Dr Jefyll wrote:
BTW you haven't explained that your '670 is used as part of a memory mapping scheme... a job for which it is eminently suited!
With A14 A15 on WA, WB the '670 would provide four bits for each 16K region of a 64K address range. Each of the 4 bits could be "extended" address bits or some combination of address and control, depending on the region. I haven't worked out all the details yet, but I'm striving for something that wouldn't be overkill for a VIC-20, unlike other schemes I've thought up, which were both overkill and
extremely pricey! For now I'm just putting out ideas that might be useful or spark suggestions or criticisms.
The 512bytes of RAM in IO space of my earlier mod is intended to be memory that wouldn't be accessed by code
not written by me. It could hold short routines for managing my enhancements, or wedges to add or alter Basic operations, etc. I've come up with two other ways to make 74_670 Write registers readable. One is specific my expanded RAM-in-I/O modification, the other is more generalized.
Below, instead of having an IO slot also appear in the small RAM space, accessing locations of that RAM also access the '670 registers. The '670 Read and Write will be at the same addresses, which would be a big improvement over my earlier post.
Attachment:
670+688.GIF [ 10.9 KiB | Viewed 1832 times ]
Besides carving out four locations (plus four more as images) for the '670, the '688 also qualifies the raw cpu R/W. (The IO RAM gets VIC's normal VR/W.) It's important to note that until '670 ER goes low its Q outputs will be tristated, presumably pulled up to 1111, which will establish the Mapping/Banking/whatever powerup default state. /MBEN
In the circuit below, two '670s are used, one in the expected manner, the 2nd as readback of what was written to the 1st. It's assumed that /WR and /RD specific to the '670s' location have been decoded and qualified.
Attachment:
670x2.GIF [ 9.22 KiB | Viewed 1832 times ]
I'm pretty sure I've seen something similar, or possibly exactly like it, elsewhere in this forum.
Another germane thread is here: ram banking
viewtopic.php?f=4&t=7591(Waiting for the resident '816 devotees to pooh pooh all the above folderol.)