STZ: Difference between revisions

Latest revision as of 06:42, 22 August 2024


Basic Info
Addressing Mode	Opcode	Length	Speed
Absolute	9C	3 bytes	4 cycles*
Direct Page	64	2 bytes	3 cycles*
Absolute Indexed by X	9E	3 bytes	5 cycles*
Direct Page Indexed by X	74	2 bytes	4 cycles*


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

STZ (Store Zero) is a 65c816 instruction that zeros out memory. The size of the accumulator determines how many zero bytes are written.

STZ has the same effect as STA when the accumulator is zero. It was not available on the original NMOS 6502 and was probably added to the CMOS version in order to make code faster/smaller because zero is such a common value to store.

No flags are affected.

Syntax

STZ addr
STZ dp
STZ addr, X
STZ dp, X

Cycle Penalties

STZ takes one additional cycle if the accumulator is 16 bits wide, in all addressing modes.
In direct page addressing modes only, STZ takes another additional cycle if the low byte of the direct page register is nonzero.

External Links

Eyes & Lichty, page 507 on STZ
"Storing Zero to Memory," page 103, lbid.
Labiak, page 188 on STZ
snes9x implementation of STZ: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L1317
undisbeliever on STZ: https://undisbeliever.net/snesdev/65816-opcodes.html#stz-store-zero-to-memory

@@ Line 1: / Line 1: @@
-'''STZ''' (Store Zero) is a [[65c816]] instruction that zeros out memory.
+{| class="wikitable" style="float:right;clear:right;width:40%"
-{| class="wikitable" style="width:30%"
 !colspan="8"|Basic Info
 |+
@@ Line 9: / Line 7: @@
 |'''Speed'''
 |+
-|absolute
+|[[Absolute]]
 |9C
 |3 bytes
-|4 cycles
+|4 cycles*
 |+
-|direct page
+|[[Direct Page Addressing|Direct Page]]
 |64
 |2 bytes
-|3 cycles
+|3 cycles*
 |+
-|absolute indexed X
+|[[Absolute Indexed by X]]
 |9E
 |3 bytes
-|5 cycles
+|5 cycles*
 |+
-|direct page indexed X
+|[[Direct Page Indexed by X]]
 |74
 |2 bytes
-|4 cycles
+|4 cycles*
 |}
 {| class="wikitable" style="float:right;clear:right;width:30%"
-!colspan="9"|Flags Clobbered
+!colspan="9"|Flags Affected
 |+
-|N
+|[[Negative Flag|N]]
-|V
+|[[Overflow Flag|V]]
-|M
+|[[M Flag|M]]
-|X
+|[[X Flag|X]]
-|D
+|[[Decimal Flag|D]]
-|I
+|[[I Flag|I]]
-|Z
+|[[Zero Flag|Z]]
-|C
+|[[Carry Flag|C]]
 |+
 |.
@@ Line 51: / Line 49: @@
 |.
 |}
+'''STZ''' (Store Zero) is a [[65c816]] instruction that zeros out memory.  The size of the [[accumulator]] determines how many zero bytes are written.
+STZ has the same effect as [[STA]] when the accumulator is zero.  It was not available on the original NMOS 6502 and was probably added to the CMOS version in order to make code faster/smaller because zero is such a common value to store.
+No flags are affected.
+==== Syntax ====
+<pre>
+STZ addr
+STZ dp
+STZ addr, X
+STZ dp, X
+</pre>
+==== Cycle Penalties ====
+* STZ takes one additional cycle if the accumulator is 16 bits wide, in all [[addressing modes]].
+* In [[direct page addressing]] modes only, STZ takes another additional cycle if the low byte of the [[direct page register]] is nonzero.
 === See Also ===
-* [[STA]]
 * [[STX]]
 * [[STY]]
+=== External Links ===
+* [[Eyes & Lichty]], [https://archive.org/details/0893037893ProgrammingThe65816/page/507 page 507] on STZ
+* "Storing Zero to Memory," [https://archive.org/details/0893037893ProgrammingThe65816/page/103 page 103], lbid.
+* [[Labiak]], [https://archive.org/details/Programming_the_65816/page/n198 page 188] on STZ
+* snes9x implementation of STZ: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L1317
+* undisbeliever on STZ: https://undisbeliever.net/snesdev/65816-opcodes.html#stz-store-zero-to-memory
 [[Category:ASM]]
 [[Category:65c02 additions]]
+[[Category:Load/Store Instructions]]