Atari 400 Technical Reference Manual page 223

User Call Parameters
SIO call parameters are passed primarily through a Device Control
Block; although an additional "noisy bus" option exists that is
selectable through a separate variable.
Device Control Block
H1 DCB [0300,12] -- Device Control Block
DCB is the name applied collectively to the 12 bytes at locations
0300-030B. These bytes provide the parameter passing mechanism for SIO
and are described individually below.
H2
DDEVIC [0300,1] -- Device bus ID
See Section 9.
H3
DUNIT [0301,1] -- Device unit number
See Sec tion 9.
H4
DCOMND [0302,1] -- Device command
See Section 9.
H5 DSTATS (0303,1] -- Device status
See Sec tion
9.
H6 DBUFLO,DBUFHI [0304,2] -- Handler buffer address
See Section 9.
H7 DTIMLO [0306,1] -- Device timeout
See Section 9.
H8 DBVTLO,DBVTHI [0308,2] -- Buffer length
I
byte count
See Section
9.
OPERATING SYSTEM C016555 -- Appendix L
238