prindle@NADC.ARPA (07/02/84)
To clear up a "very" popular misconception, the limiting factor on the read/ write speed of the 1541 disk drive is "not" the serial bus! Witness either the 4 minute backup program (4MINUT.IMG from Compuserve C64 Programmers SIG) or SUPERCOPY from Richvale Telecommunications (warning some versions of this have a bug). The former (faster) of these reads/writes approximately 6 or 7 blocks per second with the screen blanked, while the latter approaches 5 blocks per second without blanking the screen. Both execute special programs in the drive instead of the DOS. It would appear that the DOS spends considerable time between bytes transmitted over the serial channel doing ???something??? which is not particularly necessary. If appropriate software were substituted at both the 1541 and C64 ends, a load of a 100 block program (currently about a 1 minute operation) could be done in 15 seconds; while this still may not be blinding speed, it does show that the serial channel is not currently being used at it's limit. I suppose I don't know all the ramifications of running the bus full speed when several devices are connected, although SUPERCOPY claims to work with two drives on the bus. Another point: The UI+ and UI- 1541 commands, as can be seen from a disas- sembly of the disk ROM, affect only the timing between individual bits of a byte transmitted via the bus from 1541 to C64, and have no effect on the time between bytes - this is why their use normally has only a miniscule effect on read/write time. The real secret to improved speed is reducing the inter-byte time. Frank Prindle Prindle@NADC.ARPA