Transcript IBM Brand Template

Channel Programming
Goals
Offload data movement work from Processor (like DMA on PCs)
Minimize Interrupts (very disruptive)
Improve device utilization
How
“Program” the device and channel so that they accomplish very
complex activities without the processor being involved
Channel Programming
Channel Command Words (CCWs)
Each one is an instruction to the channel and/or device
Channel Program
A sequence of CCWs to do an IO function
“A program for the channel to execute”
Note: a unconditional branch op code (TIC) allows the program
to transfer to another real memory location
An IO operation starts by assigning a hardware channel to a channel
program.
At the end of the program, an IO interrupt occurs which has status
information about the operation.
Channel Programming
CCW format (see fig IO.4)
Op code – command to channel and device e.g. print and space
a line
Count – The number of bytes to move
Address – the location of the bytes in memory. This is a real
Address and cannot be paged out until the IO is complete
Flag Byte – additional command information (next foil)
Channel Programming
CCW Flag Byte – (most significant bits)
- Command Chain- after this CCW execute next CCW
- Data Chain – after count of current CCW is used, continue data
movement using count and address field from next CCW.
e.g. Scatter/Gather data to/from memory
- Program Controlled Interrupt(PCI) – Cause an IO Interrupt when
this CCW is executed (but continue with the operation).
e.g. Notify OS about Channel position in the Channel Program
- Indirect Data Address List(IDAW)- The address of a list of data
addresses which are used when the data transfer crosses
page boundaries i.e. use next address in list when data
transfer crosses a real page boundary.
e.g. an IO buffer is contiguous in Virtual memory but crosses
into multiple non-contiguous real pages
Channel Programming
IO Interrupt Information
- Identity of the device that caused the interrupt
- Completion Status
Channel end (CE)- channel ended it's operation
Device end(DE) – device ended it's operation
Unit Exception(UE)- Exception condition at device
e.g. Printer out of paper
Unit Check – Problem at device
e.g. Paper jam
Multiple conditions can be reported on one interrupt
e.g. CE and DE are a normal situation
Channel Programming
Example
Spooling Subsystem
1.During Program Execution
-OS provides a “virtual” printer
-Lines sent to printer are packed into a buffer
Assume packed 40 lines to 1 4K buffer
-When buffer is filled,it's written to disk (Spool area)
-Double buffered so program doesn't wait for disk write
-Impact
Fewer interrupts 1 disk IO every 40 lines
Execution time lower – disks faster than printer
Channel Programming
Example
2. During Printing
Lines are printed from Buffer
-by a chain of CCWs created by Spooling subsystem
-40 lines printed with 2 IO Interrupts
(at end of printout and disk read)
-Double buffer so printer doesn't wait for disk
Impact
-Fewer Interrupts ( one printer interrupt for 40 lines)
-Printer runs fast (never waits for application)
- able to share 1 printer between multiple jobs
Note: can dynamically modify channel program
for even fewer interrupts
Channel Programming
Goals