LDY: Difference between revisions

Latest revision as of 06:32, 22 August 2024


Basic Info
Addressing Mode	Opcode	Length	Speed
Immediate	A0	2/3 bytes	2 cycles*
Absolute	AC	3 bytes	4 cycles*
Direct Page	A4	2 bytes	3 cycles*
Absolute Indexed by X	BC	3 bytes	4 cycles*
Direct Page Indexed by X	B4	2 bytes	4 cycles*


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

LDY (Load Y) is a 65x instruction that loads a value into the Y index register. In immediate addressing only, LDY takes a total of 3 bytes when the index registers are 16 bits wide.

Syntax

LDY #const
LDY addr
LDY dp
LDY addr, X
LDY dp, X

Cycle Penalties

In direct page addressing modes, LDY takes another extra cycle if the low byte of the direct page register is nonzero.

External Links

Eyes & Lichty, page 464 on LDY
Labiak, page 151 on LDY
7.1 on MCS6500 Manual, page 96 on LDY
Carr, page 264 on LDY
Leventhal, page 3-74 on LDY
snes9x implementation of LDY: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L817
undisbeliever on LDY: https://undisbeliever.net/snesdev/65816-opcodes.html#ldy-load-index-register-y-from-memory
Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#LDY

@@ Line 1: / Line 1: @@
-'''LDY''' (Load Y) is a 65x instruction that loads the [[Y index register]].
+{| class="wikitable" style="float:right;clear:right;width:40%"
-{| class="wikitable" style="float:right;clear:right;width:30%"
 !colspan="8"|Basic Info
 |+
@@ Line 9: / Line 7: @@
 |'''Speed'''
 |+
-|immediate
+|[[Immediate]]
 |A0
-|2 bytes
+|2/3 bytes
-|2 cycles
+|2 cycles*
 |+
-|absolute
+|[[Absolute]]
 |AC
 |3 bytes
-|4 cycles
+|4 cycles*
 |+
-|direct page
+|[[Direct Page Addressing|Direct Page]]
 |A4
 |2 bytes
-|3 cycles
+|3 cycles*
 |+
-|absolute indexed X
+|[[Absolute Indexed by X]]
 |BC
 |3 bytes
-|4 cycles
+|4 cycles*
 |+
-|direct page indexed X
+|[[Direct Page Indexed by X]]
 |B4
 |2 bytes
-|4 cycles
+|4 cycles*
 |}
 {| class="wikitable" style="float:right;clear:right;width:30%"
-!colspan="9"|Flags Clobbered
+!colspan="9"|Flags Affected
+|+
+|[[Negative Flag|N]]
+|[[Overflow Flag|V]]
+|[[M Flag|M]]
+|[[X Flag|X]]
+|[[Decimal Flag|D]]
+|[[I Flag|I]]
+|[[Zero Flag|Z]]
+|[[Carry Flag|C]]
 |+
 |N
-|V
-|M
-|X
-|D
-|I
-|Z
-|C
-|+
-|
 |.
 |.
 |.
 |.
-|
 |.
+|Z
 |.
 |}
+'''LDY''' (Load Y) is a 65x instruction that loads a value into the [[Y index register]].  In [[immediate addressing]] only, LDY takes a total of 3 bytes when the index registers are 16 bits wide.
+==== Syntax ====
+<pre>
+LDY #const
+LDY addr
+LDY dp
+LDY addr, X
+LDY dp, X
+</pre>
+==== Cycle Penalties ====
+* In [[direct page addressing]] modes, LDY takes another extra cycle if the low byte of the [[direct page register]] is nonzero.
 === See Also ===
+* [[STY]]
 * [[LDA]]
 * [[LDX]]
+* [[TAY]]
+* [[TXY]]
 === External Links ===
-* [[Eyes & Lichty]] page on LDY: https://archive.org/details/0893037893ProgrammingThe65816/page/n489
+* [[Eyes & Lichty]], [https://archive.org/details/0893037893ProgrammingThe65816/page/n490 page 464] on LDY
+* [[Labiak]], [https://archive.org/details/Programming_the_65816/page/n161 page 151] on LDY
+* 7.1 on [[MCS6500 Manual]], [https://archive.org/details/mos_microcomputers_programming_manual/page/n114 page 96] on LDY
+* [[Carr]], [https://archive.org/details/6502UsersManual/page/n277 page 264] on LDY
+* [[Leventhal]], [https://archive.org/details/6502-assembly-language-programming/page/n123 page 3-74] on LDY
+* snes9x implementation of LDY: https://github.com/snes9xgit/snes9x/blob/master/cpuops.cpp#L817
+* undisbeliever on LDY: https://undisbeliever.net/snesdev/65816-opcodes.html#ldy-load-index-register-y-from-memory
+* Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#LDY
 [[Category:ASM]]
 [[Category:Group Three Instructions]]
 [[Category:Inherited from 6502]]
+[[Category:Load/Store Instructions]]