I/O Channel IBM 370

Ѫ I/O Channel is a line of communication between the I/O Bus or memory to the CPU or
computer peripherals.
Ѫ I/O Processor in the IBM 370 computer is called a Channel.
Ѫ Each System configures a number of channels & attached with One or more Input/output
devices.
Ѫ CPU communicates directly with the channels via dedicated Control Lines & indirectly via
reserved storage areas in memory.

Multiplexer :
 Connected to Slow & Medium Speed Devices
 Capable of operating with a no.of devices simultaneously.
Selector :
 Designed to handle one I/O operation at a time.
 Commonly used to control one High Speed Device.
Block-Multiplexer :
 Combinational features of Multiplexer and Selector.
 Can connect a no.of high speed devices, but transfer as entire block of
data.
Types of Channels :

Operation Code :
‼ Specifies one of the 8 I/O instructions. Such as :
 Start I/O
 Start I/O fast release
 Test I/O
 Clear I/O
 Half I/O
 Half Device
 Test Channel
 Store channel identification
‼ Channel & Device Address :
 Respond to each of the I/O instructions & executes it.
 Sets one of the 4 condition codes in the processor register called PSW.
 In general it specifies the whether the channel or the device is busy.
 If the I/O operation had successfully started ,and whether the status word was stored in
memory by the channel.
Instruction Format
Device
Address
Channel
Address
Operation
Code

 Key is a protection technique used to prevent from other users to use it.
 Address field in the status word gives the address of the last command used by
the channel.
 Status field identifies the error occurred during the transfer.
 The count field gives the residual count when the transfer was terminated &
shows Zero when the transfer is successfully done.
 Data address specifies the first address of a memory buffer & count gives the
no.of bytes used in the transfer.
 Command specifies an I/O operation & Flag bits provides additional
information for the channel.
Channel Status Word Format
AddressKey CountStatus
CountFlags
Data
Address
Command
Code

 Command field operates with one of the six basic types of I/O operations:
 Write – transfer data from memory to I/O device
 Read – transfer data from I/O device to memory
 Read backward – read magnetic tape in backward direction
 Control – process such as rewinding or disk access management
 Sense – informs the channel to transfer its status word to memory
location.
 Transfer – gives the command word to be next executed.

Location of information in memory for I/O operations in IBM 370
Channel status word 64
Channel address word 72
Channel command word 1
Start I/O instruction
CPU
program
I/O channel
program
Memory Unit

The operation begins when the CPU program encounters a start I/O instruction.
The IOP the goes to memory location 72 to obtain a channel address word. This word contains the
starting address of the I/O channel program. The channel the proceeds to execute the program specified
by the channel command words. The channel constructs a status word during the transfer and stores it I
the location 64. Upon interruption, the CPU can refer to memory location 64 for the status word.

I/O Channel IBM 370

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