Section 6 describes system actions for the various interrupt causing events, defines the many RAM vectors and provides recommended procedures for dealing with interrupts.
The 6502 microcomputer processes three general interrupt types:
chip-reset, nonmaskable interrupts (NMI) and maskable interrupts (IRQ). The IRQ interrupt type can be enabled and disabled using the 6502 CLI and SEI instructions. The NMI type cannot be disabled at the processor level; but the NMI interrupts other than [SYSTEM.RESET] key can be disabled at the ANTIC chip.
The system events that can cause interrupts are listed below:
Figure 6-1 List of System-Interrupt Events
chip-reset – power-up
NMI –
Display list interrupt (unused by OS)
vertical-blank (50/60 Hz)
[SYSTEM.RESET] keyIRQ –
Serial bus output ready
Serial bus output complete
Serial bus input ready
Serial bus proceed line (unused by system)
Serial bus interrupt line (unused by system)
POKEY timers 1, 2 and 4
Keyboard key
[BREAK] key
6502 BRK instruction (unused by OS)
Whenever a key is read from the keyboard, the Keyboard Handler sets WSYNC [D40A] repeatedly while generating the audible click an the console speaker. A problem occurs when interrupts are generated during the wait-for-sync period; the processing of such interrupts will be delayed by one horizontal scan line. This condition cannot be prevented. You can work around the condition by examining the line count VCOUNT [D40B] and delaying interrupt processing by one line when no HSYNC delay has occurred.
The following pages contain process flowcharts showing the main events that occur in the NMI and IRQ interrupt processes.
