ADC: Difference between revisions

Latest revision as of 17:09, 22 October 2024


Basic Info
Addressing Mode	Opcode	Length	Speed
Immediate	69	2/3 bytes	2 cycles*
Absolute	6D	3 bytes	4 cycles*
Absolute Long	6F	4 bytes	5 cycles*
Direct Page	65	2 bytes	3 cycles*
Direct Page Indirect	72	2 bytes	5 cycles*
Direct Page Indirect Long	67	2 bytes	6 cycles*
Absolute Indexed by X	7D	3 bytes	4 cycles*
Absolute Long Indexed by X	7F	4 bytes	5 cycles*
Absolute Indexed by Y	79	3 bytes	4 cycles*
Direct Page Indexed by X	75	2 bytes	4 cycles*
Direct Page Indexed Indirect by X	61	2 bytes	6 cycles*
Direct Page Indirect Indexed by Y	71	2 bytes	5 cycles*
Direct Page Indirect Long Indexed by Y	77	2 bytes	6 cycles*
Stack Relative	63	2 bytes	4 cycles*
Stack Relative Indirect Indexed by Y	73	2 bytes	7 cycles*


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

ADC (Add with Carry) is the main 65x addition instruction. The accumulator is always both one of the addends and where the sum is stored. The operand serves as the other addend.

As there is no add-without-carry instruction, a CLC should be run (or the carry flag otherwise ensured to be clear) before an ADC, or else the sum will be one greater than expected.

After ADC runs, the carry flag will indicate whether the addition caused an unsigned overflow (sum greater than $FF or $FFFF depending on how wide the accumulator is). The zero flag will be set if the sum is zero and cleared otherwise.

If the decimal flag is set, then binary coded decimal addition is performed (with the sum being adjusted automatically), otherwise binary addition.

Syntax

ADC #const
ADC addr
ADC long
ADC dp
ADC (dp)
ADC [dp]
ADC addr, X
ADC long, X
ADC addr, Y
ADC dp, X
ADC (dp, X)
ADC (dp), Y
ADC [dp], Y
ADC sr, S
ADC (sr, S), Y

Cycle Penalties

ADC takes an extra cycle for each of the following:

if the accumulator is 16 bits wide, in all addressing modes
when utilizing the direct register, if the low byte of the direct register is nonzero
in certain addressing modes if adding an index crosses a page boundary

If you only need to add one, consider INC instead. Unlike the 65c02, the 65c816 does not have a cycle penalty when adding in decimal mode.

External Links

Eyes & Lichty, page 423 on ADC
Labiak, page 115 on ADC
2.2.1 on MCS6500 Manual, page 7 on ADC
B-3, lbid.
Carr, page 245 on ADC
Leventhal, page 3-38 on ADC
undisbeliever on ADC: https://undisbeliever.net/snesdev/65816-opcodes.html#adc-add-with-carry
Pickens, John. http://www.6502.org/tutorials/6502opcodes.html#ADC
Clark, Bruce. http://www.6502.org/tutorials/65c816opcodes.html#6.1.1.1

@@ Line 139: / Line 139: @@
 * in certain addressing modes if adding an index crosses a [[page]] boundary
-If you only need to add one, consider [[INC]] instead.
+If you only need to add one, consider [[INC]] instead.  Unlike the 65c02, the 65c816 does not have a cycle penalty when adding in decimal mode.
 === See Also ===