Editing Commodore 1541 (section)

==Throughput and software==
''[[IEEE Spectrum]]'' in 1985 stated that:<ref name="ieee85">{{cite journal |author-last2=Wallich |author-first2=Paul |date=March 1985 |title=Design case history: the Commodore 64 |url=https://spectrum.ieee.org/ns/pdfs/commodore64_mar1985.pdf |archive-url=https://web.archive.org/web/20120513181613/http://spectrum.ieee.org/ns/pdfs/commodore64_mar1985.pdf |archive-date=2012-05-13 |url-status=dead |journal=[[IEEE Spectrum]] |pages=48–58 |issn=0018-9235 |author-last1=Perry |author-first1=Tekla S. |access-date=2011-11-12}}</ref>

{{Blockquote|The one major flaw of the C-64 is not in the machine itself, but in its disk drive.  With a reasonably fast disk drive and an adequate disk-operating system (DOS), the C-64 could compete in the business market with the Apple and perhaps with other business computers.  With the present disk drive, though, it is hard-pressed to lose its image as a toy.}}

The C-64's designers blamed the 1541's slow speed on the marketing department's insistence that the computer be compatible with the 1540, which is slow because of a flaw in the 6522 VIA interface controller.{{r|ieee85}} Initially, Commodore intended to use a hardware [[shift register]] (one component of the 6522) to maintain fast drive speeds with the new [[serial interface]]. However, a hardware bug with this chip prevents the initial design from working as anticipated, and the ROM code was hastily rewritten to handle the entire operation in software. According to [[Jim Butterfield]], this causes a speed reduction by a factor of five;<ref>{{Cite web|url=http://www.binarydinosaurs.co.uk/Museum/Commodore/c64/c64notes.php|title=Binary Dinosaurs - C64 Notes}}</ref> had 1540 compatibility not been a requirement, the disk interface would have been much faster. In any case, the C64 normally cannot work with a 1540 unless the [[MOS Technology VIC-II|VIC-II]] display output is disabled via a register write to the DEN bit (register $D011, bit 4), which stops the halting of the CPU during certain video lines to ensure correct serial timing.<ref>{{Cite web|url=https://www.cebix.net/VIC-Article.txt|title=The MOS 6567/6569 video controller (VIC-II) and its application in the Commodore 64 by Christian Bauer|access-date=2023-04-24|location=2.4.3 Memory access of the 6510 and VIC}}</ref>

As implemented on the VIC-20 and C64, Commodore DOS transfers 512 [[bytes]] per second, compared to the [[Atari 810]]'s 1,000 bytes per second, the Apple [[Disk II]]'s 15,000 bytes per second,{{r|ieee85}} and the 300-[[baud]] data rate of the Commodore [[Datasette]] storage system. About 20 minutes are needed to copy one disk&mdash;10 minutes of reading time, and 10 minutes of writing time.  However, since both the computer and the drive can easily be reprogrammed, third parties quickly wrote more efficient firmware that would speed up drive operations drastically.  Without hardware modifications, some "[[fast loader]]" utilities (which bypassed routines in the 1541's onboard ROM) managed to achieve speeds of up to 2.5&nbsp;kilobytes per second.{{r|ieee85}}  The most common of these products are the [[Epyx Fast Load]], the [[The Final Cartridge III|Final Cartridge]], and the [[Action Replay]] plug-in [[ROM cartridge]]s, which all have [[machine code monitor]] and [[disk editor]] software on board as well.  The popular Commodore computer magazines of the era also entered the arena with type-in fast-load utilities, with ''[[Compute!'s Gazette]]'' publishing ''TurboDisk'' in 1985 and ''[[Run (magazine)|RUN]]'' publishing ''Sizzle'' in 1987.

Even though each 1541 has its own on-board disk controller and disk operating system, it is not possible for a user to command two 1541 drives to copy a disk (one drive reading and the other writing) as with older dual drives like the [[Commodore 4040|4040]] that was often found with the PET computer, and which the 1541 is [[backward-compatible]] with (it can read 4040 disks but not write to them as a minor difference in the number of [[header (computing)|header]] bytes makes the 4040 and 1541 only read-compatible). Originally, to copy from drive to drive, software running on the C64 was needed and it would first read from one drive into computer memory, then write out to the other. Only when [[Fast Hack'em]] and, later, other disk backup programs were released, was true drive-to-drive copying possible for a pair of 1541s. The user could, if they wished, unplug the C64 from the drives (i.e., from the first drive in the daisy chain) and do something else with the computer as the drives proceeded to copy the entire disk.