Maths (Arithmetic): SPS2

       Name: SPS2                                                    [Show more]
       Type: Subroutine
   Category: Maths (Arithmetic)
    Summary: Calculate X = A / 16
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * SP2 calls SPS2


 Calculate the following, where A is a sign-magnitude 8-bit integer and the
 result is a signed 8-bit integer:

   X = A / 16


 Returns:

   C flag               The C flag is cleared


.SPS2

 TAY                    ; Copy the argument A to Y, so we can check its sign
                        ; below

 AND #%01111111         ; Clear the sign bit of the argument A

 LSR A                  ; Set A = A / 16
 LSR A
 LSR A
 LSR A

 ADC #0                 ; Round the result up to the nearest integer by adding
                        ; the bit we just shifted off the right (which went into
                        ; the C flag)

 CPY #%10000000         ; If Y is positive (i.e. the original argument was
 BCC LL163              ; positive), jump to LL163

 EOR #$FF               ; Negate A using two's complement
 ADC #0

.LL163

 TAX                    ; Copy the result in A to X

 RTS                    ; Return from the subroutine