Chapter 4
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using basic colours, it selects between the ink bytes (when D is 0) and the paper bytes
(when D is 1). In half cell mode using extra colours, it selects between the ink bytes for
the right half of each cell (when D is 0) and the ink bytes for the left half of each cell
(when D is 1). In single byte colour mode, D is always 0.
Single line mode using double byte colour produces a composite screen consisting of
two areas of different line resolutions – 128 single pixel height lines followed by 32
double pixel height lines. Both areas support the same colour palette, as specified by
the basic / extra colours bit of the display mode register.
The size of the attributes file is dictated only by the line height and single / double
byte colour bits of the display mode register. The memory usage for each attribute
mode is shown in the following table.
Summary of attribute mode memory usage
In double byte colour mode, the attributes file consists of two distinct areas. The first
generally holds the ink bytes for each cell and the second holds the paper bytes for
each cell (the exception to this is half cell mode with extra colours and using double
byte colour, and the format it uses is described later in this chapter). This partitioning
comes about because of the addressing schemes used, but has the advantage that it
becomes easy to find the corresponding paper byte for an ink byte, and vice versa. It
also makes it easy for a program to operate only on the ink or paper bytes instead of
having to manipulate both, and this could be exploited to achieve improved
performance.
As previously stated, single line mode using double byte colour produces a display
that consists of two different line heights. The attributes file spans locations $5800 to
$7FFF, of which the single line data occupies $6000 to $7FFF and the dual line data
occupies $5800 to $5FFF. Both areas are divided in two, with the first half defining the
ink colours and the second half defining the paper colours.
The total number of bytes used by each attribute mode is shown by the number in
square brackets, and excludes any unused region in between the two attribute areas.
This region, if it exists, is available for use by a program.
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