Re: Video's 60 Frame Per Second

[mjmahon@aol.com (Michael J. Mahon)]

Bryan Parkoff wrote:

>Apple II Video is always 60Hz vertical line that it looks like it is
>60 frames per second using 6502 MPU 1 MHz.  If 6502 MPU 1 MHz is
>increased to 2 MHz, what will it happen?  Will 60 frames per second be
>decreased to 30 frames per second or be increased to 120 frames per
>second.  What about 6502 MPU 4 MHz that will cause to decrease to 15
>frames per second or to increase to 240 frames per second.
>    I don't know the answer yet.  Timing Generator Reference says that
>MPU can only be affected by increasing from 1 MHz to 8 MHz while 3.5
>MHz, 7 MHz, and 14 MHz for video are always not affected.  I suspect
>that 60 frames per second is always the same when 6502 MPU's speed is
>increased or decreased.
>    What do you think?

All timing in the Apple II is derived from a single 14.318MHz crystal
(NTSC version).  If this frequency is changed, all frequencies generated
by the Apple are changed proportionately.  This is, in general, a very
bad idea, since video frequencies are fixed by standards.

The only way to increase the clock frequency of the processor without
disturbing the video frequencies and the frequencies on the bus (which
peripheral cards depend on) is to use a separate processor clock domain
and synchronize it with the rest of the Apple whenever the processor
needs to communicate with anything on the Apple board.  This implies
that, to be useful, the processor clock domain must include at least
memory, so that the processor can compute without constant re-
synchronization with the 1MHz Apple memory.

The faster processor-memory clock domain can be implemented as
a "mirror" of Apple memory or as a cache of part of Apple memory.

The synchronization mechanism must detect whenever a processor
reference is made to a softswitch, graphics memory, or the peripheral
bus, so that it can synchronize and carry out the appropriate slower
speed operation.

As you can see, fitting a faster processor to an Apple II is a non-trivial
task, requiring a substantial amount of additional logic.  This can be
seen by examining any Apple II accelerator.

-michael

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