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:
GN_Aab is called to allocate space for an alarm block. An extended pointer to the allocated memory is returned in BHL.
The parameters of the alarm should be copied into the allocated alarm block, following the format shown above.
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.
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:
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 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
oz OS_MBP ; bind in bank containing block
and @00111111 ; mask out old segment
or MM_S1 ; force into segment 1
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
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)