MOS Technology VIA
The Versatile Interface Adapter 6522 ( VIA ) is a flexible input / output module from MOS Technology . It has two 16-bit interval timers, a serial-to-parallel / parallel-to-serial shift register and two individually programmable I / O ports with 8 bits each. To control these functions, there is an interrupt flag register , an interrupt enable register and a pair of function control registers.
The 6522 was also manufactured under license by Rockwell Semiconductor and GTE .
Register set
A VIA contains a register set that is addressed during communication between the VIA and the processor via a data bus with 8 bits and an address bus with 4 bits (i.e. via 16 addresses).
The shift register bug
The shift register contained has a design flaw , which is why it could not be used to implement the CBM bus ("serial IEC bus") in the Commodore VC 20 , as originally planned . Instead, this functionality was implemented entirely in software. As a result, this bus could only work at about 1/6 the speed than if you could have resorted to the hardware support.
With the CIAs 6526 of the successor model C64 the mentioned problem was fixed, but in the floppy VC1540 (and later the VC1541 ) the 6522 chips were still used. In addition, the then Commodore boss Jack Tramiel decided to classify the backward compatibility with the VC20 as more important than the floppy speed, and so the same technology was retained with its low speed. The originally planned technology, the so-called burst mode, was not implemented until the C128 in conjunction with the VC1571 floppy .
