CPY

Basic Info
Addressing Mode	Opcode	Length	Speed
Immediate	C0	2/3 bytes	2 cycles*
Absolute	CC	3 bytes	4 cycles*
Direct Page	C4	2 bytes	3 cycles*

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

CPY (Compare Y) is a 65x instruction that compares the value of the Y index register to something. Internally, this is done via a subtraction, but the difference is not stored anywhere. Thus, the purpose of CPY is to setup the flags. The index register Y is the minuend. Depending on the addressing mode used, the subtrahend will be either an immediate value or the value stored at the effective address specified by the operand. The contents at the effective address (when not using immediate data) and the Y index register both remain unchanged.

CPY assumes its operands are unsigned.

Syntax

CPY #const
CPY addr
CPY dp

In immediate addressing, the 65c816 expects the number of bytes of immediate data to match the current size of the index registers.

Cycle Penalties

• CPY takes an extra cycle in all three addressing modes when the index registers are 16 bits wide.
• In direct page addressing only, CPY takes an extra cycle when the low byte of the direct register is nonzero.

See Also

• CPX
• CMP
• SBC

External Links

• Eyes & Lichty, page 450 on CPY
• Labiak, page 137 on CPY
• MCS6500 Manual page 99 on CPY: https://archive.org/details/mos_microcomputers_programming_manual/page/n117
• Carr, page 257 on CPY
• Leventhal page 3-59 on CPY: https://archive.org/details/6502-assembly-language-programming/page/n108
• snes9x implementation of CPY: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L442
• undisbeliever on CPY: https://undisbeliever.net/snesdev/65816-opcodes.html#cpy-compare-index-register-y-with-memory
• Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#CPY
• Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.1.1.2