XCE: Difference between revisions

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]

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

@@ Line 7: / Line 7: @@
 |'''Speed'''
 |+
-|implied (type 2)
+|[[Implied]] (type 2)
 |FB
 |1 byte
@@ Line 14: / Line 14: @@
 {| class="wikitable" style="float:right;clear:right;width:30%"
-!colspan="9"|Flags Affected
+!colspan="10"|Flags Affected
 |+
-|N
+|mode switching into
-|V
+|[[Negative Flag|N]]
-|M
+|[[Overflow Flag|V]]
-|X
+|[[M Flag|M]]
-|D
+|[[X Flag|X]]
-|I
+|[[Decimal Flag|D]]
-|Z
+|[[I Flag|I]]
-|C
+|[[Zero Flag|Z]]
+|[[Carry Flag|C]]
+|[[Emulation Mode Flag|E]]
+|+
+|emulation
+|.
+|.
+|
+|B
+|.
+|.
+|.
 |E
+|1
 |+
+|native
 |.
 |.
-|
+|1
-|
+|1
 |.
 |.
 |.
-|
+|E
-|
+|0
 |}
-'''XCE''' is an instruction that exchanges 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]].
+'''XCE''' is a [[65c816]] instruction
+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 (with [[SEC]] and [[CLC]])
+* it is used less frequently than the [[Negative Flag|negative]] and [[zero flag]]s
+* it can be tested with [[BCC]] and [[BCS]]
+==== Syntax ====
+<pre>
+XCE
+</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.<sup>[1]</sup>
+==== Examples ====
+Switching to native mode:
+<pre>
+CLC
+XCE
+</pre>
+Switching to emulation mode:
+<pre>
+SEC
+XCE
+</pre>
+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.<sup>[2]</sup>
+[[File:xce.png]]
 === See Also ===
@@ Line 45: / Line 87: @@
 === External Links ===
-* [[Eyes & Lichty]] page on XCE: https://archive.org/details/0893037893ProgrammingThe65816/page/n551
+# [[Eyes & Lichty]], [https://archive.org/details/0893037893ProgrammingThe65816/page/525 page 525] on XCE
-* [[Labiak]] page on XCE: https://archive.org/details/Programming_the_65816/page/n215
+# lbid, [https://archive.org/details/0893037893ProgrammingThe65816/page/277 page 277]
-* snes9x implementation of XCE: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L2517
+# "Exchanges" on [https://archive.org/details/0893037893ProgrammingThe65816/page/102 page 102], lbid
-* undisbeliever on XCE: https://undisbeliever.net/snesdev/65816-opcodes.html#xce-exchange-carry-and-emulation-bits
+# [[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
+# 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
 [[Category:ASM]]
 [[Category:65c816 additions]]
+[[Category:Exchange Instructions]]
+[[Category:One-byte Instructions]]
+[[Category:Implied Instructions]]
+[[Category:Two-cycle Instructions]]