STA

Basic Info
Addressing Mode	Opcode	Length	Speed
Absolute	8D	3 bytes	4 cycles*
Absolute Long	8F	4 bytes	5 cycles*
Direct Page	85	2 bytes	3 cycles*
Direct Page Indirect	92	2 bytes	5 cycles*
Direct Page Indirect Long	87	2 bytes	6 cycles*
Absolute Indexed by X	9D	3 bytes	4 cycles*
Absolute Long Indexed by X	9F	4 bytes	5 cycles*
Absolute Indexed by Y	99	3 bytes	4 cycles*
Direct Page Indexed by X	95	2 bytes	4 cycles*
Direct Page Indexed Indirect by X	81	2 bytes	6 cycles*
Direct Page Indirect Indexed by Y	91	2 bytes	5 cycles*
Direct Page Indirect Long Indexed by Y	97	2 bytes	6 cycles*
Stack Relative	83	2 bytes	4 cycles*
Stack Relative Indirect Indexed by Y	93	2 bytes	7 cycles*

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

STA (Store Accumulator) is a 65x instruction that stores the value of the accumulator. If the accumulator is 16 bits wide, its low byte is stored at the effective address and its high byte is stored at the effective address plus one.

No flags are affected.

Syntax

STA addr
STA long
STA dp
STA (dp)
STA [dp]
STA addr, X
STA long, X
STA addr, Y
STA dp, X
STA (dp, X)
STA (dp), Y
STA [dp], Y
STA sr, S
STA (sr, S), Y

Cycle Penalties

• In all addressing modes, STA takes one extra cycle when the accumulator is 16 bits wide.
• In direct page addressing modes, STA takes another extra cycle if the low byte of the direct page register is nonzero.

See Also

• LDA
• STX
• STY
• STZ

External Links

• Eyes & Lichty page 503, on STA: https://archive.org/details/0893037893ProgrammingThe65816/page/503
• Labiak, page 184 on STA
• MCS6500 Manual page 5 on STA: https://archive.org/details/mos_microcomputers_programming_manual/page/n20
• Carr page 273 on STA: https://archive.org/details/6502UsersManual/page/n286
• Leventhal page 3-96 on STA: https://archive.org/details/6502-assembly-language-programming/page/n145
• snes9x implementation of STA: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L1219
• undisbeliever on STA: https://undisbeliever.net/snesdev/65816-opcodes.html#sta-store-accumulator-to-memory
• Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#STA