Here is a subroutine multiplying 16-bit A with 8-bit Y, with the lower 16-bit result in A and the top 8-bits result in Y.
multiplication: sep #$20 sta $4202 sty $4303 nop nop nop nop lda $4216 ldy $4217 xba sta $4202 nop nop tya clc adc $4216 ldy $4217 bcc carry_bit iny carry_bit: xba rep #$20 rts
This uses the Mode-7 registers. If you are already using Mode-7 registers, you can use this subroutine instead. This will produce errors if Mode-7 graphics are currently being rendered by the PPU.
multiplication: sep #$20 sta $211b xba sta $211b sty $211c sty $211c rep #$20 lda $2134 ldy $2136 rts