IBM 1 Series - Modifier Bits; Device Address

Function Bits
Modifier Bits
Device Address
6-4 GA34-0033
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
bits Function
012
000 Read data
001 Read data
010 Read status
O11 Reserved*
100 Write data
101 Write data
110 Write control
111 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).
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, when used with read status or write
control, have system functions and must be implemented by all attached
devices. These commands are defined as follows:
Function Modifier bits Command
0123
Read status 0000 Read ID
Write control 1111 Device Reset
The modifier bit values for device-directed commands are
device-dependent, with the exception of the preceding formats, and are
defined as follows:
Function Modifier
bits bits Command
012 012 3
00X XX X X Read Data
010 XX X X Read Status
10xX XX X X Write Data
110 XX XX Write Control
Note: X=0 or 1.
‘Device address’ is a set of four outbound lines. These lines contain four
bits of encoded device address to select the device that is to respond to
the current operation. ‘Device address’ is used during both an I/O active
sequence or an interrupt-service active sequence. When the DPC adapter
is configured to attach less than 16 I/O devices, the leading bit of each
device-address is always 0.