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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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