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ADC: Difference between revisions

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'''ADC''' (Add with Carry) is 65x instruction that performs an addition.  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.
'''ADC''' (Add with Carry) is 65x instruction that performs an addition.  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.


The accumulator is always one of the addends.  The sum is stored in the accumulator.
The accumulator is always both one of the addends and where the sum is stored.


After ADC runs, the [[carry flag]] will indicate whether the addition caused an unsigned overflow.  The [[zero flag]] will be set if the sum is zero and cleared otherwise.
After ADC runs, the [[carry flag]] will indicate whether the addition caused an unsigned overflow.  The [[zero flag]] will be set if the sum is zero and cleared otherwise.

Revision as of 18:34, 4 August 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 65x instruction that performs an addition. 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.

The accumulator is always both one of the addends and where the sum is stored.

After ADC runs, the carry flag will indicate whether the addition caused an unsigned overflow. 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, 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

See Also

External Links

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