Development
Soon after the C64's release, some astute programmers realized that Commodore's bit-banging serial KERNAL routines were unnecessarily sluggish. Since the CPU in the C64 ran at approximately the same speed as that in the 1541 disk drive, it was sufficient to synchronize only at the beginning of each byte, rather than at each individual bit. Moreover, this transfer method allowed two bits to be sent simultaneously, one over the standard DATA line and one over the CLK line (which was normally used to perform the handshaking). On the C64, this required very careful timing to avoid interference from interrupts and from the VIC-II graphics chip, which could "steal" CPU cycles. Some fast loaders disabled interrupts and blanked the screen for this reason. A fast loader would generally "wedge" itself into the LOAD vector at $0330, thus intercepting any calls to the KERNAL LOAD routine. Next, the fast loader would transfer the necessary code into the drive RAM and order its execution, then receive the file sent by the altered transfer code. Depending on the exact nature of the routines used, the loading speed could be improved by as much as a factor of five.
This technique was used for a few of the many fast-load systems made (such as JiffyDOS). Others were simply more efficient in I/O and file handling, offering marginal to good improvement. Other products added parallel hardware.
Read more about this topic: Fast Loader, Floppy Disks
Famous quotes containing the word development:
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