WDM: Difference between revisions

Latest revision as of 16:18, 6 August 2024


Basic Info
Addressing Mode	Opcode	Length	Speed
Implied (type 3)^[4]	42	2 bytes	2 cycles


Flags Affected
N	V	M	X	D	I	Z	C
.	.	.	.	.	.	.	.

WDM (the initials of William David Mensch, the designer of the 65c816) is an instruction that reserves its signature byte for future expansion of the instruction set. None of these extra 256 opcodes were ever implemented, so WDM functions essentially as a two-byte NOP.

No flags are affected, but future expanded WDM opcodes may.

WDM "works" even in emulation mode.

Syntax

WDM
WDM sig

ca65 for example requires the signature byte to be specified in the assembly source.

Directly specifying the opcode 42h can be used to skip the next instruction.^[5]

Trivia

WDM is the only implied addressing instruction that is more than one byte long
The Labiak textbook does not describe the WDM instruction.

External Links

Eyes & Lichty, page 523 on WDM
snes9x implementation of WDM: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L3335
undisbeliever on WDM: https://undisbeliever.net/snesdev/65816-opcodes.html#wdm-reserved-for-future-expansion
Table 5-4 Opcode Matrix of official 65c816 datasheet
Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.7

@@ Line 1: / Line 1: @@
-'''WDM''' (the initials of William David Mensch, the designer of the [[65c816]]) is an instruction that reserves its signature byte for future expansion of the instruction set.  None of these extra 256 opcodes were ever implemented, so WDM functions essentially as a two-byte [[NOP]].
+{| class="wikitable" style="float:right;clear:right;width:40%"
+!colspan="8"|Basic Info
+|+
+|'''Addressing Mode'''
+|'''Opcode'''
+|'''Length'''
+|'''Speed'''
+|+
+|[[Implied]] (type 3)<sup>[4]</sup>
+|42
+|2 bytes
+|2 cycles
+|}
 {| class="wikitable" style="float:right;clear:right;width:30%"
-!colspan="8"|Flags Clobbered
+!colspan="8"|Flags Affected
 |+
-|N
+|[[N Flag|N]]
-|V
+|[[V Flag|V]]
-|M
+|[[M Flag|M]]
-|X
+|[[X Flag|X]]
-|D
+|[[D Flag|D]]
-|I
+|[[I Flag|I]]
-|Z
+|[[Z Flag|Z]]
-|C
+|[[C Flag|C]]
 |+
 |.
@@ Line 22: / Line 34: @@
 |.
 |}
+'''WDM''' (the initials of [https://themenschfoundation.org/who-is-william-d-mensch-jr-aka-bill-mensch/ William David Mensch], the designer of the [[65c816]]) is an instruction that reserves its [[signature byte]] for future expansion of the instruction set.  None of these extra 256 opcodes were ever implemented, so WDM functions essentially as a two-byte [[NOP]].
+No flags are affected, but future expanded WDM opcodes may.
+WDM "works" even in [[emulation mode]].
+==== Syntax ====
+<pre>
+WDM
+WDM sig
+</pre>
+ca65 for example requires the signature byte to be specified in the assembly source.
+Directly specifying the opcode 42h can be used to skip the next instruction.<sup>[5]</sup>
+=== Trivia ===
+* WDM is the only [[implied addressing]] instruction that is more than one byte long
+* The [[Labiak]] textbook does not describe the WDM instruction.
+=== See Also ===
+* [[COP]]
+* [[BRK]]
+=== External Links ===
+# [[Eyes & Lichty]], [https://archive.org/details/0893037893ProgrammingThe65816/page/523 page 523] on WDM
+# snes9x implementation of WDM: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L3335
+# undisbeliever on WDM: https://undisbeliever.net/snesdev/65816-opcodes.html#wdm-reserved-for-future-expansion
+# Table 5-4 Opcode Matrix of official 65c816 datasheet
+# Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.7
 [[Category: ASM]]
+[[Category:65c816 additions]]
+[[Category:Two-byte Instructions]]
+[[Category:Implied Instructions]]
+[[Category:Two-cycle Instructions]]