# Difference between revisions of "Day Of Week in 6502 machine code"

Based on code at http://6502.org/source/misc/dow.htm by Paul Guertin.

```   \ This routine works for any date from 1900-03-01 to 2155-12-31.
\ No range checking is done, so validate input before calling.
\
\ It uses the formula
\     Weekday = (day + offset[month] + year + year/4 + fudge) mod 7
\ where the value of fudge depends on the century.
\
\ On entry, A=day, 1..31
\           X=month, 1..12
\           Y=year-1900, 0..255
\ On exit,  A=day of week 0..6 for Sun..Sat, Carry will be Set
\           Needs incrementing with ADC #0 after calling to
\           become standard 1..7 range

.DayOfWeek
CPX #3:BCS dow_march    :\ Year starts in March to bypass leap year problem
DEY                     :\ If Jan or Feb, decrement year
.dow_march
EOR #&7F                :\ Invert A so carry works right
CPY #200                :\ Carry will be 1 if 22nd century
ADC dow_months-1,X      :\ A is now day+month offset
STA dow_tmp
TYA:JSR dow_mod7        :\ Get the year MOD 7
SBC dow_tmp:STA dow_tmp :\ Combine with day+month
TYA:LSR A:LSR A         :\ Get the year DIV 4
CLC:ADC dow_tmp         :\ Add it to y+m+d and fall through to MOD 7
.dow_mod7
ADC #7:BCC dow_mod7     :\ Reduce A to A MOD 7
RTS
.dow_months
EQUB 1:EQUB 5:EQUB 6:EQUB 3  :\ Month offsets
EQUB 1:EQUB 5:EQUB 3:EQUB 0
EQUB 4:EQUB 2:EQUB 6:EQUB 4
.dow_tmp
EQUB 0                  :\ Temporary storage

```

You can test this with:

```   FOR Y%=1 TO 255
FOR X%=1 TO 12
FOR A%=1 TO 31
PRINT A%;"/";X%;"/";1900+Y%;" ";
PRINT MID\$("SunMonTueWedThuFriSat",((USRDayOfWeek)AND&FF)*3+1,3)
NEXT A%:NEXT X%:NEXT Y%
```

I must say that this is an impressive bit of code!

Jgharston 01:56, 29 September 2007 (BST)