# Day Of Week in 6502 machine code

From BeebWiki

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)