The keyboard consists of 36 keys which are polled on interrupt at a variable interval, initially set to 50ms. The keys auto-repeat at a variable rate and key presses are stored in a 16 character type-ahead buffer. The keyboard lookup table, the polling, the key translation and the shift status can all be altered through the use of the keyboard vectors and variables. The SHIFT key can be disabled and the keys for CAP and NUM can be changed.

The keyboard interrupts also control alarm checking and display timing, and increment a frame counter.

While testing for a key, the datapacks or the machine itself may switch off and a low battery test is made.

KEYBOARD VARIANTS

Variants on the standard 36-key keyboard were also produced for particular customer requirements. These are either all-numeric or a special alphanumeric layout called "Alpha-POS". The Alpha-POS layout "inverts" the Alpha and Numeric keys by re-labeling the template around the keys, and the keycaps.

standard keyboard	Alpha-POS keyboard
numeric keyboard

OPERATING THE KEYBOARD

The normal keyboard consists of a 6 by 6 matrix of keys, as shown above.

The letter keys normally produce upper-case letters and return the corresponding ')"; onMouseout="hideddrivetip()"> ASCII value. By holding down the SHIFT key and pressing one of the letters you can access the symbols and numbers marked above the keys.

To select lower-case letters, hold down the SHIFT key and press the CAP key. Repeat this to return the keyboard to upper-case mode.

Pressing the SHIFT and NUM keys, puts you permanently in 'shift' mode so that the function of the SHIFT key is now reversed. The functions of SHIFT NUM, SHIFT CAP and SHIFT DEL, however, are not affected (DEL deletes characters to the left and SHIFT DEL deletes characters from the right).

The top row of keys and some on the bottom row are special. These keys return the following values:

	KEY	VALUE
	ON/CLEAR	1
	MODE	2
	UP	3
	DOWN	4
	LEFT	5
	RIGHT	6
	SHIFT DEL	7
	DEL	8
	EXE	13

The SHIFT key, the CAP key and the NUM key do not return values but immediately carry out their function.

The keyboard on the LZ has been extended to allow SHIFT-EXE, SHIFT-SPACE and SHIFT-RIGHT-ARROW. These SHIFT-KEY functions can be disabled by setting bit 7 of KBB_SPEC.

Shift EXE
To allow the use of SHIFT-EXE, bit 0 of KBB_SPEC must be set. When set, SHIFT-EXE will be returned as value 14.
Shift Space
To allow the use of SHIFT-SPACE, bit 1 of KBB_SPEC must be set. When set, SHIFT-SPACE will be returned as value 15.
Shift Right-arrow
SHIFT-RIGHT-ARROW is used to select foreign characters for the next key press only. For example, when the language is set to French, SHIFT-RIGHT arrow followed by 'a' produces the character 'á'.

KEYBOARD SCANNING

The ON/CLEAR key is polled independently of the others. The remaining 35 keys are polled on a 5 by 7 matrix.

THE ON/CLEAR KEY

Testing for the ON/CLEAR key is done by reading bit 7 of port 5 (address $15). If the key is pressed, bit 7 will be set, otherwise bit 7 is clear. Hence, the ON/CLEAR key can be tested directly and very quickly. The following routine waits for it to be pressed:

 TESTKEY:
        LDA     A,POB_PORT5:    ;READ ADDRESS $15 
        BPL     TESTKEY         ;BRANCH IF BIT 7 IS CLEAR

In some applications it is necessary to protect against 'key bounce' and it is recommended that a delay of approximately 50ms is used. The following routine waits for the ON/CLEAR key to be released and then pauses for 50ms:

DEBOUNCE:
        LDA     A,POB_PORT5:    ;READ ADDRESS $15 
        BMI     DEBOUNCE        ;BRANCH IF BIT 7 IS SET 
        LDX     #11600          ;FOR DELAY OF 50MS 
1$:     DEX 
        BNE     1$

Note that if keyboard interrupts are enabled while running the above procedure, holding down the ON/CLEAR key will fill up the keyboard buffer.

A system service, KB$BREK, is provided to test if the ON/CLEAR key is pressed or if an ON/CLEAR key is in the keyboard buffer.

THE KEY MATRIX

The key matrix consists of 7 columns, controlled by the SEMI-CUSTOM-CHIP 'COUNTER', and 5 rows which can be read as bits 2 to 6 of PORT 5 (the ON/CLEAR key uses bit 7). The layout is as follows (see also System Board - Keyboard):

input line	port 5 bits
	KBD5	KBD4	KBD3	KBD2	KBD1
	6	5	4	3	2
K7	D	J	P	V	S
K6	F	L	R	X	EXE
K5	G	K	Q	W	DEL
K4	C	I	O	U	Z
K3	B	H	N	Y	Y
K2	A	G	M	S	SHIFT
K1	right arrow	left arrow	down arrow	up arrow	MODE

To control the COUNTER lines, there are two significant addresses:

SCA_COUNTERRESET (address $300) - sets all lines to zero.
SCA_COUNTERCLOCK (address $340) - increments COUNTER address by one.

To carry out these functions, simply read or write to the respective address. The following will set the contents of the counter to $3F, i.e. K1 to K6 high and K7 low:

        TST     SCA_COUNTERRESET        ;SET COUNTER TO ZERO 
        LDA     A,#$3F 
1$:     TST     SCA_COUNTERCLOCK        ;INCREMENT COUNTER 
        DEC     A 
        BNE     1$

When a key is pressed, a connection is made from one of K1 to K7 to one of the PORT 5 lines. The lines on PORT 5 are pulled high, so by setting one of K1 to K7 low, a specific key press can be detected. For example, if K7 only is low, the 'D' key can be detected by bit 6 of PORT 5 going low.

Polling the entire keyboard involves setting each of K1 to K7 low in turn, reading PORT 5, and decoding the key. By setting all 7 lines low simultaneously, a quick check for any key can be made. The following routine will test for any key press:

        TST     SCA_COUNTERRESET        ;SET K1 TO K7 LOW 
        LDA     A,POB_PORT5:            ;READ PORT 5 
        BMI     CLRKEY                  ;BRANCH IF ON/CLEAR KEY IS PRESSED 
        AND     A,#$7C                  ;IGNORE BITS 7,1 AND 0 
        CMP     A,#$7C                  ;CHECK IF ANY BIT IS LOW 
        BNE     AKEY                    ;BRANCH IF KEY PRESS DETECTED

The method of keyboard scanning is explained in the next section.

KEY SCANNING

The keyboard is scanned as fast as possible using the routine pointed to by the ram vector BTA_POLL. For maximum efficiency, the following method is used:

Check if ON/CLEAR key is pressed and exit if it is.
Check quickly if any other key is pressed and exit if not.
Determine which key is pressed and check if the SHIFT key is also pressed.

Steps 1 and 2 are described above. Step 3 involves setting each of the SEMI-CUSTOM-CHIP COUNTER lines (K1 to K7) low in turn. This is done most efficiently by setting K7 low first then K6 down to K1:

K7	K6	K5	K4	K3	K2	K1	COUNTER VALUE
0	1	1	1	1	1	1	$3F
1	0	1	1	1	1	1	$5F
1	1	0	1	1	1	1	$6F
1	1	1	0	1	1	1	$77
1	1	1	1	0	1	1	$7B
1	1	1	1	1	0	1	$7D
1	1	1	1	1	1	0	$7E

The following code will set the values $3F,$5F... etc. on the COUNTER:

        TST     SCA_COUNTERRESET        ;READ ADDRESS $300 TO ZERO COUNTER
        LDA     B,#$40
        PSH     B
        DEC     B                       ;INITIAL VALUE $3F
        BRA     CLOCKB
NEXTCOL:
        PSH     B
CLOCKB:
        TST     SCA_COUNTERCLOCK        ;READ ADDRESS $340 TO INC COUNTER
        DEC     B                       ;DO IT B TIMES
        BNE     CLOCKB
        BSR     READPORT5               ;READ PORT 5 HERE AND DECODE KEY
        PUL     B
        LSR     B
        BNE     NEXTCOL                 ;REPEAT FOR COLUMNS K7 to K1

The SHIFT key is in column K2 so scanning of the keyboard must continue, even after one key has been found, to check if SHIFT is also pressed.

This method results in the following approximate keyboard scan times:

KEYBOARD STATE	TIME TAKEN TO SCAN KEYBOARD
no key pressed	0.3ms
on/clear key pressed	0.2ms
any other key pressed	1.9ms

Note that the time taken for the entire keyboard interrupt may be increased by one or more of the following:

The variable key click, 1ms by default
The 'buffer-full' beep which lasts 10ms.
Alarm checking and/or ringing which may last up to 2 mins (see Alarm Checking Interrupts).

KEYBOARD INTERRUPTS

The keyboard scanning routine is called at regular intervals from an interrupt generated by the processor's TIMER 1 counter. This is a 16 bit FREE RUNNING COUNTER (FRC) (address $09,$0A) incremented by the processor clock.

The interrupt is generated when the value in the FRC matches a value set up in the TIMER 1 OUTPUT COMPARE REGISTER 1 (address $0B,$0C) and is directed to the address contained in the ROM at address $FFF4. The ram vector BTA_OCI is then used to jump to the interrupt service routine, i.e.:

        LDX     BTA_OCI 
        JMP     0,X

The code in the interrupt service routine handles the following:

Polling the keyboard using a routine at BTA_POLL
Translating any key found using a routine at BTA_TRAN
Producing a key 'click'
Checking for alarms
Incrementing a frame-counter
Decrementing a display-timer

INITIALISING

Keyboard interrupts are fully initialised on a cold start of the machine only, using the system service KB$INIT. When each interrupt occurs, the first task is to set up the FREE RUNNING COUNTER and OUTPUT COMPARE REGISTER for the next interrupt, i.e.:

        LDA     A,POB_TCSR1:    ;MUST BE READ SO THAT A SUBSEQUENT WRITE TO
        CLR     A               ; - OCR1 WILL CLEAR THE OUTPUT COMPARE FLAG
        CLR     B               ; - SEE ACCOMPANYING 6301 BOOK
        STD     POW_FRC:        ;RESET FREE RUNNING COUNTER TO ZERO
        LDD     KBW_TDEL:       ;GET VALUE FOR KEYBOARD INTERRUPT RATE
        STD     POW_OCR1:       ;SET OUTPUT COMPARE REGISTER

When the machine switches off, the value in TIMER CONTROL STATUS REGISTER 1 and the state of the INTERRUPT MASK are saved so that on a warm start they can be restored. Also, the keyboard buffer is flushed on a warm start using system service KB$FLSH.

POLLING

The routine to poll the keyboard is called through the ram vector BTA_POLL. The function of this routine is to scan the keyboard and return the key pressed in the A register and to set any flags required by the translating routine at ram vector BTA_TRAN. The following code can be used to poll the keyboard and will return the key pressed in the A register as a value between 0 and 36 (0 means no key):

KB_POLL:
        TST     SCA_COUNTERRESET        ;SET COUNTER TO ZERO
        AIM     #<$FF-KY_SHFT>,KBB_STAT:        ;NO SHIFT KEY YET
        LDA     B,POB_PORT5:            ;TEST FOR ON/CLEAR KEY
        BPL     NOTCLR                  ;BRANCH IF NOT PRESSED
        LDA     A,#36                   ;36 REPRESENTS THE ON/CLEAR KEY
        RTS

NOTCLR:
        CLR     A                       ;NO KEY YET
        STA     A,KBB_KNUM:             ;KEY NUMBER BECOMES 0
        PSH     A                       ;PSH 0
        BSR     LOOKKEY                 ;ANY KEY IS PRESSED? SETS Z IF NOT
        PUL     A                       ;DOES NOT AFFECT Z FLAG
        BEQ     ENDKYBD                 ;BRANCH IF NO KEY FOUND IN LOOKKEY
        LDA     B,#$40
        PSH     B
        DEC     B
        BRA     CLOCKB

NEXTCOL:
        PSH     B
CLOCKB:
        PSH     A
1$:     TST     SCA_COUNTERCLOCK        ;READ ADDRESS $340 TO INC COUNTER
        DEC     B                       ;DO IT B TIMES
        BNE     1$
        BSR     LOOKKEY                 ;SETS B TO ROW NUMBER,IF KEY FOUND
        PUL     A                       ;IN THIS COLUMN
        BEQ     NOPSH                   ;BRANCH IF NO KEY IN THIS COLUMN
        ADD     B,KBB_KNUM:             ;GOT A KEY BUT MUST CONTINUE,
        TBA                             ;TO CHECK IF SHIFT IS ALSO PRESSED
NOPSH:  LDA     B,KBB_KNUM:
        ADD     B,#5
        STA     B,KBB_KNUM:             ;POINT TO NEXT COLUMN
        PUL     B
        LSR     B                       ;SELECT NEXT COLUMN
        BNE     NEXTCOL
ENDKYBD:
        RTS                             

LOOKKEY:
                ;SETS B TO ROW NUMBER (1-5) OF KEY IN CURRENT COLUMN
                ;B WILL BE ZERO IF NO KEY IN THIS COLUMN (OR JUST SHIFT)
                ;SETS KY_SHFT FLAG IF SHIFT IS PRESSED
                ;SETS Z ON B
                ;PRESERVES X

        LDA     B,#5            ;CHECK 5 ROWS
        LDA     A,POB_PORT5:    ;READ PORT 5
        ASL     A               ;IGNORE BIT 7
NEXTROW:ASL     A               ;ROTATE KEY INTO CARRY,CLR IF KEY PRESSED
        BCC     GOTKEY          ;BRANCH IF KEY PRESS DETECTED
        DEC     B
        BNE     NEXTROW         ;CHECK NEXT ROW
        RTS                     ;RETURN WITH B=0 IF NO KEY FOUND

GOTKEY:                         ;B IS POSITION IN ROW
        TST     KBB_SHFK        ;TEST SHIFT ENABLE FLAG
        BNE     11$             ;EXIT IF SHIFT DISABLED, B IS NOT ZERO
        PSH     A
        LDA     A,KBB_KNUM:     ;GET CURRENT KEY NUMBER
        CMP     A,#25           ;IS IT 6TH COLUMN, CONTAINING SHIFT KEY?
        PUL     A
        BNE     11$             ;EXIT IF NOT, B IS NOT ZERO
        DEC     B               ;COMPARE B WITH ROW 1 (ROW WITH SHIFT KEY)
        BEQ     10$             ;BRANCH IF THIS IS THE SHIFT KEY (B IS 0)

        PSH     B               ;ELSE CHECK IF SHIFT ALSO PRESSED
1$:     ASL     A
        DEC     B
        BNE     1$
        PUL     B
        INC     B               ;RESTORE B TO REAL KEY PRESSED
        BCS     11$             ;BRANCH IF C SET (FROM ASL A)

10$:    OIM     #KY_SHFT,KBB_STAT:      ;SET SHIFT-PRESSED FLAG
        TST     B                       ;SET Z FLAG ON B
11$:    RTS

The auto-repeat of the keys is accomplished using KBB_PREV, KBB_DLAY, KBB_REPT and KBB_CNTR in the following manner:

        LDX     BTA_POLL                ;POLL KEYBOARD (SEE ABOVE)
        JSR     0,X                     ;RETURN KEY PRESS IN A
        CMP     A,KBB_PREV:             ;IS SAME KEY STILL PRESSED ?
        BEQ     1$                      ;BRANCH IF YES
        STA     A,KBB_PREV:             ;SAVE NEW KEY
        LDA     B,KBB_DLAY:             ;DELAY BEFORE AUTO-REPEATING BEGINS
        BRA     2$

1$:     LDA     B,KBB_REPT:             ;DELAY BETWEEN KEYS DURING REPEAT
        TST     KBB_CNTR                ;TEST DELAY COUNTER
        BEQ     2$                      ;BRANCH IF TIME TO RETURN KEY
        LDA     B,KBB_CNTR:
        DEC     B                       ;TO DECREMENT DELAY COUNTER
        CLR     A                       ;NO KEY UNTIL COUNTER IS ZERO

2$:     STA     B,KBB_CNTR:             ;SET DELAY COUNTER
        LDX     BTA_TRAN
        JSR     0,X                     ;TRANSLATE KEY IN A REGISTER

        TIM     #KY_CPNM,KBB_STAT:      ;IS IT CAP OR NUM.
        BNE     DOCLICK                 ;IF SO, JUST EMIT KEY CLICK
        
        TST     A                       ;TEST IF KEY PRESSED
        BEQ     END                     ;END OF INTERRUPT IF NOT

                ;INSERT KEY INTO BUFFER - SEE BELOW
DOCLICK:
                ;EMIT KEY CLICK         - SEE BELOW

END:    RTI                             ;RETURN FROM INTERRUPT

TRANSLATING

The ram vector BTA_TRAN points to the routine used to translate the key returned from BTA_POLL. The function of this routine is to translate the key number passed in the A register into an ')"; onMouseout="hideddrivetip()"> ASCII character and return it in the A register.

The following code can be used to translate a key number (0 to 36) into the ')"; onMouseout="hideddrivetip()"> ASCII key it represents on the standard Organiser. It uses KBB_STAT to decide whether 'shifted' characters, or lower-case characters are returned and also to refresh the cursor in the correct state (block or line). If the key pressed is SHIFT CAP or SHIFT NUM, no key is returned (i.e. the A register is 0) but the KY_CPNM flag is set.

KB_TRAN:
        LDA     B,KBB_STAT:             ;GET KEYBOARD STATUS FLAGS
        AND     B,#<$FF-KY_CPNM>        ;CLEAR CAP/NUM FLAG
        BPL     3$                      ;BRANCH IF NOT SHIFT,NOT CAP OR NUM
        CMP     A,KBB_CAPK              ;IS IT THE CAP KEY ?
        BNE     1$                      ;BRANCH IF NOT
        EOR     B,#KY_CAPS              ;TOGGLE CAPS FLAG
        BRA     2$
1$:     CMP     A,KBB_NUMK              ;IS IT THE NUM KEY ?
        BNE     3$                      ;BRANCH IF NOT
        EOR     B,#KY_NUMB              ;TOGGLE NUM FLAG
2$:     ORA     B,#KY_CPNM              ;SET CAP/NUM FLAG
        CLR     A                       ;RETURN NO KEY

3$:     PSH     A                       ;SAVE KEY PRESSED
        OS      KB$STAT                 ;SET KEYBOARD STATE
                                        ;PRESERVES B
        TBA                             ;SET A TO KBB_STAT
        PUL     B                       ;B IS KEY PRESSED
        TST     B
        BEQ     9$                      ;BRANCH IF NO KEY PRESSED
        PSH     B
        LDX     BTA_TABL                ;ADDRESS OF KEYBOARD LOOKUP TABLE
        DEX                             ;SO KEY 1 IS FIRST KEY IN TABLE
        LDA     B,KBB_STAT:             ;GET KEYBOARD STATUS
        ASL     B                       ;EXCLUSIVE OR KY_SHFT WITH KY_NUMB
        BVC     4$                      ;BRANCH IF NOT 'SHIFT' MODE
        LDA     B, 36                   
        ABX                             ;SKIP TO 'SHIFTED' LOOKUP TABLE
4$:     PUL     B
        ABX                             ;INDEX INTO TABLE
        LDA     B,0,X                   ;GET 
					')";
					onMouseout="hideddrivetip()">
					ASCII KEY FROM TABLE
        LSR     A                       ;TEST CAPS FLAG
        BCS     5$                      ;IF SET LEAVE AS LOWER CASE
        CMP     B,#^a/a/
        BCS     5$
        CMP     B,#^a/z/
        BHI     5$
        ADD     B,#^a/A/-^a/a/          ;CONVERT B TO UPPER CASE
5$:
        CMP     B,#K_DEL                ;IS IT DELETE KEY
        BNE     9$                      ;BRANCH IF NOT
        ASL     A                       ;TEST SHFT FLAG
        BPL     9$                      ;BRANCH IF NOT SHIFT
        DEC     B                       ;SHIFT DEL IS ALWAYS DEL RIGHT
9$:     TBA                             ;RETURN TRANSLATED KEY IN A
        RTS

The keyboard table pointer KBA_TABL points to the following table by default:

KBT_TABL:
        
					')";
					onMouseout="hideddrivetip()">
					ASCII  /zvpjd/
        .BYTE   K_EXE
        
					')";
					onMouseout="hideddrivetip()">
					ASCII  /xrlf/
        
					')";
					onMouseout="hideddrivetip()">
					ASCII  / wqke/
        
					')";
					onMouseout="hideddrivetip()">
					ASCII  /yuoic/
        .BYTE   K_DEL
        
					')";
					onMouseout="hideddrivetip()">
					ASCII  /tnhb/
        
					')";
					onMouseout="hideddrivetip()">
					ASCII  /?smga/         ;(?=SHIFT - NOT RETURNED EVER !)
        .BYTE   K_MODE
        .BYTE   K_UP,K_DOWN
        .BYTE   K_LEFT,K_RGHT
        .BYTE   K_AC            ;ON/CLEAR KEY
                
        ;SHIFTED CHARACTERS
        ;==================

        
					')";
					onMouseout="hideddrivetip()">
					ASCII  /.258)/
        .BYTE   K_EXE
                   $
        
					')";
					onMouseout="hideddrivetip()">
					ASCII  $+-$
        
					')";
					onMouseout="hideddrivetip()">
					ASCII  $/$
        
					')";
					onMouseout="hideddrivetip()">
					ASCII  / 369%/ 
        
					')";
					onMouseout="hideddrivetip()">
					ASCII  /0147(/
        .BYTE   K_DEL
        
					')";
					onMouseout="hideddrivetip()">
					ASCII  /:$">/
        
					')";
					onMouseout="hideddrivetip()">
					ASCII  /?;,=

BUFFERING

When a key press is detected, the ')"; onMouseout="hideddrivetip()"> ASCII character for that key is placed in a 16 byte wrap-around buffer, KBT_BUFF. An offset into the buffer KBB_BACK and a count of the number of characters in the buffer KBB_NKYS are used to implement the buffering. If the buffer is already full, the character is not stored, but a 'buffer-full' beep is emitted lasting 10ms. The following code can be used to buffer the character in the A register:

        LDA     B,KBB_NKYS:             ;GET NUMBER OF KEYS IN BUFFER
        LDX     #10                     ;10MS FOR 'BUFFER-FULL' BEEP
        CMP     B,#16                   ;IS BUFFER FULL ?
        BEQ     DOBUZ                   ;BRANCH IF YES
        LDX     #KBT_BUFF               ;ADDRESS OF KEYBOARD BUFFER
        ADD     B,KBB_BACK:             ;OFFSET INTO BUFFER
        AND     B,#$0F                  ;WRAP AROUND
        ABX                             
        STA     A,0,X                   ;STORE 
					')";
					onMouseout="hideddrivetip()">
					ASCII KEY IN BUFFER
        INC     KBB_NKYS                ;INCREMENT NUMBER OF KEYS
DOCLICK:
        LDA     B,KBB_CLIK              ;LENGTH OF KEY CLICK    
        BEQ     END                     ;NO CLICK IF ZERO
        DEC     B                       ;LENGTH 1 WILL GIVE < 1MS CLICK
        CLR     A
        XGDX                            ;DURATION IN X
DOBUZ:  LDD     TMW_TCNT                ;SIZE OF SWITCH OFF TIME OUT
        STD     TMW_TOUT:               ;RESET TIMEOUT COUNT IF KEY PRESSED
        LDD     #56                     ;A GOOD NOTE
        OS      BZ$TONE                 ;BEEP FOR X MS
END:

The keyboard buffer can be cleared out using system service KB$FLSH and there is a facility for inserting a key into a 1 byte buffer KBB_WAIT by using system service KB$UGET. KBB_WAIT is always tested for a key before looking in the main keyboard buffer KBT_BUFF.

Keys can also be inserted into KBT_BUFF, but keyboard interrupts must be prevented while this is done. The following code will insert the key contained in the A register:

PUTA:
        SEI                             ;STOP INTERRUPTS OCCURRING IN HERE
        LDA     B,KBB_NKYS:             ;GET NUMBER OF KEYS IN BUFFER
        CMP     B,#16                   ;IS BUFFER FULL ?
        BEQ     NOPUT                   ;BRANCH IF YES
        LDX     #KBT_BUFF               ;ADDRESS OF KEYBOARD BUFFER
        ADD     B,KBB_BACK:             ;OFFSET INTO BUFFER
        AND     B,#$0F                  ;WRAP AROUND
        ABX                             
        STA     A,0,X                   ;STORE 
					')";
					onMouseout="hideddrivetip()">
					ASCII KEY IN BUFFER
        INC     KBB_NKYS                ;INCREMENT NUMBER OF KEYS
NOPUT:  CLI                             ;RESTORE INTERRUPTS

KEY CLICK

The key click is produced in the keyboard interrupt by using the routine for system service BZ$TONE, using the following code:

 KEYCLICK: 
        LDA     B,KBB_CLIK              ;LENGTH OF KEY CLICK 
        BEQ     END                     ;NO CLICK IF ZERO 
        DEC     B                       ;LENGTH 1 WILL GIVE < 1MS CLICK 
        CLR     A 
        XGDX                            ;DURATION IN X 
        LDD     #56                     ;A GOOD NOTE 
        JSR     BZ_TONE                 ;BEEP FOR X MS 
 END:

Note: BZ_TONE is JSR'ed to and not called as an operating system service. Hence, intercepting operating system calls to BZ$TONE (by re-vectoring SWI) will not affect the key click. JSR'ing or JMP'ing to operating system services can only be done within the operating system and not by external routines.

The length of the key click, KBB_CLIK, can be changed. Zero will disable the click and any value up to $FF will specify the length of the click in milliseconds.

Warning: BZ_TONE toggles the alarm line to produce the click (using addresses SCA_ALARMHIGH and SCA_ALARMLOW) but if SOE_B is high, it is possible that 21v will appear on any devices present, see low level pack access. For example if an ')"; onMouseout="hideddrivetip()"> EPROM device is selected, a byte may be blown on the pack. Since the click occurs on interrupt, care must be taken to ensure interrupts are disabled whenever SOE_B is set high.

ALARM CHECKING

Alarms and diary alarms are checked for only when the flag AMB_DOIT is set (e.g. by the NMI on a minute boundary):

        TST     AMB_DOIT                ;TEST DOIT FLAG 
        BEQ     1$                      ;BRANCH IF CLEAR
        ;ALARM CHECKING CODE GOES HERE
        CLR     AMB_DOIT                ;CLEAR DOIT FLAG 
1$:

The method of alarm checking is described in chapter 19. AMB_DOIT is cleared when alarm checking is finished to prevent it being done on the next interrupt.