STZ

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.

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.

See Also

• STA
• STX
• STY

External Links

• Eyes & Lichty page 507, on STZ: https://archive.org/details/0893037893ProgrammingThe65816/page/n533
• "Storing Zero to Memory," page 103, lbid. https://archive.org/details/0893037893ProgrammingThe65816/page/n129
• Labiak page 188 on STZ: https://archive.org/details/Programming_the_65816/page/n198
• 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