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Decimal Mode: Difference between revisions
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Decimal mode is one of the [[SNES]] CPU's processor flags. 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) | Decimal mode is one of the [[SNES]] CPU's processor flags. 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.<sup>[2]</sup> | |||
== Processes == | == Processes == | ||
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The decimal mode has been documented more thoroughly in Ersanio's ASM tutorial V2.1. | The decimal mode has been documented more thoroughly in [https://www.smwcentral.net/?p=section&a=details&id=4750&r=0 Ersanio's ASM tutorial V2.1]. | ||
=== See Also === | |||
* [[DAA]] | |||
* [[DAS]] | |||
* [[Half-carry Flag]] | |||
=== Reference === | |||
# Labiak, William. Appendix C, page 357 conversion table: https://archive.org/details/Programming_the_65816/page/n367 | |||
# [[Eyes & Lichty]] page 262, https://archive.org/details/0893037893ProgrammingThe65816/page/n288 | |||
[[Category:ASM]] | |||
[[Category:Flags]] | |||
Latest revision as of 12:49, 15 December 2023
Decimal mode is one of the SNES CPU's processor flags. 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]
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
Reference
- Labiak, William. Appendix C, page 357 conversion table: https://archive.org/details/Programming_the_65816/page/n367
- Eyes & Lichty page 262, https://archive.org/details/0893037893ProgrammingThe65816/page/n288