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    Atari Assembler Course Session 9
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    Atari Operating System Overview
    
      The 10K Operating System in the Atari computers has five main
    functions: power-up Monitor, Interrupt Processor, I/O Utility,
    Vertical-Blank Processor and Floating-Point routines. The ROM also
    includes the default character set. The system routines that are
    available to us are vectored through ROM "Jump Tables".
    
    
    9.1  Power-up Monitor
    
      The Monitor initializes the memory, resident handlers and hardware
    when the computer is turned on; this is refered to as "Coldstart". The
    <System/Reset> key causes a "Warmstart" which initiates much the same
    as coldstart. We may "steal" the System/Reset vector for our own
    initialization purposes.
      The power-up routines also check for cartridges, the disk drive or a
    cassette in that order. The details of how to use these features are
    given in the Atari PCS O/S User's Manual.
    
    
    9.2  O/S RAM and ROM Vectors 
    
       The interaction of our program and the O/S must take place through
    the system vectors. The RAM vectors hold the address of the O/S
    routines (or ours!) for interrupt-level processing. The Display list,
    Vertical Blank, Serial I/O and POKEY-timer interrupts are all vectored
    here.
       The ROM vectors give us a gate-way to the O/S routines. These
    three-byte entries are actually JMP instruction to the real routine in
    the ROM. This table will always be at the same addresses in all
    revisions of the O/S to insure proper entry. The routines that we will
    be most interested in are the Resident Disk Handler, the Central I/O
    Utility and the Vertical Blank routines.
      An outline of these vectors is given in "De Re Atari" on pages 8-12
    thru 8-21.
    
    
    9.3  Central I/O Utility and Device Handlers
    
       The CIO Subsystem is the device-independent Utility that handles
    input and output for all devices. This capabilty was modeled after I/O
    systems in larger computers and handles byte and record transfers,
    multiple open device/files and error handling independent of device
    type.
       CIO takes our commands and passes control to the appropriate device
    handler which performs the actual I/O. The O/S contains Resident
    Handlers for Cassette, Keyboard, Screen, Editor and Printer. The best
    feature of CIO is the ability to add non-resident devices to its list
    such as the disk, modem and other "add-ons".
       To use CIO, we must converse with it through one of eight I/O
    control Blocks or "IOCB'S". These RAM blocks of 16 bytes each begin at
    $340 and contain values for the command, status, buffer length, buffer
    location, and auxiliary info for our I/O operation. After supplying
    this data to the IOCB, we then call the CIO routine with the value of
    the IOCBx16 in the X-register. A JSR CIOV ($E456) will perform the I/O
    and return the status in the Y-register. The programmer must remember
    what IOCBs he his using and must open them before and close them after
    use. Refer to pages 8-22 thru 8-34 in "De Re Atari".