IBM 1 Series - Page 20

Address Gate. This is an outbound tag used during DPC sequences. This
tag signals a device that it can respond to initial selection and begin
execution of the command specified by bits 0-7 of the address bus.
Address Gate Return. This is an inbound tag used by the selected device
during a DPC sequence. This tag signals: (1) the reception of ‘address
gate,’ (2) the activation of ‘condition code in bus,’ and (3) the activation
of ‘data bus’ for a read sequence.
Service Gate. This outbound tag signals the device that last captured a poll
that a cycle-steal or interrupt-service sequence can begin. The I/O device
detects the leading edge of this tag following a poll capture to begin the
sequence; this is called service gate capture.
Service Gate Return. This is an inbound tag used by a device to signal a
service gate capture and activation of ‘address bus’ (on a cycle-steal
sequence), ‘data bus,’ ‘condition code in bus,’ and other tags as required
by the particular cycle-steal or interrupt-service sequence.
Condition Code In Bus. This is a three-bit, binary encoded inbound bus
used by the I/O device on DPC, interrupt, and cycle-steal sequences.
On DPC and interrupt-service sequences, the ‘condition code in bus’
corresponds to the condition code indicators in the level status register
(LSR) as follows:
Condition LSR
code in bit indicator
0 Even
1 Carry
2 Overflow
On cycle-steal service sequences, the ‘condition code in bus’ is used by the
device to pass the address key to the channel. On cycle-steal data
transfers, condition code bits O—2 are logically equal to the cycle-steal
address key. This key is bits 5—7 of the DCB control word previously
fetched by the device. During cycle-steal transfers for fetching the DCB
and for reporting residual status, a value of logical 0 is used for the
address key. During IPL cycle-steal transfers, a value of logical O is also
used for the address key.
Cycle Input Indicator. This is an inbound tag used by the device on a
cycle-steal service sequence. This tag signals the channel that the
cycle-steal is either: (1) an output from storage or (2) an input to storage.
When the indicator is a logical 0, an output from storage is indicated;
when the indicator is a logical 1, an input to storage is indicated.
Cycle Byte Indicator. This is an inbound tag used by the device on a
cycle-steal service sequence. This tag signals the channel that the
cycle-steal is either: (1) a word transfer or (2) a byte transfer. When the
indicator is a logical 0, a word transfer is indicated; when the indicator is a
logical 1, a byte transfer is indicated.
Processor I/O Channel 2-9