Basic
I/O
System (BIOS) Functions
5-8
WRITE
CHARACTER
TO
PUNCH
DEVICE
ENTRY
CL: ASCII
Character
FUNCTION
6
PUNCH
RETURN
This function sends the ASCII character in register
CL
to
the currently assigned
punch
device with RS 232C interface.
READ
READER
DEVICE
ENTRY
FUNCTION
7
READER
RETURN
AL:
Character
This function reads the next character from the currently assigned reader device
into
register AL.
An
end-of-file condition
is
reported by returning
an
ASCII
CONTROL-Z
(lAH).
IOBYTE Function
The
IOBYTE function creates a mapping oflogical
to
physical devices which can be
altered during
CP
IM-86
processing. The definition
of
the IOBYTE function
corresponds
to
the Intel
standard
as follows: a single location in the BIOS is
maintained, called IOBTYE, which defines the logical
to
physical device
mapping
which
is
in effect
at
a particular time.
The
mapping
is
performed by splitting the
IOBYTE into four distinct fields
of
two bits each, called the CONSOLE,
READER,
PUNCH,
and
LIST fields, as shown below.
most significant least significant
IOBTYE
LIST
PUNCH
READER
CONSOLE
bits
6,
7
bits 4, 5 bits 2, 3 bits
0, 1
The
value assigned in each field can be in the range 0-3, defining the assigned source
or
destination
of
each logical device. The values which
can
be assigned
to
each field
are given in Table 5-3.
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