Commodore 1541

[BillyHW]

Could they not have included a fastloader cart in every box to ensure VIC-20 capability, as well as C64 performance?

 

Alternatively, could they not have sold the 1541 as a VIC-20 device with promised future compatibility with the C64 if the customer ever decided to upgrade their machine (albeit at a reduced speed)? Then they could have also released a C64-only 1542 for higher performance? The VIC-20 people would be happy because they wouldn't need to buy a new drive and the C64 people would be happy because they wouldn't have to wait 20 minutes each time they wanted to play Bruce Lee.

 

Does this not all seem logical? What am I missing?

[Rybags]

FastLoad came somewhat later and was from Epyx, not C=

 

Numerous stories/reasons, major one being JT put the big rush on and cared more about just delivering something to market than performance.

 

Commodore were pretty lite-on with their OS and firmware development/upgrade up until the Plus4 and 128 era (ie after the Tramiels left).

Before that, Vic-20 and C-64 were running on not much more than what the PETs had, the C64 OS in particular had zero support for stuff like SID sounds or anything Vic-2 could generate other than colour attributes.

 

In a way that helped innovation from outsiders - you had next to nothing to begin with although the C= documentation of the C64 hardware was very good for the time. Given the Basic language was pretty weak and few people bothered to upgrade via cart or loadable extensions, the incentive was there to just develop in Assembler from the get-go.

[carlsson]

The story goes that already the 1540 would have a burst mode, but due to bugs in the 6522 VIA at the time (found in both the VIC-20 and the 1540/1541), they couldn't make it work and had to settle for slower speeds. The C64 with its 6526 CIA fixed the bugs, so the question was whether to implement the burst in the 1541 but as mentioned, it would have to fall back to slower speeds for VIC users. Actually the 1541, due to timing the C64 bad lines I presume, runs a bit slower than a 1540 unless you issue the UI- command to set the drive into 1540 mode. Of course if the C64 always closed the display while loading, it would not be an issue.

[+SpiceWare]

Yep, Jim Butterfield spells it out here:

starting with the VIC-20 the serial bus was born. It was intended to be just as fast as the IEEE-488 it replaced. 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.

When the 64 came out, the problem VIA 6522 chip had been replaced by the CIA 6526. This did not have the shift register problem which had caused trouble on the VIC-20, and at that time it would have been possible to restore plan 1, a fast serial bus. Note that this would have called for a redesign of the 1540 disk drive, which also used a VIA. As best I can estimate - and an article in the IEEE Spectrum magazine supports this - the matter was discussed within Commodore, and it was decided that VIC-20 compatibility was more important than disk speed. Perhaps the prospect of a 1541 redesign was an important part of the decision, since current inventories needed to be taken into account. But to keep the Commodore 64 as a "bit-banger", a new problem arose. The higher-resolution screen of the 64 (as compared to the VIC-20) could not be supported without stopping the CPU every once in a while. To be exact: Every 8 screen raster lines (each line of text), the CPU had to be put into a WAIT condition for 42 microseconds, so as to allow the next line of screen text and color nybbles to be swept into the chip. (More time would be needed if sprites were being used). But the bits were coming in on the serial bus faster than that: a bit would come in about every 20 microseconds! So the poor CPU, frozen for longer than that, would miss some serial bits completely! Commodore's solution was to slow down the serial bus even more. That's why the VIC-20 has a faster serial bus than the 64, even though the 64 was capable, technically, of running many times faster. Fast disk finally came into its own with the Commodore 128.