System information

The definition of the IOBYTE function corresponds to the Intel standard as follows: a single
location in memory, currently location 0003H, is maintained, called IOBYTE, which defines the
logical-to-physical device mapping that 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 in the following figure.
MOST SIGNIFICANT LEAST SIGNIFICANT
IOBYTE AT 003H LIST PUNCH READER CONSOLE
BITS 6,7 BITS 4,5 BITS 2,3 BITS 0,1
Figure 6-1. IOBYTE Fields
The value in each field can be in the range 0-3, defining the assigned source or destination of
each logical device. Table 6-4 gives the values that can be assigned to each field.
Table 6-4. IOBYTE Field Values
Value Meaning
CONSOLE field (bits 0,I)
0 console is assigned to the console printer device (TTY:)
1 console is assigned to the CRT device (CRT:)
2 batch mode: use the READER as the CONSOLE input, and the LIST device as
the CONSOLE output (BAT:)
3 user-defined console device (UCI:)
READER field (bits 2,3)
0 READER is the teletype device (TTY:)
1 READER is the high speed reader device (PTR:)
2 user-defined reader #1 (UR1:)
3 user-defined reader #2 (UR2:)
6.6 BIOS Entry Points CP/M Operating System Manual
6-18