(A401 and A403) contain the I/O subsystem, interrupt processing,
initialization, Power-Up and Reset. The 2K ROM chip (A402) contains
the floating point arithmetic package.
I/O Subsystem. The I/O subsystem contained in the ROM Operating
System provides a high-level interface between the user programs and
the Console and peripheral hardware. All peripheral devices capable
of dealing with character data have symbolic names (such as K, D, P,
E) and may be accessed using a Central I/O (CIO) routine.
Interrupt Processing. All hardware interrupts are handled by the
interrupt subsystem in the Operating System. Vectored addresses
contained in RAM memory point to subroutines in ROM to handle each
type of interrupt.
Initialization. There are two levels of initialization provided by
the system, Power-Up and Reset. Power-Up initialization is performed
each time the system power is turned on, and Reset initialization is
performed each time the Reset key is pressed.
Whenever system power is turned on, the Operating System examines and
notes the configuration of the unit. The Operating System performs
the following actions at Power-Up:
• Determines the highest RAM address
• Clears all RAM to zeroes
• Establishes all RAM interrupt vectors
• Initializes the ROM Cartridges
• Sets-up the video screen (24 x 40 text mode)
• Boots the cassette if desired
• Checks ROM Cartridge for disk boot instructions
• Boots the disk if desired and a disk drive is attached
• Transfers control to the ROM cartridge, booted program, or
"blackboard" program.
Whenever the Reset key is pressed, the Operating System performs
some, but not all, of the functions performed at Power-Up. The
Operating System performs the following actions after the Reset key
is pressed:
• Clears the Operating System portion of RAM memory
• Re-establishes all RAM interrupt vectors
• Formats the Handler Address table
• Initializes the ROM Cartridges
ATARI Personal Computers 4-15
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