Atari 400 Technical Reference Manual page 49

+---------+ +---------+
COLOR
0
•
COLPFO
I
=
=--)1
COLPFl
COLOR
1
COLPF2
COLOR 2
COLPF3
COLOR 3 COLBK
COLOR 4
+---------+
+---------+
Figure 5-8 Screen Display Block Diagram
The following relationships are present in the preceding diagram:
1. Data base variables SDLSTL/SDLSTH contain the address of
the current display list. This address is stored in the
hardware display list address registers DLISTL and DLISTH
as part of the VBLANK process.
2.
The display list itself defines the characteristics of the
screen to be displayed and points to the memory containing
the data to be displayed.
3.
Data base variable CHBAS contains the MSB of the base address
of the character representations for the character data (text
modes only).
64
The default value for this variable is $EO. This variable
declares that the character representations start at memory
address EOOO (the character set provided by the OS in ROM).
Each character is defined as an
8X8
bit matrix, requiring
8
bytes per character. 1024 bytes are re,uired to define the
largest set, since a character code contains
up
to
7
significant bits (set of 128 characters),
The OS ROM contains
the default set in the region from EOOO to E3FF.
All character codes are converted by the handler from ATASCII
to an internal code (and vice versa), as shown below:
ATASCII
CODE
00-IF
20-3F
40-5F
60-7F
80-9F
AO-SF
CO-DF
EO-FF
INTERNAL
CODE
40-5F
00-IF
20-3F
60-7F
CO-OF
80-9F
AO-BF
EO-FF
OPERATING SYSTEM C016555 -- Section 5