Atari ST series Technical Reference Manual page 65

XBIOS Device and System Functions
where timernum is number from 0-3 that represents an MFP
timer (0 = A, 1 = B, 2 = C, 3 = D). Control represents the
value to place in the timers control register. This is an 8-bit
register that controls the timer mode. For Timers A and B,
possible values for this register include:
Control
Value
0 Timer off
1 Delay mode, clock divided by 4 .
2 Delay mode, clock divided by 10
3 Delay mode, clock divided by 16
4 Delay mode, clock divided by 50
5 Delay mode, clock divided by 64
6 Delay mode, clock divided by 100
7 Delay mode, clock divided by 200
8 Event Count Mode
9 Pulse Length mode, clock divided by 4
10 Pulse Length mode, clock divided by 10
11 Pulse Length mode, clock divided by 16
12 Pulse Length mode, clock divided by 50
13 Pulse Length mode, clock divided by 64
14 Pulse Length mode, clock divided by 100
15 Pulse Length mode, clock divided by 200
Data represents the value stored in the timer data register. Fi­
nally, vector is the address for the interrupt handler routine
associated with this timer. To install your own timer inter­
rupt routine for Timer A, set this value to the address of
your interrupt code.
Miscellaneous System Routines
The remaining XBIOS functions perform a number of miscel­
laneous functions. The first of these is used to execute a sub­
routine in the 68000's supervisor mode. Normally, ST pro­
grams operate in what's known as user mode. Some
privileged operations, however, can only be performed while
in supervisor mode. For example, the address space from
0-2048 ($0-$800), and the I/O space from 16,744,448
($FF80000) up is protected. If you attempt to access these
memory areas while in user mode, you'll cause a bus error.
Your program will display two bombs and it will crash.
Therefore, if you wish to access any of the system variables
in low memory, or any of the I/O registers, do so while in
Timer Mode