Cba Register; Input/Output Instructions - RCA 70/46 Reference Manual

Assembly /Status
Register
CBA Register
INPUT/OUTPUT
INSTRUCTIONS
Start Device Instruction
Block
1
Block 2
Input/Output Operation
Data Bytes
I
Standard Device Byte
I
o
23 24 31
Bit Positions 0 through
31
are for equipment use only.
When the device status is stored as a result of an input/output opera-
tion, bit positions 24 through 31 of the assembly/status register are used
to store the standard device byte. The bits of the standard device byte
supply status information pertaining to the device control electronics and
the input/output device and are defined as follows:
Bit 24 - External Device Request Interrupt Pending
Bit 25 - Terminating Interrupt Pending
Bit 26 - Device Busy
Bit 27 - Control Busy (not applicable)
Bit 28 - Device End
Bit 29 - Second Indicator
Bit 30 - Device Inoperable
Bit 31 - Status Modifier
(For a detailed description of the above, see Standard Device Byte
section, page 65.)
• Not modified by channel, equals CBA originally fetched from 76-79.
• There are four processor instructions which are concerned with input/
output operations. They are Start Device, Halt Device, Check Channel and
Test Device. These instructions are executed by the processor and the
results, in the form of condition codes, are available upon instruction com-
pletion. It should be noted that the condition code settings indicate the
results of the instruction and not the results of the input/output operation
that the instruction may be initiating. The channel continues off-line to
accomplish the input/output operation as specified by the instruction.
However, during this time the processor continues executing subsequent
instructions.
• The Start Device instruction is a privileged operation and, therefore,
can be executed only if the mode bit (bit position 15 of the Interrupt Status
register for the current state) is set to zero. This instruction is executed
in the normal mode. Continuation of progranl execution is delayed until
the Start Device instruction has been terminated.
Upon execution of a Start Device instruction, the following events
occur. (See figure 5.)
• If the privileged mode bit (bit position 15 of the Interrupt Status
register) for the current state is not set to zero, the privileged operation
bit is set in the Interrupt Flag register and an interrupt occurs (if
permi tted) .
• If the specified channel is a selector channel that is not available on
the system, the condition code is set to 3, the Start Device instruction is
terminated and program control is transferred to the next instruction.
The input/output operation is not initiated.
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