; assembly version

 lda my     ;multiply my by 32
 sta ptr_h  ;through shifting
 ldy #0     ;a 16-bit var (ptr_h,ptr_l)
 sty ptr_l  ;to the right for three times
dup 3
 lsr ptr_h  ;shift
 ror ptr_l
edup
 lda ptr_l  ;add mx as 16-bit value
 clc
 adc mx
 bcc @1
 inc ptr_h
@1:
 clc
 adc #<map  ;add map offset
 sta ptr_l
 lda ptr_h
 adc #>map
 sta ptr_h
 lda [ptr_l],y  ;read the value


// C version

n=map[(my<<5)+mx];