Atari 400 Technical Reference Manual page 221

Zero-Page IOCB
G13
ZIOCS" (IOCSAS) [0020,16] -- Zero-page IOCB
The Zero-page IOCB is an exact c~py (except as noted in the
discussions that follow) of the IOCB specified by the 6502 X
register upon entry to CIO; CIa copies the outer level IOCB to
the Zero-page IOCB, performs the indicated function, moves the
(possibly altered) Zero-page IOCB back to the outer level IOCB,
and then returns to the caller.
Although both the outer level IOCB and the Zero-page IOCB are
defined to be 16 bytes in size, only the first 12 bytes are moved
by CIO.
G14 ICHIDZ [0020, lJ -- Handler index number
See Section 5. Set to $FF on CLOSE.
G15 ICDNOZ [0021,1J -- Device drive number
See Section 5.
016 ICCOMZ [0022,1] -- Command byte
See Section 5.
017 ICSTAZ [0023,1] -- Status byte
See Section 5.
G1S ICBALZ,ICBALH [0024,2] -- Buffer address
See Section 5. This pointer variable is modified by CIa in the
course of processing some commands; however, the original value
is restored before returning to the caller.
G19 ICPTLZ, ICPTHZ
See Section 5. Set to point to CIO's "IOCB not OPEN" routine on
CLOSE.
G20 ICBLLZ,ICBLHZ [0028,2J -- Buffer length
I
byte count
See Section 5. This double-byte variable, which starts out
representing the buffer length, is modified by CIO in the course
OPERATING SYSTEM C016555 -- Appendix L
236