There are four types of interrupts which can occur with the 6502
1. chip reset
2. IRQ, interrupt request (maskable)
3. MNI (non-maskable interrupt)
4. software interrupt (BRK instruction)
On the 400/800 the chip reset occurs only upon power-up and causes the
computer to do a cold start. On later models, pressing [SYSTEM RESET]
will cause a chip reset but the computer then does a warm start. On
the 400/800, the [SYSTEM RESET] key generates a NMI interrupt.
This is a synopsis of the cold start routine.
The warm start flag [$0008] is set to 0 (false)
If a cartridge slot contains a diagnostic cartridge, control is handed
to the cartridge.
The end of RAM is determined by trying to complement the first byte of
each 4K block of memory.
Hardware registers at $D000 - $D4FF (except $D100 - $D1FF) are
RAM is cleared from $0008 to the top of ram.
The user program jump vector, DOSVEC [$000A] is set to point to the
black board mode (Atari logo display mode in XL/XE models).
The screen margins are set to 2 and 39
Interrupt vectors are initialized.
Bottom of free RAM pointer, MEMLO [$02E7], is set to point to $0700.
Resident CIO handlers are initialized.
If the [START] key is pressed the cassette boot request flag, CKEY
[$004A], is set.
The CIO device table is initialized.
If a cartridge is present it is initialized.
Channel 0 is opened to the screen editor. The top-of-free-RAM
pointer, MEMTOP [$02E5], is set to point below the screen region. The
computer then waits for the screen to be established before
If the cassette boot flag is set the cassette is booted.
If there is no cartridge present or a cartridge doesn't prevent it,
the disk is booted.
The cold start flag is reset.
If there is a cartridge present, the computer jumps to the cartridge's
If there is no cartridge present the computer jumps through the vector
DOSVEC [$000A (10)]. DOSVEC will point to either a booted program,
the memo pad routine (400/800) or the logo display routine (XL/XE).
The warm start flag is set to $7F (true).
cold start steps 2 - 4 are executed.
RAM is cleared from $0010 - $007F and $0200 - $03FF.
Cold start steps 7 - 14 are executed.
If cassette booted software is present the computer JSRs through
If disk booted software is present the computer JSRs through DOSINI
The difference between cold start and warm start is the condition of
the warm start flag, WARMST, [$0008]. If this flag is 0 a complete
cold start is executed. If the flag is anything other than 0 then
only the warm start part of the warm start/cold start code is
NON-MASKABLE INTERRUPTS (NMI)
NMI interrupts are generated by the following conditions:
1. Display list interrupt, generated by the ANTIC chip.
2. TV vertical blank interrupt, generated by the ANTIC
3. [SYSTEM RESET] key (400/800).
When an NMI interrupt occurs, the hardware register NMIST [$D40F] is
examined to determine what type of interrupt occurred. The computer
is then directed through the proper ram vector to service the
DISPLAY LIST INTERRUPTS (DLIs)
The computer makes no use of DLIs. The ram vector points to an RTI
VERTICAL BLANK INTERRUPTS (VBIs)
There are two stages to the VBI service routine. The second stage is
only done if a critical function was not interrupted.
Stage 1 (VBI)
The real time clock, RTCLOK [$0012 - $0014], is incremented.
The attract mode variables are processed.
System timer 1 is decremented. If it goes to zero the computer JSRs
through system time-out vector 1.
Stage 2 (VBI)
The hardware registers are loaded with the data in their shadow
System timer 2 is decremented. If it goes to zero the computer JSRs
through the system time-out vector 2.
System timers 3, 4, and 5 are decremented. If a timer goes to zero
the computer sets system timer flags 3, 4, and/or 5.
If auto-repeat is active, the auto-repeat process is done.
The keyboard debounce timer is decremented if not 0.
Information at the controller port registers is read, processed and
placed in the proper shadow registers.
[SYSTEM RESET] INTERRUPT
If a [SYSTEM RESET] interrupt is generated on the 400/800 the computer
jumps to the warm start routine.
INTERRUPT REQUESTS (maskable interrupts (IRQs))
When an IRQ interrupt occurs the hardware register IRQST [$D20E], the
PIA status registers, PACTL [$D302] and PBCTL [$D303] are examined to
determine what caused the interrupt.
For each interrupt, the 6502 accumulator is pushed to the stack. The
computer is then directed to the proper ram vector to service the
SOFTWARE INTERRUPT (BRK instruction)
The operating system doesn't use software interrupts. The software
interrupt vector points to a PLA followed by an RTI.
Label address type function
VDSLST $0200 NMI DLI Points to an RTI
VVBLKI $0222 NMI stage 1 VBI
VVBLKD $0224 NMI return-from-interrupt routine
CDTMA1 $0226 NMI time-out 1 (used by SIO)
CDTMA2 $0228 NMI time-out 2 (not used by OS)
VPRCED $0202 IRQ not used (points to PLA,RTI)
VINTER $0204 IRQ not used (PLA,RTI)
VKEYBD $0208 IRQ keyboard interrupt
VSERIN $020A IRQ used by Serial I/O routine
VSEROR $020C IRQ used by SIO
VSEROC $020E IRQ used by SIO
VTIMR1 $0210 IRQ not used by OS (PLA,RTI)
VTIMR2 $0212 IRQ not used by OS (PLA,RTI)
VTIMR4 $0214 IRQ ?
VIMIRQ $0216 IRQ main IRQ code
VBREAK $0206 BRK unused by OS (PLA,RTI)
The following timers are updated during vertical blank (VBI) as noted
above. If a timer is decremented to 0 the computer jumps through it's
associated vector or sets it's associated flag.
Label address flag/vector
RTCLOK $0012 3 byte clock ($0012 = MSB)
CDTMV1 $0218 CDTMA1 $0226 vector (SIO time-out)
CDTMV2 $021A CDTMA2 $0228 vector
CDTMV3 $021C CDTMF3 $022A flag
CDTMV4 $021E CDTMF4 $022C flag
CDTMV5 $0220 CDTMF5 $022E flag
HARDWARE INTERRUPT CONTROL
There are two registers on the antic chip which control interrupts.
These registers can be used to disable interrupts if necessary. There
are also two associated interrupt status registers.
The IRQ enable and status registers use the same address. The result
is that reading the register does not reveal the enabled interrupts
but the interrupts pending. IRQ interrupt enable data should usually
be written to the OS shadow first. Reading the OS shadow tells which
interrupts are enabled.
Non maskable interrupt enable
7 6 5 4 3 2 1 0
| | | not used |
bit 7 1 = DLI enabled
6 1 = VBI enabled
Non maskable interrupt status
7 6 5 4 3 2 1 0
| | | | not used|
bit 7 1 = DLI pending
6 1 = VBI pending
5 1 = [SYSTEM RESET] key pending
Interrupt request enable
7 6 5 4 3 2 1 0
| | | | | | | | |
bit 7 1 = [BREAK] key interrupt enable
6 1 = keyboard interrupt enable
5 1 = serial input interrupt enable
4 1 = serial output interrupt enable
3 1 = serial output-finished interrupt enable
2 1 = timer 4 interrupt enable
1 1 = timer 2 interrupt enable
0 1 = timer 1 interrupt enable
IRQEN has a shadow register, POKMSK [$0010 (A)].
Interrupt request status
7 6 5 4 3 2 1 0
| | | | | | | | |
bit 7 1 = [BREAK] key interrupt pending
6 1 = keyboard interrupt pending
5 1 = serial input interrupt pending
4 1 = serial output interrupt pending
3 1 = serial output-finished interrupt pending
2 1 = timer 4 interrupt pending
1 1 = timer 2 interrupt pending
0 1 = timer 1 interrupt pending
WAIT FOR HORIZONTAL SYNC
Writing any number to WSYNC [$D40A (54282)] will cause the computer to
stop and wait for the next TV horizontal sync.
It is wise to use DLIs one TV line before needed then writing to
WSYNC. This will keep other interrupts from causing DLIs to be
serviced late. This can cause a DLI to change something in the middle
of a scan line.
Useful database variables and OS equates
POKMSK $0010 (16): IRQEN shadow
IRQEN $D20E (53774): enables IRQs when written to
IRQST $D20E (53774); gives IRQs waiting when read
PACTL $D302 (54018): bit 7 (read) peripheral A interrupt status
bit 0 (write) peripheral A interrupt enable
PBCTL $D303 (54019): bit 7 (read) peripheral B interrupt status
bit 0 (write) peripheral B interrupt enable
WSYNC $D40A (54282): wait for horizontal sync
NMIEN $D40E (54286): NMI enable
NMIST $D40F (54287): NMI status