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WAI: Difference between revisions
From SnesLab
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'''WAI''' (WAit for Interrupt) is a 65x instruction that waits until an interrupt is received. The processor consumes less power while waiting. The [[RDY pin]] is pulled low during the third cycle and the bus is freed up. The interrupt is serviced immediately. | '''WAI''' (WAit for Interrupt) is a 65x instruction that waits until an interrupt is received. The processor consumes less power while waiting. The [[RDY pin]] is pulled low during the third cycle and the bus is freed up. The interrupt is serviced immediately. | ||
If the [[interrupt disable flag]] is set, then the way servicing occurs is for control to simply fall through to the instruction following WAI. | If the [[interrupt disable flag]] is set, then the way servicing occurs is for control to simply fall through to the instruction following WAI. This is a very efficient way to service because the ISR is inlined and there is no need to [[RTI]]. | ||
If that flag is clear, control instead jumps through a vector into the [[interrupt handler]]. | If that flag is clear, control instead jumps through a vector into the [[interrupt handler]]. Use RTI at the end of this ISR to return control to the instruction following WAI. | ||
No flags are affected. | No flags are affected. | ||
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=== See Also === | === See Also === | ||
* [[IRQ]] | * [[IRQ]] | ||
* [[STP]] | * [[STP]] | ||
* [[NOP]] | * [[NOP]] | ||
Revision as of 15:23, 22 July 2024
| Basic Info | |||||||
|---|---|---|---|---|---|---|---|
| Addressing Mode | Opcode | Length | Speed | ||||
| Implied (type 3) | CB | 1 byte | 3 cycles | ||||
| Flags Affected | ||||||||
|---|---|---|---|---|---|---|---|---|
| N | V | M | X | D | I | Z | C | |
| . | . | . | . | . | . | . | . | |
WAI (WAit for Interrupt) is a 65x instruction that waits until an interrupt is received. The processor consumes less power while waiting. The RDY pin is pulled low during the third cycle and the bus is freed up. The interrupt is serviced immediately.
If the interrupt disable flag is set, then the way servicing occurs is for control to simply fall through to the instruction following WAI. This is a very efficient way to service because the ISR is inlined and there is no need to RTI.
If that flag is clear, control instead jumps through a vector into the interrupt handler. Use RTI at the end of this ISR to return control to the instruction following WAI.
No flags are affected.
Syntax
WAI
See Also
External Links
- Eyes & Lichty page 522, on WAI: https://archive.org/details/0893037893ProgrammingThe65816/page/522
- Labiak page 203 on WAI: https://archive.org/details/Programming_the_65816/page/n213
- snes9x implementation of WAI: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L3313
- undisbeliever on WAI: https://undisbeliever.net/snesdev/65816-opcodes.html#wai-wait-for-interrupt
- Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.9