Description Of Hardware; Antic And Ctia - Atari 400 Technical Reference Manual

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DESCRIPTION OF HARDWARE
A. ANTIC AND CTIA
TV Display: The ANTIC and CTIA chips generate the television
display at the rate of 60 frames per second on the NTSC (US) system.
The PAL (European) system is different and is described in the section
on NTSC vs PAL. Each frame consists of 262 horizontal TV lines and each
line is made up of 228 color clocks, as shown in figure VI-3. The 6502
microprocessor runs at 1.79 MHz. This rate was chosen so that one
machine cycle is equivalent in length to two color clocks. One clock
is approximately equal in width to two TV lines.
In any graphics mode, the display is divided up into small squares
or rectangles called pixels (picture elements). The highest resolution
graphics mode has a pixel size of 1/2 color clock by 1 TV line. A
sample display list is given in section IV.
The current TV line may be determined by reading the vertical counter
(VCOUNT). This register gives the line count divided by 2. There are 262
lines per frame so VCOUNT runs from 0 to 130 (0 to 155 on the PAL system).
The 0 point occurs near the end of vertical blank (see figure VI.S).
Vertical blank (VBLANK) is the time during which the electron beam returns
back to the top of the screen in preparation for the next frame. The
Atari 800 does not do interlacing, so each frame is identical unless
the program which is being executed changes the display. Vertical sync
(VSYNC) occurs during the fourth through sixth lines of vertical blank
(VCOUNT
=
hex 7D through 7F). This tells the TV set where each frame
starts. After VSYNC, there are 16 more lines of VBLANK for a total of 22
lines of VBLANK. The display list jump and wait instruction (to be
described later) causes the display list graphics to start at the end of
VBLANK.
Operating System (OS): The ATARI 400/800 comes with a 10K Operating
System (OS) in ROM. The OS affects some of the hardware registers, so
it will be mentioned from time to time in this manual. Refer to the OS
manual for more details. The OS descriptions in this manual apply to the
version that was being distributed when this manual was written.
The OS supports most of the hardware graphics modes (BASICS, GRAPHICS,
PLOT, and DRAWTO commands). The OS always displays 24 background lines after
the end of vertical blank. This convention is used at Atari to compensate
for television sets which overscan. Most TV's are designed so that the
edges of the picture are cut off. This is fine for ordinary broadcasts,
but with a computer it is essential for all important information to be
displayed on the screen. It is fairly common for four to eight color
clocks at the right or left edge of the picture to overscan. A TV set
that has excessive overscan may have to readjusted to obtain a satisfactory
display.
11.1