origin of C64 HMOS version?

From: Nicolas Welte (welte_at_chemie.uni-konstanz.de)
Date: 2000-06-20 10:23:23

Reading about all those circuit board and chip revisions, I had to think
about when the HMOS version of the C64 was originally created or
planned. There are several strange facts when I look at the release
dates of the other 8bit machines and compare it to several C64 facts:

1984: 264 series released, HMOS-1 or HMOS-2 
1985: 128 released, HMOS-2
1986: C64C released, NMOS, but the case is clearly prepared to mount the
HMOS-2 equipped C64E board. Not only that, the case is optimized for the
C64E board, the C64B doesn't mount too well in there.
1987: finally, the HMOS-2 C64E board is released.

The list of video chip type numbers suggests something else:
8563 C128 VDC
8564 C128 VIC-II NTSC
8565 C64E VIC-II PAL
8566 C128 VIC-II PAL
8567 never seen this, does it exist?
8568 C128CR/DCR VDC

Is it just a coincidence, or why do the C64 chips have lower numbers
than the C128 chips, if they actually were introduced later? Also, PAL
versions have higher numbers than NTSC versions, but this is normal.

So I come to the conclusion, that CBM was already working on redesigning
the C64 chips when they decided to push out the 128. C64 redesign was
delayed until the 128 was finished and even then CBM had to fight
further difficulties, e.g. the redesign of the CIA chip, which failed
more than once. Evidence is the only shortly used 8521 CIA, that only
appeared in some C64 and C128 machines from 1986, and only in one of the
two CIA sockets. Something must be wrong with that chip ... The 6526
chips that were manufactured after 1987 must be other redesigned
versions, because their IRQ timing is one cycle off, which leads to
problems with certain software. The 8521 didn't have that problem. I
also take the C64B3 board as a desperate attempt to cut down costs by
using the new memory chips while the C64E board was waiting to be
equipped with a working chipset. 

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