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XCE: Difference between revisions
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{| class="wikitable" style="float:right;clear:right;width:30%" | {| class="wikitable" style="float:right;clear:right;width:30%" | ||
!colspan=" | !colspan="10"|Flags Affected | ||
|+ | |+ | ||
|mode switching into | |||
|[[Negative Flag|N]] | |[[Negative Flag|N]] | ||
|[[Overflow Flag|V]] | |[[Overflow Flag|V]] | ||
| Line 26: | Line 27: | ||
|[[Emulation Mode Flag|E]] | |[[Emulation Mode Flag|E]] | ||
|+ | |+ | ||
|emulation | |||
|. | |. | ||
|. | |. | ||
| | | | ||
|B | |B | ||
|. | |. | ||
|. | |. | ||
|. | |. | ||
|E | |E | ||
| | |1 | ||
|+ | |||
|native | |||
|. | |||
|. | |||
|1 | |||
|1 | |||
|. | |||
|. | |||
|. | |||
|E | |||
|0 | |||
|} | |} | ||
'''XCE''' is a [[65c816]] instruction | '''XCE''' is a [[65c816]] instruction | ||
that exchanges the [[carry]] and [[Emulation Mode Flag|emulation]] bits. It typically appears soon after the [[Reset Vector]] following a [[CLC]], to switch the [[S-CPU]] into [[65c816 native mode]]. The carry flag was chosen because: | that exchanges the values of the [[carry]] and [[Emulation Mode Flag|emulation]] bits. It typically appears soon after the [[Reset Vector]] following a [[CLC]], to switch the [[S-CPU]] into [[65c816 native mode]]. The carry flag was chosen because: | ||
* it is easy to set and clear | * it is easy to set and clear (with [[SEC]] and [[CLC]]) | ||
* it is used less frequently than the negative and zero | * it is used less frequently than the [[Negative Flag|negative]] and [[zero flag]]s | ||
* it can be tested with [[BCC]] and [[BCS]] | * it can be tested with [[BCC]] and [[BCS]] | ||
| Line 49: | Line 62: | ||
</pre> | </pre> | ||
The low bytes of both index registers (X & Y) are unaffected by a mode change. The high byte B of the [[accumulator]] is also unaffected. | The low bytes of both index registers (X & Y) are unaffected by a mode change. The high byte B of the [[accumulator]] is also unaffected.<sup>[1]</sup> | ||
==== Examples ==== | ==== Examples ==== | ||
| Line 74: | Line 87: | ||
=== External Links === | === External Links === | ||
# [[Eyes & Lichty]] | # [[Eyes & Lichty]], [https://archive.org/details/0893037893ProgrammingThe65816/page/525 page 525] on XCE | ||
# lbid, | # lbid, [https://archive.org/details/0893037893ProgrammingThe65816/page/277 page 277] | ||
# "Exchanges" on | # "Exchanges" on [https://archive.org/details/0893037893ProgrammingThe65816/page/102 page 102], lbid | ||
# [[Labiak]] | # [[Labiak]], [https://archive.org/details/Programming_the_65816/page/n215 page 205] on XCE | ||
# snes9x implementation of XCE: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L2517 | # snes9x implementation of XCE: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L2517 | ||
# undisbeliever on XCE: https://undisbeliever.net/snesdev/65816-opcodes.html#xce-exchange-carry-and-emulation-bits | # undisbeliever on XCE: https://undisbeliever.net/snesdev/65816-opcodes.html#xce-exchange-carry-and-emulation-bits | ||
Latest revision as of 14:35, 7 August 2024
| Basic Info | |||||||
|---|---|---|---|---|---|---|---|
| Addressing Mode | Opcode | Length | Speed | ||||
| Implied (type 2) | FB | 1 byte | 2 cycles | ||||
| Flags Affected | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| mode switching into | N | V | M | X | D | I | Z | C | E |
| emulation | . | . | B | . | . | . | E | 1 | |
| native | . | . | 1 | 1 | . | . | . | E | 0 |
XCE is a 65c816 instruction that exchanges the values of the carry and emulation bits. It typically appears soon after the Reset Vector following a CLC, to switch the S-CPU into 65c816 native mode. The carry flag was chosen because:
- it is easy to set and clear (with SEC and CLC)
- it is used less frequently than the negative and zero flags
- it can be tested with BCC and BCS
Syntax
XCE
The low bytes of both index registers (X & Y) are unaffected by a mode change. The high byte B of the accumulator is also unaffected.[1]
Examples
Switching to native mode:
CLC XCE
Switching to emulation mode:
SEC XCE
When switching to emulation mode to call existing 6502 code, it is recommended to clear the direct page register and point the stack pointer to page one.[2]
See Also
External Links
- Eyes & Lichty, page 525 on XCE
- lbid, page 277
- "Exchanges" on page 102, lbid
- Labiak, page 205 on XCE
- snes9x implementation of XCE: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L2517
- undisbeliever on XCE: https://undisbeliever.net/snesdev/65816-opcodes.html#xce-exchange-carry-and-emulation-bits
- Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.10.4