IBM Series 1 User Manual page 186

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The function bits are three outbound lines that specify the
general type of I/O operation to be performed. They are
defined as follows:
Function bit-1Q111
000
001
010
011
100
101
110
111
Read data
Read data
Read status
Reserved·
Write data
Write data
write control
Reserved*
*These are system reserved functions for cycle steal and
should not be implemented
by
devices. Devices should return
command reject to these functions, subject to Operate I/O
instruction condition code precedence as defined in the
appropriate processor description manual (see Preface).
Modifi~!: Bit~
The modifier bits are four outbound lines that are used in
conjunction with the function bits for further definition of
the
I/O operation to be performed.
Certain modifier values
.hen used with "read status" or "write control" have system
functions and must be iaplemented by all attached devices.
These commands are defined as follows:
Read status
Write control
0000
1111
Command
Read 1D
Device reset
The modifier bit values for device directed commands
are device dependent with the exception of the above formats
and are defined as follows:
Function bits
.1Q.liL-__ _
oox
010
10X
110
Modifier bits
1.Q.123L _ _
XXXI
XXXI
XXXX
XXIX
Read data
Read status
write data
Write control
Not~.
Where
X
could be equal to a zero or one.
Device address is a set of four outbound lines. These lines
contain 4 bits of encoded device address to select the
Customer DPC Adapter 6-5