Cycle stealing I/O controller with programmable offline mode of operation

In a data processing system containing a primary processing subsystem, a device and a secondary processing subsystem, interfacing between the primary subsystem and device, an improved secondary subsystem comprising:

• a secondary microprocessor;
• cycle steal adapter means for exchanging data with a directly accessible memory in said primary subsystem;
• device adapter means for exchanging data with said device in a variable format;
• means interconnecting said adapter means for passing data bidirectionally between said device and said primary subsystem memory;
• means connecting said microprocessor with both said adapter means for transferring primary command information from said primary subsystem to a memory in said microprocessor, and including means for transferring data and other information between said memory in said micoprocessor and either said memory in said primary subsystem or said device;
• said microprocessor containing means responsive to primary commands received one at a time from said primary subsystem for conditioning said secondary subsystem to operate in either a high speed cycle stealing mode or a programmable offline mode;
• said adapter means comprising means conditioned by said microprocessor in said high speed mode for operating said adapter means autonomously relative to said microprocessor to transfer an amount of data specified in said primary subsystem and said device;
• and
• means conditioned by said microprocessor in said programmable offline mode for operating said adapter means under direct control of said microprocessor to transfer data between said memory in said microcessor and either said device or said memory in said primary subsystem.

A method of operating a data processing system containing primary and secondary processing subsystems comprising:

• storing a program of chainable primary commands variably defining high speed and programmable offline modes in response to a setting of a mode bit;
• interpreting said commands in said secondary subsystem in a predetermined sequence and establishing operating modes defined by said commands;
• performing I/O data transfers, under the direction of said secondary subsystems, between a memory in said primary subsystem and a peripheral device while said high speed mode is effective;
• storing an ordered array of multiple secondary data processing commands in said secondary subsystem;
• and
• directing said secondary subsystem to perform one or more programs of secondary data transfer and data processing operations defined by commands in said list array when said offline mode is effective;
• said programs being performed without assistance or interference from said primary subsystem.