TxQSize,D0
 BNE    @1
 MOVE   #0,D0
@1  MOVED0,TxByteIn(A5)
 MOVE.B #0,TxQEmpty(A5) ; reset queue empty flag
 BRA    TxExit   ; and exit
 
FirstByte
 MOVE   DataOffset(A5),D0 ; get index to data
 MOVE.L (SP),(SP); delay
 MOVE.B 9(A6),0(A1,D0)  ; write data to SCC
 MOVE.L (SP),(SP); Delay
 

TxExit  
 MOVEM.L(SP)+,D0/A0-A2  ; Restore registers
 MOVE   (SP)+,SR ; Restore interrupts
 UNLK   A6; release frame pointer
 MOVE.L (SP)+,A1 ; save return address
 ADD.L  #2,SP    ; move past data word
 MOVE.L A1,-(SP) ; put address back on stack
 RTS    ; and return

; This is the routine to receive a byte of MIDI data.  To use this
; routine treat it like a Pascal function.  Leave space on the
; stack for a word of data before BSR'ing to this routine.  If the
; routine executes is $FFFF there was no MIDI data available.
; If the upper byte is clear then a valid MIDI byte is in the lower 
; 8 bits.

RxMIDI
 LINK   A6,#0    ; set frame pointer
 MOVE   SR,-(SP) ; Save interrupts
 MOVEM.LD0-D1/A0-A2,-(SP) ; Save registers
 ORI    #$0300,SR; disable interrupts

 TST.B  RxQEmpty(A5) ; any data available?
 BEQ    @1; if so, branch
 MOVE   #$FFFF,8(A6) ; if not, return with $FFFF
 BRA    RxExit
@1  MOVERxByteOut(A5),D0  ; get index to byte out
 LEA    RxQueue(A5),A2  ; point to queue
 MOVE.L #0,D1    ; clear data register
 MOVE.B 0(A2,D0),D1; get MIDI data
 MOVE   D1,8(A6) ; place it on stack for return 
 ADDQ   #1,D0    ; update index
 CMP    #RxQSize,D0
 BNE    @2
 MOVE   #0,D0
@2   MOVE D0,RxByteOut(A5)
 MOVE   RxByteIn(A5),D1
 CMP    D0,D1    ; is queue empty?
 BNE    RxExit   ; if not exit
 MOVE.B #$FF,RxQEmpty(A5) ; if empty, set flag

RxExit  
 MOVEM.L(SP)+,D0-D1/A0-A2 ; Restore registers
 MOVE   (SP)+,SR ; restore interrupts
 UNLK   A6
 RTS    ; and return
 
; This is the interrupt routine for receiving a byte of MIDI data. 
; It places the received byte in a circular queue to be
; accessed later by the application.


RxIntHand
 ORI    #$0300,SR; disable interrupts
 MOVEM.LD0-D1/A0-A2/A5,-(SP); save registers
 MOVE.L CurrentA5,A5 ; make sure A5 is correct
 MOVE.L #sccRBase,A0 ; get SCC address
 MOVE.L #sccWBase,A1
 
 MOVE   DataOffset(A5),D0 ; get data offset
 MOVE.B 0(A0,D0),D1; read data from SCC
 MOVE.L (SP),(SP); Delay
 LEA    RxQueue(A5),A2  ; point to queue
 MOVE   RxByteIn(A5),D0 ; get offset to next cell
 MOVE.B D1,0(A2,D0); put byte in queue
 MOVE.B #0,RxQEmpty(A5) ; reset queue empty flag
 ADDQ   #1,D0    ; update index
 CMP    #RxQSize,D0
 BNE    @1
 MOVE   #0,D0
@1   MOVE D0,RxByteIn(A5)
 
 MOVEM.L(SP)+,D0-D1/A0-A2/A5; restore registers
 ANDI   #$F8FF,SR; enable interrupts
 RTS    ; and return
 
; This is the interrupt routine for transmitting a byte of MIDI
; data.  It  checks to see if there is any data to send.  If there is
; it sends it to the SCC.  If there isn't it resets the TBE interrupt 

; in the SCC and exits.


TxIntHand
 ORI    #$0300,SR; disable interrupts
 MOVEM.LD0-D1/A0-A2/A5,-(SP); save registers
 MOVE.L CurrentA5,A5
 MOVE.L #sccRBase,A0 ; get SCC address
 MOVE.L #sccWBase,A1
 
 TST.B  TxQEmpty(A5) ; Is queue empty?
 BEQ    @1; if not branch
 MOVE   CtlOffset(A5),D0  ; get offset for control
 MOVE.B #$28,0(A1,D0); if so, reset TBE 
 ; interrupt
 MOVE.L (SP),(SP); Delay
 BRA    TxIExit  ; and exit
@1  MOVETxByteOut(A5),D0  ; get index to next data 
 ; byte
 LEA    TxQueue(A5),A2  ; point to queue
 MOVE   DataOffset(A5),D1 ; get data offset
 MOVE.B 0(A2,D0),0(A1,D1) ; write data to SCC
 MOVE.L (SP),(SP); Delay
 ADDQ   #1,D0    ; update index
 CMP    #TxQSize,D0
 BNE    @2
 MOVE   #0,D0
@2   MOVE D0,TxByteOut(A5)
 MOVE   TxByteIn(A5),D1
 CMP    D0,D1    ; is TxQueue empty?
 BNE    TxIExit  ; if not exit
 MOVE.B #$FF,TxQEmpty(A5) ; if empty set flag

TxIExit 
 MOVEM.L(SP)+,D0-D1/A0-A2/A5; restore registers
 ANDI   #$F8FF,SR; enable interrupts
 RTS    ; and return
 
; This routine must be called when the application quits or the
; system will crash due to the interrupt handling pointers 
; becoming invalid.

SCCResetA
 MOVE   #aCtl,CtlOffset(A5) ; set up globals for Chn A
 MOVE   #aData,DataOffset(A5)
 MOVE.B #%10000000,ChnReset(A5)
 MOVE   #24,RxIntOffset(A5)
 MOVE   #16,TxIntOffset(A5)
 MOVE   #28,SpecRecCond(A5)
 BRA    SCCReset

SCCResetB
 MOVE   #bCtl,CtlOffset(A5) ; set up globals for Chn B
 MOVE   #bData,DataOffset(A5)
 MOVE.B #%01000000,ChnReset(A5)
 MOVE   #8,RxIntOffset(A5)
 MOVE   #0,TxIntOffset(A5)
 MOVE   #12,SpecRecCond(A5)

SCCReset
 MOVE   SR,-(SP) ; Save interrupts
 MOVE.L A0,-(SP) ; Save register
 ORI    #$0300,SR; Disable interrupts
 
 MOVE.L #sccWBase,A0 ; Get base Write address
 ADD    CtlOffset(A5),A0  ; Add offset for control
 MOVE.B #9,(A0)  ; pointer for SCC reg 9
 MOVE.L (SP),(SP); Delay
 MOVE.B ChnReset(A5),(A0) ; Reset channel
 MOVE.L (SP),(SP); Delay
 MOVE.B #15,(A0) ; pointer for SCC reg 15
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00001000,(A0) ; Enable DCD int
 MOVE.L (SP),(SP); Delay
 MOVE.B #0,(A0)  ; pointer for SCC reg 0
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00010000,(A0) ; Reset EXT/STATUS
 MOVE.L (SP),(SP); Delay
 MOVE.B #0,(A0)  ; pointer for SCC reg 0
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00010000,(A0) ; Reset EXT/STATUS 
 MOVE.L (SP),(SP); Delay
 MOVE.B #1,(A0)  ; pointer for SCC reg 1
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00000001,(A0) ; Enable mouse 
 ; interrupts
 MOVE.L (SP),(SP); Delay
 MOVE.B #9,(A0)  ; pointer for SCC reg 9
 MOVE.L (SP),(SP); Delay
 MOVE.B #%00001010,(A0) ; Set master int enable
 MOVE.L (SP),(SP); Delay
 
 MOVE.L (SP)+,A0 ; Restore register
 MOVE   (SP)+,SR ; Restore interrupts
 RTS    ; and return

; this is the space for a special condition interrupt routine

Stub
 RTS
;-------------------------------MIDI Globals--------------------------------
CtlOffset DS.W 1 ; offset for channel control
DataOffsetDS.W 1 ; offset for channel data
ChnResetDS.B1  ; SCC channel reset select
RxIntOffset DS.W 1 ; offset for dispatch table
TxIntOffset DS.W 1 ; offset for dispatch table
SpecRecCond DS.W 1 ; offset for dispatch table

TxQueue DS.BTxQSize; transmitted data queue
TxQEmptyDS.B1  ; Transmit queue empty flag
TxByteInDS.W1  ; index to next cell in
TxByteOut DS.W 1 ; index to next cell out

RxQueue DS.BRxQSize; received data queue
RxQEmptyDS.B1  ; receive queue empty flag
RxByteInDS.W1  ; index to next cell in 
RxByteOut DS.W 1 ; index to next cell out 

End