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⌂ ∞ ▽ Elite on the 6502

Sprites: SPRITE4_BYTE

[Commodore 64 version, Sprites]

       Name: SPRITE4_BYTE                                            [Show more]
       Type: Macro
   Category: Sprites
    Summary: Macro definition for a four-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 four-colour sprite pixel row, with four
 pixels per byte (i.e. four 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:

   bit67_1 = (%01 * -(ASC(MID$(pixel_byte, 1, 1)) = ASC("/"))) << 6

 does the same as this longer statement:

   IF ASC(MID$(pixel_byte, 1, 1)) = ASC("/")
    bit67_1 = %01 << 6
   ELSE
    bit67_1 = 0
   ENDIF

 In other words, bit67_1 gets set to %01 << 6 (i.e. %01000000) if character 1
 of pixel_byte is "/", otherwise it gets set to 0.



 Arguments:

   pixel_byte           A string containing a row of 4 pixels, where each pixel
                        is defined as follows:

                          * "/" denotes colour 1

                          * "*" denotes colour 2

                          * "@" denotes colour 3

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



MACRO SPRITE4_BYTE pixel_byte

 bit67_1 = (%01 * -(ASC(MID$(pixel_byte, 1, 1)) = ASC("/"))) << 6
 bit67_2 = (%10 * -(ASC(MID$(pixel_byte, 1, 1)) = ASC("*"))) << 6
 bit67_3 = (%11 * -(ASC(MID$(pixel_byte, 1, 1)) = ASC("@"))) << 6

 bit45_1 = (%01 * -(ASC(MID$(pixel_byte, 2, 1)) = ASC("/"))) << 4
 bit45_2 = (%10 * -(ASC(MID$(pixel_byte, 2, 1)) = ASC("*"))) << 4
 bit45_3 = (%11 * -(ASC(MID$(pixel_byte, 2, 1)) = ASC("@"))) << 4

 bit23_1 = (%01 * -(ASC(MID$(pixel_byte, 3, 1)) = ASC("/"))) << 2
 bit23_2 = (%10 * -(ASC(MID$(pixel_byte, 3, 1)) = ASC("*"))) << 2
 bit23_3 = (%11 * -(ASC(MID$(pixel_byte, 3, 1)) = ASC("@"))) << 2

 bit01_1 = (%01 * -(ASC(MID$(pixel_byte, 4, 1)) = ASC("/"))) << 0
 bit01_2 = (%10 * -(ASC(MID$(pixel_byte, 4, 1)) = ASC("*"))) << 0
 bit01_3 = (%11 * -(ASC(MID$(pixel_byte, 4, 1)) = ASC("@"))) << 0

 bit67 = bit67_1 + bit67_2 + bit67_3
 bit45 = bit45_1 + bit45_2 + bit45_3
 bit23 = bit23_1 + bit23_2 + bit23_3
 bit01 = bit01_1 + bit01_2 + bit01_3

 EQUB bit67 + bit45 + bit23 + bit01

ENDMACRO