Drawing pixels: GetScreenAddress

       Name: GetScreenAddress                                        [Show more]
       Type: Subroutine
   Category: Drawing pixels
    Summary: Return the screen address for a specified screen coordinate
    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * UpdateDashboard calls GetScreenAddress
             * PrintCharacter calls via GetScreenAddress-2


 Arguments:

   A                    The screen x-coordinate in pixels (0 to 159)

   Y                    The screen y-coordinate in pixels


 Returns:

   (Q P)                The address of the character block containing the screen
                        coordinates

   Y                    The pixel row within the character block containing the
                        screen coordinates


 Other entry points:

   GetScreenAddress-2   Treat the x-coordinate as a character column number
                        rather than a pixel coordinate (0 to 39)


 ASL A                  \ Set A = A << 2
 ASL A                  \       = x-coord << 2
                        \
                        \ so in the following, (Q P) gets set to x-coord << 3,
                        \ or x-coord * 8, which gives us the correct byte number
                        \ for this coordinate on the character row, as each
                        \ character block contains eight bytes

.GetScreenAddress

 STA P                  \ Set (Q P) = A << 1
 LDA #0                 \           = x-coord << 1
 ASL P                  \           = x-coord * 2
 ROL A                  \
 STA Q                  \ so (Q P) contains the correct byte number for this
                        \ coordinate as an offset from the start address of the
                        \ character row, as each character row contains 320
                        \ bytes, and the x-coordinate in A is in the range 0 to
                        \ 160 (i.e. each character block is two pixels wide)

 TYA                    \ Set X = Y
 LSR A                  \       = y-coord >> 3
 LSR A                  \
 LSR A                  \ so X is the character row number for this coordinate
 TAX

                        \ The X-th entry in the (yLookupHi yLookupLo) table
                        \ contains the screen address of the start of character
                        \ row X in the custom screen, so we now add this to
                        \ (Q P) to get the screen address of the correct
                        \ character block on this row

 LDA yLookupLo,X        \ Set (Q P) = (Q P) + X-th yLookup entry
 CLC                    \
 ADC P                  \ starting with the low bytes
 STA P

 LDA yLookupHi,X        \ And then the high bytes
 ADC Q
 STA Q

 TYA                    \ Set Y = Y mod 8, to set it to the pixel row within the
 AND #7                 \ character block for the coordinate
 TAY

 RTS                    \ Return from the subroutine