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REP: Difference between revisions
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'''REP''' (Reset Status Bits) is a [[65c816]] instruction that clears bits in the [[status register]] that correspond to set bits in the operand. | '''REP''' (Reset Status Bits) is a [[65c816]] instruction that clears bits in the [[status register]] that correspond to set bits in the operand. | ||
REP is the only way to clear the m and x flags directly (but [[PLP]] and [[RTI]] may clear them too.)<sup>[1]</sup> | REP is the only way to clear the m and x flags directly without disturbing the [[stack]] (but [[PLP]] and [[RTI]] may clear them too.)<sup>[1]</sup> But, REP can't clear those two flags in [[emulation mode]] because they are being forced to be set. | ||
==== Syntax ==== | ==== Syntax ==== | ||
<pre> | <pre> | ||
REP #nvmxdizc | REP #%nvmxdizc | ||
</pre> | </pre> | ||
If you only want to clear one flag, [[CLC]], [[CLV]], [[CLD]], and [[CLI]] are smaller/faster than REP. Early 65c816 chips had a timing problem with REP and [[SEP]] which required a [[NOP]] to be inserted after them, or to stretch the clock.<sup>[5]</sup> | |||
=== See Also === | === See Also === | ||
* [[TRB]] | * [[TRB]] | ||
* [[Derailment]] | * [[Derailment]] | ||
=== References === | === References === | ||
# [[Eyes & Lichty]] | # [[Eyes & Lichty]], [https://archive.org/details/0893037893ProgrammingThe65816/page/489 page 489] on REP | ||
# [[Labiak]] | # [[Labiak]], [https://archive.org/details/Programming_the_65816/page/n183 page 173] on REP | ||
# snes9x implementation of REP: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L3145 | # snes9x implementation of REP: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L3145 | ||
# undisbeliever on REP: https://undisbeliever.net/snesdev/65816-opcodes.html#rep-reset-status-bits | # undisbeliever on REP: https://undisbeliever.net/snesdev/65816-opcodes.html#rep-reset-status-bits | ||
# http://forum.6502.org/viewtopic.php?f=4&t=5196 | |||
[[Category:ASM]] | [[Category:ASM]] | ||
Latest revision as of 20:35, 17 August 2024
| Basic Info | |||||||
|---|---|---|---|---|---|---|---|
| Addressing Mode | Opcode | Length | Speed | ||||
| Immediate | C2 | 2 bytes | 3 cycles | ||||
| Flags Affected | ||||||||
|---|---|---|---|---|---|---|---|---|
| N | V | M | X | D | I | Z | C | |
| emulation mode | N | V | . | . | D | I | Z | C |
| native mode | N | V | M | X | D | I | Z | C |
REP (Reset Status Bits) is a 65c816 instruction that clears bits in the status register that correspond to set bits in the operand.
REP is the only way to clear the m and x flags directly without disturbing the stack (but PLP and RTI may clear them too.)[1] But, REP can't clear those two flags in emulation mode because they are being forced to be set.
Syntax
REP #%nvmxdizc
If you only want to clear one flag, CLC, CLV, CLD, and CLI are smaller/faster than REP. Early 65c816 chips had a timing problem with REP and SEP which required a NOP to be inserted after them, or to stretch the clock.[5]
See Also
References
- Eyes & Lichty, page 489 on REP
- Labiak, page 173 on REP
- snes9x implementation of REP: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L3145
- undisbeliever on REP: https://undisbeliever.net/snesdev/65816-opcodes.html#rep-reset-status-bits
- http://forum.6502.org/viewtopic.php?f=4&t=5196