The ATARI 400 and ATARI 800™ computers support a wide variety of configurations, each with a unique operating environment:
The OS accounts for all of these variables without requiting a change in the resident OS itself (see Section 2). The machine configuration is checked when power is first turned on and then is not checked again, unless system reset is used. A general discussion of some of the valid configurations follows.
The OS allows one of four program types to be in control at any point in time:
Control choice is based upon information in the cartridge(s), upon whether or not a disk drive is attached, and upon operator keyboard inputs. The exact algorithms are discussed in detail in Section 7.
In blackboard mode, the screen is established as a 24 x 40 text screen. Anything entered from the keyboard goes to the screen without being examined, although all of the screen editing functions are supported. Blackboard mode is the lowest priority environment. You go there only by command from a higher priority environment, or by default, if there is no other reasonable environment for the OS to enter. For example, typing BYE in BASIC causes the OS to enter the blackboard mode. The blackboard mode can be exited by pressing the [SYSTEM.RESET] key if it was entered from a higher environment.
An inserted cartridge normally provides the main control after initialization is complete (for example: ATARI BASIC, SUPER BREAKOUT™, BASKETBALL, COMPUTER CHESS, and others. All these cartridge programs interface directly with you in some way). Although a cartridge can provide a supporting function for some other program environment, this has not yet been done. Some cartridges (particularly keyboard-oriented ones) can change environments by entering special commands (such as "BYE") to go to blackboard mode or "DOS" to enter the disk utility. Other cartridges cannot change environments. Note that a hardware interlock prevents the removal or insertion of a cartridge with the power on; this feature causes the entire system to reinitialize with every cartridge change.
The diskette may or may not be booted when the system powers up with diskette-bootable software. This paragraph assumes that a diskette boot did occur. See Section 7 for boot condition explanations.
The diskette-booted software can take control as the Disk Utility Program (DUP) does under certain conditions, or can provide a supporting function as the File Management System (FMS) does. This environment is so flexible that it is difficult to generalize an its capabilities and restrictions. The only machine requirement (other than the disk drives) is that sufficient RAM be installed to support the program being booted.
The cassette-boot environment is similar to the diskette-boot environment, although the cassette is limited as an I/O device. It is slow and can access only one file at a time in sequence. Note that the cassette-boot facility has no relation to the use of cassettes to store high-level language programs (e.g., programs written in ATARI BASIC), nor to the use of cassettes to store data.
Although you can expand RAM noncontiguously in the ATARI 800 Home Computer, the OS will only recognize RAM that is contiguous starting from location 0. Installation directions are provided with the purchased RAM modules. RAM can be added until it totals 48K. After 32K additional RAM overlays first the right-cartridge addresses (32K to 40K) and then the left-cartridge addresses (40K to 48K). Note that in cases of conflict, the inserted cartridge has higher priority and disables the conflicting RAM in 8K increments. See Section 4 for a detailed discussion of system memory.
As a result of power-up, the OS will generate two pointers that define the lowest available RAM location and the highest available RAM location. The OS and diskette or cassette-booted software will determine the location of the lowest available RAM, while the number of RAM modules and the current screen mode will determine the highest available RAM.
Peripheral devices of several types can be added to the system using standard cables to either the serial bus or the connectors at the front of the computer console. The most common types deal with either transmission of bytes of data (usually serial bus) or transmission of sense information (usually game controllers).
The OS periodically senses (50 or 60 times per second) the standard game controllers (Paddles and Joysticks) and the values read are stored in RAM. You can plug in, remove. and rearrange these controllers at will without affecting system operation, because the system will always try to read all of these controllers.
The Driving Controllers are read, but not decoded, by the OS. Special instructions are required to read the keyboard controller (see Section 11).
The ATARI 410™ Program Recorder is a special peripheral. It uses the serial bus to send and receive data, but does not conform to the protocol of the other peripherals that use the serial bus. The Program Recorder must also be the last device on the serial bus, because it does not have a serial bus extender connector as the other peripherals do. There can never be more than one Program Recorder connected to any system for the same reason. The system cannot sense the presence or absence of the Program Recorder so it can be connected and disconnected at will.
A serial bus device conforms to the serial I/O bus protocol as defined in Section 9, but this does not include the Program Recorder. Each serial bus device has two identical connectors: a serial bus inputs and a serial bus extender. Either connector can be used for either purpose. Peripherals can be "daisychained" by cabling them together in a sequential fashion. There are usually no restrictions on the cabling order because each device has a unique identifier. Where restrictions exist, they will be mentioned in Section 5.