Drawing the screen: SCTBH

       Name: SCTBH                                                   [Show more]
       Type: Variable
   Category: Drawing the screen
    Summary: Lookup table for converting a character row number to the address
             of the top pixel line in that character row (high byte)
    Context: See this variable in context in the source code
 References: This variable is used as follows:
             * clearrow uses SCTBH
             * CPIX uses SCTBH
             * HLOIN uses SCTBH
             * letter2 uses SCTBH
             * LOIN (Part 2 of 7) uses SCTBH
             * LOIN (Part 4 of 7) uses SCTBH
             * LOIN (Part 5 of 7) uses SCTBH
             * PIXEL uses SCTBH


 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.


.SCTBH

 EQUB HI($2000)
 EQUB HI($2080)
 EQUB HI($2100)
 EQUB HI($2180)
 EQUB HI($2200)
 EQUB HI($2280)
 EQUB HI($2300)
 EQUB HI($2380)
 EQUB HI($2028)
 EQUB HI($20A8)
 EQUB HI($2128)
 EQUB HI($21A8)
 EQUB HI($2228)
 EQUB HI($22A8)
 EQUB HI($2328)
 EQUB HI($23A8)
 EQUB HI($2050)
 EQUB HI($20D0)
 EQUB HI($2150)
 EQUB HI($21D0)
 EQUB HI($2250)
 EQUB HI($22D0)
 EQUB HI($2350)
 EQUB HI($23D0)

 EQUD $20202020         ; These bytes appear to be unused
 EQUD $20202020