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BBC Micro Elite

Drawing pixels: CTWOS (6502SP version)

Name: CTWOS [View in context] Type: Variable Category: Drawing pixels Summary: Ready-made single-pixel character row bytes for mode 2
Ready-made bytes for plotting one-pixel points in mode 2 (the bottom part of the split screen). In mode 2, each character row is one byte, which is two pixels. Rows 0 and 1 of the table contain a character row byte with just the left pixel plotted, while rows 2 and 3 contain a character row byte with just the right pixel plotted. In other words, looking up row X will return a character row byte with pixel X/2 plotted (if the pixels are numbered 0 and 1). There are two extra rows to support the use of CTWOS+2,X indexing in the CPIX2 routine. The extra rows are repeats of the first two rows, and save us from having to work out whether CTWOS+2+X needs to be wrapped around when drawing a two-pixel dash that crosses from one character block into another. See CPIX2 for more details.
.CTWOS EQUB %10101010 EQUB %10101010 EQUB %01010101 EQUB %01010101 EQUB %10101010 EQUB %10101010