Atari 400 Technical Reference Manual page 81

The byte-count field contains the number of data bytes in the
sector.
Non-CIO 110
Some portions of the 1/0 subsystem are accessed independently of
the Central 1/0 Utility (CIO);
this section discusses those
areas.
Resident Device Handler Vectors
All of the OS ROM resident device handlers can be accessed via
sets of vectors that are part of the
as
ROM. These ,vectors
increase the speed of 1/0 operations that utilize fixed device
assignments, such as output to the Display Handler. For each
resident Handler there is a set of vectors ordered as shown
below:
+---------------+
+-
OPEN
-+ +0
+---------------+
+- CLOSE
-+
+2
+---------------+
+-
GET BYTE -+ +4
+---------------+
+-
PUT BYTE
-+ +6
+---------------+
+-
GET STATUS
-+ +8
+---------------+
+- SPECIAL
-+
+10
+---------------+
+-
JMP
-+ +12
+-
INIT
-+
+---------------+
+-
SPARE
-+
+-
BYTE
-+
+---------------+
Figure 5-18
Resident Device Handler Vectors
See Section 9 for a detailed description of the data interface
for each of these Handler entry points.
Each of the vectors contains the address (lo,hi) of the Handler
entry point minus 1. A technique similar to the one shown below
is required to access the desired routines:
OPERATING SYSTEM C016555 -- Section 5
96