The SCTBL variable - Elite on the 6502

       Name: SCTBL                                                   [Show more]
       Type: Variable
   Category: Drawing the screen
    Summary: Lookup table for converting a character row number to the address
             of the top or bottom pixel line in that character row (low byte)
  Deep dive: Drawing pixels in the Apple II version
    Context: See this variable in context in the source code
 References: This variable is used as follows:
             * clearrow uses SCTBL
             * CPIX uses SCTBL
             * HLOIN uses SCTBL
             * letter2 uses SCTBL
             * LOIN (Part 2 of 7) uses SCTBL
             * LOIN (Part 3 of 7) uses SCTBL
             * LOIN (Part 4 of 7) uses SCTBL
             * LOIN (Part 5 of 7) uses SCTBL
             * LOIN (Part 6 of 7) uses SCTBL
             * LOIN (Part 7 of 7) uses SCTBL
             * PIXEL uses SCTBL
             * VLOIN uses SCTBL


 The character rows in screen memory for the Apple II high-res screen are not
 stored in the order in which they appear. The SCTBL, SCTBH and SCTBH2 tables
 provide a lookup for the address of the start of each character row.

 Also, the pixel rows within each character row are interleaved, so each pixel
 row appears $400 bytes after the previous pixel row. The address of pixel row
 n within character row Y is stored at the address given in the Y-th entry of
 (SCTBH SCTBL), plus n * $400, so the addresses are as follows:

   * Pixel row 0 is at the Y-th entry from (SCTBH SCTBL)
   * Pixel row 1 is at the Y-th entry from (SCTBH SCTBL) + $400
   * Pixel row 2 is at the Y-th entry from (SCTBH SCTBL) + $800
   * Pixel row 3 is at the Y-th entry from (SCTBH SCTBL) + $C00
   * Pixel row 4 is at the Y-th entry from (SCTBH SCTBL) + $1000
   * Pixel row 5 is at the Y-th entry from (SCTBH SCTBL) + $1400
   * Pixel row 6 is at the Y-th entry from (SCTBH SCTBL) + $1800
   * Pixel row 7 is at the Y-th entry from (SCTBH SCTBL) + $1C00

 To make life easier, the table at SCTBH2 contains the high byte for the final
 row, where the high byte has $1C00 added to the address.


.SCTBL

 EQUB LO($2000)
 EQUB LO($2080)
 EQUB LO($2100)
 EQUB LO($2180)
 EQUB LO($2200)
 EQUB LO($2280)
 EQUB LO($2300)
 EQUB LO($2380)
 EQUB LO($2028)
 EQUB LO($20A8)
 EQUB LO($2128)
 EQUB LO($21A8)
 EQUB LO($2228)
 EQUB LO($22A8)
 EQUB LO($2328)
 EQUB LO($23A8)
 EQUB LO($2050)
 EQUB LO($20D0)
 EQUB LO($2150)
 EQUB LO($21D0)
 EQUB LO($2250)
 EQUB LO($22D0)
 EQUB LO($2350)
 EQUB LO($23D0)
Drawing the screen: SCTBL

[Apple II version]
