C64 tape loading

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Bobbi
Vic 20 Afficionado
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C64 tape loading

Postby Bobbi » Fri Jan 06, 2017 3:18 pm

In anticipation of the arrival of my first ever Commodore 64 (from ebay) I am reading the C64 User Guide.

I was surprised to see that loading from tape seems to be different on C64 compared to VIC20. The (1982) C64 User Guide suggests that after you type LOAD, the screen is cleared to the border colour. When the program is found the message "FOUND ..." is displayed and apparently you have to hit the C= key to start it loading. I don't have a tape drive on any of my VICs, but this seems very different to what I recall on the VIC-20.

Is the C64 User Guide actually correct about this? And if so, why did CBM change the way of operation from the PET/VIC-20 method?

Seems very strange to me.

Aside: Interestingly the 1982 C64 User Guide explicitly refers to 'yellow function keys' ...

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Mike
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Re: C64 tape loading

Postby Mike » Fri Jan 06, 2017 3:39 pm

Unlike the VIC-I in the VIC-20, the VIC-II of the C64 needs an extra DMA at the start of each text line to retrieve 40 bytes of the text screen (and 40 nibbles of the colour RAM) into a cache inside the video chip. These rasters are called "badlines", and during this time, all CPU activity is halted.

Now, the standard KERNAL tape routines can't cope with this, for this reason screen DMA is switched off during tape operations. Only when the file name is displayed, the screen DMA is activated, and the C64 waits for the C= key as confirmation to continue tape operation; alternatively, operation resumes after roughly 5 seconds.

Most turbo tapes on the C64, especially those used for commercial games, take the badlines into account and continue display during their operation.

Bobbi
Vic 20 Afficionado
Posts: 347
Joined: Thu Oct 13, 2016 11:35 am
Location: Toronto
Occupation: Programmer

Re: C64 tape loading

Postby Bobbi » Fri Jan 06, 2017 3:43 pm

Thanks Mike - that makes sense. Was a bit of surprise to me!

The manual doesn't mention the 5 second delay if key not pressed. However in general the C64 User Guide is not a great manual, so I am not too surprised!


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