Sprites: SPRITE2_BYTE

       Name: SPRITE2_BYTE                                            [Show more]
       Type: Macro
   Category: Sprites
    Summary: Macro definition for a two-colour sprite pixel byte
    Context: See this macro in context in the source code
 References: No direct references to this macro in this source file


 This macro inserts one byte of a two-colour sprite pixel row, with eight
 pixels per byte (i.e. eight pixels per character block).

 In BeebAsm, true statements evaluate to -1 while false statements evaluate to
 0 (just as in BBC BASIC), so this statement:

   bit7 = -(ASC(MID$(pixel_byte, 1, 1)) = ASC("X")) << 7

 does the same as this longer statement:

   IF ASC(MID$(pixel_byte, 1, 1)) = ASC("X")
    bit7 = 1 << 7
   ELSE
    bit7 = 0
   ENDIF

 In other words, bit7 gets set to 1 << 7 (i.e. %10000000) if character 1 of
 pixel_byte is "X", otherwise it gets set to 0.


 Arguments:

   pixel_byte           A string containing a row of 8 pixels, where each pixel
                        is defined as follows:

                          * "X" denotes colour 1

                          * Anything else (e.g. "." or "+") denotes colour 0


MACRO SPRITE2_BYTE pixel_byte

 bit7 = -(ASC(MID$(pixel_byte, 1, 1)) = ASC("X")) << 7
 bit6 = -(ASC(MID$(pixel_byte, 2, 1)) = ASC("X")) << 6
 bit5 = -(ASC(MID$(pixel_byte, 3, 1)) = ASC("X")) << 5
 bit4 = -(ASC(MID$(pixel_byte, 4, 1)) = ASC("X")) << 4
 bit3 = -(ASC(MID$(pixel_byte, 5, 1)) = ASC("X")) << 3
 bit2 = -(ASC(MID$(pixel_byte, 6, 1)) = ASC("X")) << 2
 bit1 = -(ASC(MID$(pixel_byte, 7, 1)) = ASC("X")) << 1
 bit0 = -(ASC(MID$(pixel_byte, 8, 1)) = ASC("X")) << 0

 EQUB bit7 + bit6 + bit5 + bit4 + bit3 + bit2 + bit1 + bit0

ENDMACRO