Z88 Developers' Notes
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16. Handling Alarms

Alarms in the Z88 are organized as a linked list of 'alarm blocks' with the root at some fixed location. Each alarm block contains all the relevant data for the particular alarm; time, date, number of repeats; in fact much the same as the options given in the alarm display in the main alarm popdown. Although there are many system calls which deal with alarms the programmer need only know about four:

GN_Aab      allocate alarm block
GN_Lab      link alarm block into alarm chain
GN_Uab      unlink alarm block from chain
GN_Fab      free alarm block

The following stages are involved in using alarms:
1. Allocation           GN_Aab is called to allocate space for an alarm block.
                        An extended pointer to the allocated memory is returned
                        in BHL.

2. Setting              The parameters of the alarm should be copied into the
                        alloctated alarm block, following the format shown

3. Linking              Once the block has been set up it can be linked into
                        the alarm chain, using GN_Lab. This done, the
                        programmer can forget about the alarm. After expiry the
                        alarm will be removed from the chain and the memory
                        associated with it will be freed.

4. Removal              To remove an alarm that has been set, but has not
                        expired, the alarm block is first unlinked from the
                        chain using GN_Uab and the memory associated with the
                        block is then released with GN_Fab.

The format of the alarm block is as follows:
3 bytes     link to next block, set by system
3 bytes     time of alarm in internal format
3 bytes     date of alarm in internal format
24 bytes    command line to execute or simply a comment. This should be null-
            terminated. Note that the repeat time starts 33 bytes into the
            block, not after the terminator for the command line.
3 bytes     repeat time in days, added to value below
3 bytes     repeat time in centisecond ticks, added to value above
2 bytes     number of times to repeat
1 byte      repeat time display units:

            1           seconds
            2           minutes
            4           hours
            8           days
            16          weeks
            32          months
            64          years
            128         never repeat

1 byte      alarm status, some combination of:

            1           beep on expiry
            2           execute command line on expiry
            4           alarm has expired, set by system
            8           alarm is pending, set by system
The 'repeat time display units' byte controls what is displayed in the alarm popdown window. It is important to choose a unit appropriate to the repeat time. If you use units of hours, with a repeat time of 2 minutes, then the repeat time will be displayed as 0 hours.

NOTE: The repeat time must be at least ten seconds otherwise it may be difficult to enter the alarm popdown to clear the alarm.

The alarms set up are all lost when a system soft reset occurs, and unfortunately there is no way of saving and loading the currently set alarms. In addition, alarms are suppressed when the machine is in the alarm popdown. This is true even if the machine is in coma. This feature is included to allow a simple way to disable alarms.


The following example sets up an alarm for the entirely arbitrary date and time of "21/05/3934 08:46:20". The alarm has a repeat time of 25 seconds and will occur three time (ie. number of repeats is two). Finally the type of alarm is ALARM and the bleeping is enabled:

include "#alarm.def"             ; alarm definition calls
include "#memory.def"            ; memory management definition calls

.set_alarm  call_oz(GN_Aab)         ; allocate alarm block
            ret  c                  ; exit if error (no room)

            push bc                 ; preserve BHL which holds
            push hl                 ; the extended pointer to the block
            ld   c, MS_S1           ; segment 1 specifier
            call_oz(OS_MBP)         ; bind in bank containing block
            ld   a,h
            and  @00111111          ; mask out old segment
            or   MM_S1              ; force into segment 1

            ld   d,a
            ld   e,l                ; alarm block address in DE
            ld   hl, alm_block      ; address of data for alarm block
            ld   bc, sizeof_block
            ldir                    ; copy data into alarm block

            pop  hl
            pop  bc
            call_oz(GN_Lab)         ; link block into alarm chain
; alarm block definition to be copied into allocated block
DEFC sizeof_ablock = end_alm_block - alm_block
.alm_block  defb 0, 0, 0            ; link to next block (set up by system)
            defm "000"              ; time in internal format
            defm "000"              ; date in internal format
            defm "Message space 24 bytes." & 0
            defb 0, 0, 0            ; repeat time in days
            defb 0, 10, 0           ; repeat time in centiseconds
            defw 2                  ; times to repeat
            defb 1                  ; repeat time display unit (seconds)
            defb 1                  ; alarm status (bleep on expiry)

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