Virtual CTCA (VCTC) Support in VM/ESA

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This document describes the Virtual Channel-to-Channel (VCTC) support provided by the VM/ESA Control Program (CP) Version 2, Release 1.0 (and subsequent levels).

What is a Channel-to-Channel Adapter (you might ask)?

A Channel-to-Channel Adapter (CTCA) is an Input/Output device that can be connected to two independent I/O channel subsystems at the same time (unlike most devices which can only be connected to one I/O subsystem at any point in time). To some extent, you can think of the CTCA device as a pair of modems and a telephone line built into a single unit so one end connects to your computer, and the other end connects to a different computer.

So what is a Virtual CTCA?

VM differs from some operating systems in that it permits many users to share the same equipment by providing "virtual" devices (in contrast to operating systems that permit your application to run the real equipment directly (but for short bursts of time). VM supports several options for for device access. For example:

  • A "dedicated" device can be attached to your virtual machine so your application can control it directly. I sometimes get a tape drive temporarily attached to my virtual machine and make backup copies of my development work.
  • A real device (like a hard drive) may be shared by dividing it into logical or physical partitions. For example, many of my personal files are stored on a VM minidisk which is a 20 cylinder partition of a much larger physical disk. My "virtual" disk behaves like a real (but relatively small) disk when it interacts with any application program that I run in my virtual machine.
  • A device may be fully simulated (like a CTCA) even when no corresponding physical device exists on the system. A Virtual CTCA behaves like a real CTCA when it interacts with any application program that I run in my virtual machine.

A Virtual CTCA (or VCTC) is a simulation of the CTCA that permits two "Virtual Machines" on the same "Real Machine" to exchange data using the same application programs that would be used to operate a real CTCA device.

What is the purpose of this document?

I am responsible for portions of the VM/ESA Control Program (CP) that simulate the operation of a CTCA (Channel-to-Channel Adapter) connecting two "virtual" systems. The information in this document is moderately technical and (let's face it) pretty boring. However, you might find it useful in the following circumstances:

  • If you need general information about using a VCTC device in the VM environment, you will find usage notes in "VCTC Usage Notes".
  • If you need to understand the internals of the VCTC support within CP, you should read "VCTC Logic in CP".
  • If you need to debug a problem with an application program that uses a VCTC to connect two virtual machines, you might want to read "VCTC Diagnostic Notes".
  • If you want information about recent updates to the VCTC support, updates are summarized in "VCTC News".

The main entry point for the document is the "Table of Contents".

This document is written and maintained by: 

Dennis Musselwhite
VM/ESA CP Connectivity Development
musselwh@us.ibm.com

This document is NOT intended as a programming reference for the CTCA. Detailed specifications for Channel-to-Channel hardware are available in the following publications:

IBM Channel-to-Channel Adapter, SA22-7091
This publication defines the architecture for CTCA devices that use the parallel-I/O interface. This definition applies to:

VM Virtual CTCA (VCTC) support for devices defined as "CTCA" or "3088" emulates the behavior of the 3088 MCCU.

VM VCTC support does NOT emulate the System/360 CTCA (because a VCTC device can enter "Extended Mode"). It may, however, be possible to run a Basic CTCA application using a 3088 VCTC connection, as long as neither side issues the "Set Extended Mode" command.

ESA/390 ESCON Channel-to-Channel Adapter, SA22-7203
This publication defines the architecture for CTCA devices that use the serial-I/O interface. This definition applies to:
  • The IBM Enterprise Systems Connection Architecture (ESCON) CTCA, which can be configured to support either "Standard Mode" or "Basic Mode" operation (but cannot be switched by the application).

VM Virtual CTCA (VCTC) support for devices defined as "SCTC", "BCTC", or "CNC" emulates the behavior of the ESCON CTCA.