It's just an enlarged version of the 16-bit division. There's not enough unused page-zero locations on a C64 to have all the variables in page-zero. The remainder variable is in page-zero because is the most used.
; Executes an unsigned integer division of a 24-bit dividend by a 24-bit divisor ; the result goes to dividend and remainder variables ; ; Verz!!! 18-mar-2017 div24 lda #0 ;preset remainder to 0 sta remainder sta remainder+1 sta remainder+2 ldx #24 ;repeat for each bit: ... divloop asl dividend ;dividend lb & hb*2, msb -> Carry rol dividend+1 rol dividend+2 rol remainder ;remainder lb & hb * 2 + msb from carry rol remainder+1 rol remainder+2 lda remainder sec sbc divisor ;substract divisor to see if it fits in tay ;lb result -> Y, for we may need it later lda remainder+1 sbc divisor+1 sta pztemp lda remainder+2 sbc divisor+2 bcc skip ;if carry=0 then divisor didn't fit in yet sta remainder+2 ;else save substraction result as new remainder, lda pztemp sta remainder+1 sty remainder inc dividend ;and INCrement result cause divisor fit in 1 times skip dex bne divloop rts dividend .ds 3 divisor .ds 3 remainder .equ $fb ; remainder is in zero-page to gain some cycle/byte ($fb-$fd) pztemp .equ $fe