This code snippet will set the both the bank and the screen address registers, given simple pointers to screenChars and screenPixels.

 screenChars  = $0400 ; the 40x25 buffer
 screenPixels = $1000 ; the pixel data for font or bitmap ($1000 or $9000 are always charrom)

; Select VIC bank
 lda # ((screenChars ^ $ffff) >> 14)
 sta $dd00

; Set VIC screen and font pointers
 lda # (((screenChars & $3fff) / $0400) << 4) + (((screenPixels & $3fff) / $0800) << 1)
 sta $d018

by White Flame