Atari 400 Hardware Manual

Each fetch of a display list byte takes one cycle, so three cycles
are required for a three byte instruction.
Pla y er/missile and display list instruction fetch DMA take place
during horizontal blank, if they are required for the next line.
I n memory map modes, the graphics data is fetched as needed throughout
the first l i ne of the display list instruction, then saved by ANTIC for
use in succeeding lines. In character modes, the character codes are
fetched during the first line of each row of characters, along with the
graphics data needed for that line. On the next lines, only the graphics
data is fetched, since ANTIC remembers the character codes.
In the 40 x 24 character mode, with a standard screen width, most of
the cycles during the top line of each row of characters are requried to
fetch the character codes and data, so there is only time for one memory
refresh cycle instead of the usual nine. Less DMA is required with a narrow
screen width so two memory refresh cycles would occur in this case.
The memory refresh is done fast enough to make up for the lost cycles
in the high resolution modes. Once memory refresh starts on a line, it
occurs every four cycles unless pre-empted by DMA.
All interrupts reach the 6502 near the end of horizontal blank.
With standard or narrow screen widths, refresh DMA starts after the end of
horizontal blank.
The time at which ANTIC does cycle stealing is deterministic, but
depends on the graphics mode, screen width and whether or not horizontal
scrolling is enabled. Horizontal scrolling requires extra graphics data:
see HSCROL.
ANTIC does horizontal scrolling of an even number of color clocks by
delaying the time at which it DMA's the data. To do an odd number of color
clocks (which involves half of a machine cycle), ANTIC has a one color clock
i nternal delay.
Theoretically, it is possible to write a program which changes graphics
or colors "on the fly", i.e. during the middle of a TV line. However,
with all the DMA going on, the cycle counting gets to be quite complicated,
and is beyond the scope of this manual.
There are a number of delays associated with the display of graphics.
These occur in the ANTIC and the CTIA. The ANTIC sends data to the CITA
which adds in the color information. Thus the timing for changing colors on
the fly is different from that for changing graphics on the fly.
II. 21

Atari 400 Specifications

General

Manufacturer	Atari
Model	400
Type	Home Computer
Release Year	1979
CPU	MOS Technology 6502
CPU Speed	1.79 MHz
ROM	10 KB
Operating System	Atari OS
Keyboard	Membrane keyboard
RAM	8 KB (expandable to 48 KB)
Graphics	ANTIC and GTIA chips
Sound	4 channels
Display	RF output for connection to TV
Storage	Optional cassette tape drive or floppy disk drive
Ports	Cartridge
Display Resolution	320x192 (16 colors)