Atari 400 Technical Reference Manual page 96

the SETVBV tE45FJ routine is provided to perform the desired
update in safe manner. The calling
se~uence
is shown below:
A
=
update item indicator
1 - 5 for timers 1 - 5.
6 for immediate VBLANK vector VVBLKI.
7 for deferred VBLANK vector VVBLKD.
X
=
MSS of value to store.
Y = LSB of value to store.
JSR SETVSV
The A,X and Y registers can be altered.
The display list interrupt will always be disabled on
return, even if enabled upon entry.
It is possible to fully process a vertical-blank interrupt
during a call to this routine.
When working with the System Timers, the vectors for timers 1 and
2 and the flags for timers 3,4 and 5 should be set while the
associated timer is
e~ual
to zero, then the timer should be set
to its (nonzero) value.
Stack Content at Interrupt Vector Points
The following table shows the stack content at everyone of the
RAM interrupt vector points:
OPERATING SYSTEM C016555 -- Section 6
111