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XCE
From SnesLab
| Basic Info | |||||||
|---|---|---|---|---|---|---|---|
| Addressing Mode | Opcode | Length | Speed | ||||
| Implied (type 2) | FB | 1 byte | 2 cycles | ||||
| Flags Affected | ||||||||
|---|---|---|---|---|---|---|---|---|
| N | V | M | X | D | I | Z | C | E |
| . | . | M | B/X | . | . | . | E | C |
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: https://archive.org/details/0893037893ProgrammingThe65816/page/525
- lbid, page 277: https://archive.org/details/0893037893ProgrammingThe65816/page/277
- "Exchanges" on page 102, lbid: https://archive.org/details/0893037893ProgrammingThe65816/page/102
- Labiak page 205 on XCE: https://archive.org/details/Programming_the_65816/page/n215
- 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