We've just updated MediaWiki and its underlying software. If anything doesn't look or work quite right, please mention it to us. --RanAS

XCE: Difference between revisions

From SnesLab
Jump to: navigation, search
(dots for unclobbered flags)
(→‎External Links: hid archive URLs for E&L)
 
(47 intermediate revisions by the same user not shown)
Line 1: Line 1:
'''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]].
{| class="wikitable" style="float:right;clear:right;width:40%"
 
{| class="wikitable" style="float:right;clear:right;width:30%"
!colspan="8"|Basic Info
!colspan="8"|Basic Info
|+
|+
Line 9: Line 7:
|'''Speed'''
|'''Speed'''
|+
|+
|implied
|[[Implied]] (type 2)
|FB
|FB
|1 byte
|1 byte
Line 16: Line 14:


{| class="wikitable" style="float:right;clear:right;width:30%"
{| class="wikitable" style="float:right;clear:right;width:30%"
!colspan="9"|Flags Clobbered
!colspan="10"|Flags Affected
|+
|mode switching into
|[[Negative Flag|N]]
|[[Overflow Flag|V]]
|[[M Flag|M]]
|[[X Flag|X]]
|[[Decimal Flag|D]]
|[[I Flag|I]]
|[[Zero Flag|Z]]
|[[Carry Flag|C]]
|[[Emulation Mode Flag|E]]
|+
|+
|N
|emulation
|V
|.
|M
|.
|X
|
|D
|B
|I
|.
|Z
|.
|C
|.
|E
|E
|1
|+
|+
|native
|.
|.
|.
|.
|
|1
|
|1
|.
|.
|.
|.
|.
|.
|
|E
|
|0
|}
|}
'''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 ===
* [[XBA]]
* [[XCN]]
* [[SWAP]]
=== External Links ===
# [[Eyes & Lichty]], [https://archive.org/details/0893037893ProgrammingThe65816/page/525 page 525] on XCE
# lbid, [https://archive.org/details/0893037893ProgrammingThe65816/page/277 page 277]
# "Exchanges" on [https://archive.org/details/0893037893ProgrammingThe65816/page/102 page 102], lbid
# [[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:ASM]]
[[Category:65c816 additions]]
[[Category:Exchange Instructions]]
[[Category:One-byte Instructions]]
[[Category:Implied Instructions]]
[[Category:Two-cycle Instructions]]

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:

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
  2. lbid, page 277
  3. "Exchanges" on page 102, lbid
  4. Labiak, page 205 on XCE
  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