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A popular third party floppy disk controller for the Color Computer was the Super Controller II from Disto. It's main feature was a no-halt mode. Why was this desirable? How did the no-halt mode work?

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While in normal mode, the SCII (and the Tandy and other manufacturers' controllers) would not buffer data. The CPU would wait as the drive would read and write data to and from the computer's memory. While data transfer for the CoCo was pretty quick for the day, this process would not allow the CPU to do any other work.

The SCII had a second mode which was buffered. With the proper drivers, the CPU would command the controller to switch into buffered mode, and the buffer circuit would take over the task of waiting for the drive to transfer data to and from it's own RAM. After the data transfer was complete, the controller would signal the CPU that it was done, and the CPU could handle the data in whatever fashion it needed to.

This was a very nice feature as it freed up the CPU for other tasks while the controller took care of the buffering the data from the disk drive.

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According to this gem Tandy's Little Wonder, The Color Computer 1979-1991,

When OS-9 came out a flaw was quickly discovered in the Tandy disk controller (and all others made at that time). Although OS-9 was a multi-user, multi-tasking true DOS, the CoCo disk controller was not! The controller used a simple design that interrupted the 6809 during disk access. This was the cheapest and easiest way to build the control- lers, but it prevented OS-9 from doing anything until disk access was completed.

Essentially, the original CoCo disk controller would block the CPU during disk seeks. This was problematic due to how (relatively) slow the disk technology of the time was. After-market controllers like the DISTO SC II and Sardis Technologies "no halt" controller got around this problem by using a buffered mode that handles waiting for the floppy disk controller and writing the data into RAM (so the CPU didn't have to manage that process).

Additional reference: Disto Super Controller II Instruction Manual

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    Those most notable advantage to running in no-halt mode was that keyboard type ahead fully worked; you wouldn't lose keystrokes like you would on a stock controller (when the CPU was completely halted). The SCII also had two driver versions; polled and IRQ driven, and the IRQ version worked better for multi-tasking as well, as the CPU didn't waste time checking to see if the sector's worth of data was ready or not. – user294 May 2 '16 at 16:50

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