|New, licensed release for 2013 on VIC-20.|
Like most of the older 8-bit computers, BASIC was not only the out-of-box language you could learn to program, but it was also the operating system. In the case of the early PET series computers, the ROM code to talk to the hardware was mixed in with the ROM code for the BASIC interpreter. This was done by Microsoft.
Technically, the idea was sound: the 6522 VIA chip has a "shift register" circuit that, if tickled with the right signals (data and clock) will cheerfully collect 8 bits of data without any help from the CPU. At that time, it would signal that it had a byte to be collected, and the processor would do so, using an automatic handshake built into the 6522 to trigger the next incoming byte. Things worked in a similar way outgoing from the computer, too.
We early PET/CBM freaks knew, from playing music, that there was something wrong with the 6522's shift register: it interfered with other functions. The rule was: turn off the music before you start the tape! (The shift register was a popular sound generator). But the Commodore engineers, who only made the chip, didn't know this. Until they got into final checkout of the VIC-20.
By this time, the VIC-20 board was in manufacture. A new chip could be designed in a few months (yes, the silicon guys had application notes about the problem, long since), but it was TOO LATE!
A major software rewrite had to take place that changed the VIC-20 into a "bit-catcher" rather than a "character-catcher". It called for eight times as much work on the part of the CPU; and unlike the shift register plan, there was no timing/handshake slack time. The whole thing slowed down by a factor of approximately 5 to 6.
- Jiim Butterfield (1997)
|drop-in KERNAL replacement|
A DOS ROM upgrade is also required for one or more of your disk drives, but is included natively in µIEC and all other sd2iec-based solid state drive solutions. The performance improvement is HUGE (approximately 600%). The only disadvantage of using JiffyDOS is that when it is enabled, the tape routines are not available. The space taken by these routines is used for the JiffyDOS code. However, a switch is provided which allows JiffyDOS to be disabled, should the need arise.
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