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XCE

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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:

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]

xce.png

See Also

External Links

  1. Eyes & Lichty page 525, on XCE: https://archive.org/details/0893037893ProgrammingThe65816/page/525
  2. lbid, page 277: https://archive.org/details/0893037893ProgrammingThe65816/page/277
  3. "Exchanges" on page 102, lbid: https://archive.org/details/0893037893ProgrammingThe65816/page/102
  4. Labiak page 205 on XCE: https://archive.org/details/Programming_the_65816/page/n215
  5. snes9x implementation of XCE: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L2517
  6. undisbeliever on XCE: https://undisbeliever.net/snesdev/65816-opcodes.html#xce-exchange-carry-and-emulation-bits
  7. Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.10.4