Atari 400 Technical Reference Manual page 93

Return from the interrupt after restoring the 6502 A
register from the stack.
o If the interrupt is due to the serial I/O bus proceed line;
then clear the interruptI and Jump through global RAM vector
VPRCED (0202].
o If the interrupt is due to the serial 110 bus interrupt
line, then clear the interrupt and Jump through global RAM
vector VINTER (0204].
o If the interrupt is due to a 6502 BRK instruction, then Jump
through global RAM vector VBREAK (0206].
o If none of the above, restore the 6502 A register and return
from the interrupt <RTI).
INTERRUPT INITIALIZATION
The interrupt subsystem completely reinitializes itself whenever
the system is powered up or the (SYSTEM. RESET] key is pressed.
The OS clears the hardware registers, and sets the interrupt
global RAM vectors to the following configurations:
Vector Type Function
VDSLST (0200]
NMI
RTI
--
ignore interrupt.
VVBLKI [0222] u
System stage 1 VBLANK.
CDTMAl (0226]
II
SID timeout timer.
CDTMA2 (0228] u No system function.
VV13LKD [0224]
II
System return from interrupt.
VIMIRG [0216]
IRG
System IRG processor.
VSEROR [020C]
II
SID.
VSERIN (020A]
u
SID.
VSEROC [020E]
II
SID.
VTIMRl [0210]
II
PLA/RTI ignore interrupt.
VTIMR2 [0212]
II
PLA,RTI
ignore interrupt.
VTIMR4 [0214] u
***
doesn't matter
***
VKEYBD [0208]
II
System keyboard
interrupt handler.
VPRCED [0202]
If
PLA/RTI ignore i nterrup t.
VINTER [0204]
II
PLA,RTI ignore interrupt.
VBREAK [0206]
BRK
PLA/RTI
ignore interrupt.
Figure 6-2 Interrupt RAM Vector Initialization
OPERATING SYSTEM C016555 -- Section 6
108