Decimal Mode

Decimal mode is one of the SNES CPU's processor flags. It is bit 3 of the status register. Toggling it on/off will only affect the instructions ADC and SBC. To toggle decimal mode on/off, use SED (Set decimal mode) and CLD (clear decimal mode)

On the 65c816, it is cleared on reset.^[2] BRK and COP clear it as well.

The negative, overflow, and zero flags are invalid in decimal mode.^[3]

There are no conditional branch instructions that examine the decimal flag (such as BDS or BDC).^[5]

Processes

Let's just assume A is 8-bit and holds the value #$00. When one is doing ADC #$0C for example, the following steps will take place.

• Take the parameter after ADC
• Convert it to Decimal
• Add the conversion to A (execute ADC)
• If A turns out to have a hexadecimal number after the addition, convert it it to a decimal number again.

In this case, #$0C gets converted to 12. Since $0C equals to 12, doing ADC #$12 would give you the same result.

Same story with SBC, except SBC subtracts of course.

Example usage of ADC:

 LDA #$09  ;A = $09
 CLC       ;Clear carry flag
 ADC #$02  ;A = $11

The decimal mode has been documented more thoroughly in Ersanio's ASM tutorial V2.1.

See Also

• DAA
• DAS
• Half-carry Flag

References

1. Labiak, William. Appendix C, page 357. conversion table
2. Eyes & Lichty, page 262
3. lbid, page 44
4. Pickens, John. NMOS 6502 Opcodes. http://www.6502.org/tutorials/6502opcodes.html#DFLAG
5. Eyes & Lichty, page 148
6. Clark, Bruce. http://www.6502.org/tutorials/decimal_mode.html