Why was the Commodore 64 disk drive so slow?

2 days ago

The Commodore 64 (C64) is the best-selling home computer model ever, with estimates of up to 17 million units sold.
The C64's 1541 disk drive was notoriously slow, with speeds around 400 bytes per second, making it one of the slowest disk drives ever sold to the public.
The slow speed was due to a shift register bug in the 6522 chip, which forced Commodore to emulate the shift register in software, significantly reducing transfer speeds.
The VIC-20, C64's predecessor, used a serial bus to cut costs, but the shift register bug limited its disk transfer speed to about 1kB per second.
A hardware design error in the C64's mainboard accidentally discarded the high-speed serial bus line, locking it into the same slow transfer mode as the VIC-20.
The C64's VIC-II chip introduced memory access interleaving and caching to handle higher resolution graphics, but this caused CPU lockouts during 'badlines,' further slowing disk transfers.
Despite its slow disk drive, the C64 was a massive success, especially in the home market, where its price and capabilities overshadowed its loading speeds.
Commodore's decline began after Jack Tramiel's departure, with failed product lines and a shift in focus leading to the company's eventual bankruptcy in 1994.

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