|
Post by mirkosoft on May 29, 2015 11:48:02 GMT
Hi!
Really wrong decision of Commodore 128 designers and really only for compatibility with C64 is VIC2E chip as one of two display chips. Meaning that VIC2E can work only at 1MHz, or by IRQ can be system speed up to 1,3MHz approximatelly... When we look at TED or VDC they both are better chips and are not limited by 1MHz. TED - 121 Colors and flash attribute with resolution up to 320×496 at ~1,76MHz and VDC with separate VideoRAM, resolution up to known working 800×600 and color cells variability incl. 4 attributes flash - reverse - underline and alternate...
But can anybody eplain me this behavior of VIC2E when is Z80 active:
In case of active MOS 8502 @ 2MHz VIC cannot handle screen display.
In case of active Zilog Z80 @ 2MHz VIC can handle screen display.
What it means?
Z80 is faster - works @ 2MHz.
Does it its downclocking from 4MHz to 2MHz?
Or handles VIC screen with downclocking even to 1MHz?
Or VIC can work @ 2MHz an maybe even @ true 4MHz Z80 clock?
Can anybody explain this?
Thank you for suggestions.
Miro
|
|
|
Post by C128Man on Jun 2, 2015 14:23:10 GMT
I'ven't got explanation, only a suggestion ;-)
The Z80 works à 4Mhz in the C128, but it only use 2 cycles on 4. This means that in fact, the Z80 runs in 2Mhz. Maybe, during the 2 "deads" cycles, VIC2 can handle the screen?!
Ben
|
|
|
Post by hydrophilic on Jun 3, 2015 10:12:51 GMT
It has been a long dream of mine to hack C128 so that when VIC is in 2MHz mode, the Z80 will run at full speed (4MHz). Saddly I never made this a reality... there is rumor that Bil Herd made a prototype that runs Z80 at full speed of 4MHz, but I never saw those schematics... I look at published schematics and think (to myself) "it *almost* could be real"... but I never found a true solution...
|
|
|
Post by mirkosoft on Jun 3, 2015 15:11:46 GMT
OK, now other Q:
When Z80 runs @ 4 MHz and has 2 dead cycles, is it possible to make them alive?
Really C128 @ 4 MHz (I know that Z80) can be really fantastic and no matter if VIC2E can handle it, my favorite videochip is VDC - its variability is even more than EGA!
Miro
|
|
|
Post by hydrophilic on Jun 4, 2015 7:10:25 GMT
If you want Z80 at full 4MHz, then yes, you must turn off VIC and only use 80-column VDC. However, I still don't know how to hack my C128 to make it happen
|
|
|
Post by C128Man on Jun 4, 2015 9:56:44 GMT
Is it really possible to run the Z80 at 4Mhz?! The 8502 do certains task also un CP/M mode and the maximum speed is only 2MHz.
|
|
|
Post by hydrophilic on Jun 4, 2015 11:41:03 GMT
Looking at the hardware specs... yes Z80 can (with a good hack) run at 4MHz. I do *not* know how to hack a real C128 to make it happen...
But there was at least one prototype (more?) that allows this... I never worked for CBM, so you will have to ask Bil Herd for details... he has reported 4MHz prototype, but I (we?) have never seen "real-life" schematics...
So maybe it is real... or maybe wishful thinking... but I think only Bil Herd can say for sure!!
|
|
|
Post by mirkosoft on Jun 4, 2015 18:53:16 GMT
OK, only last post 'cause again hack like VIC2E Color RAM nibble width, summary:
1 MHz of 8502 handles operating & VIC2E screen 2 MHz of 8502 handles operating, VIC2E screen unhandled 4 MHz 2 cycles handles operating, 2 dead cycles can leave VIC2E handle screen 'cause 2 cycles @ 4 MHz = 1 cycle @ 2 MHz without dead cycle(s) Result: C128 can run @ 4 MHz on both screens - VIC2E & VDC but MIPS equals to 2 MHz, but when is VIC2E turned off and 2 dead cycles could be alive MIPS equals 4 MHz = true 4 MHz clock
Only mysteroius reason to use VIC chip in C128... in case marketting only one reason: sell as many as possible
Miro
|
|
|
Post by hydrophilic on Jun 6, 2015 7:06:55 GMT
Right... sell as many as possible... rush it to the consumer market (never develop full potential).
Wrong... C128 can not do both VIC and VDC at 4MHz. If VIC is active, 1/2 of system bus is for VIC, the other 1/2 is for CPU... if the CPU is Z80 (max 4MHz) then with only 1/2 bus for CPU (1/2 stolen by VIC) then Z80 CPU has max speed of 2MHz.
Maybe I am wrong... but from my electronic circuit analysis (and old Bil Herd comments about prototype), you can only get full 4MHz Z80 when VIC is blanked... if VIC is active, maximum Z80 speed is 2MHz.
If you know a way to make 4MHz Z80 + 2MHz 8502, then we would all love to know how!!! (well, at least me)
|
|
|
Post by robertb on Jun 6, 2015 9:05:19 GMT
mirkosoft said: > Really wrong decision of Commodore 128 designers and really only for compatibility with C64 is VIC2E chip as one of two display chips. Meaning that VIC2E can > work only at 1MHz... I do not consider it a wrong decision. I say it was the best decision Bil Herd, Dave Haynie, and the others had at the time, considering the objective of the C128 project -- to make a super C64 which eventually would go up against the Apple IIc and the PCjr, to make a computer that had nearly 100% compatibility with the C64 after the incompatibilities with the Plus/4. In "On the Edge: the Spectacular Rise and Fall of Commodore," you can read about all the massive problems they had just to create the machine with what the chips and engineering they had available. Their efforts were truly heroic. As a small side note, here's a quote from the above book (page 392): "In [Robert] Russell's opinion, MOS Technology lacked the capability to produce a VIC-III chip. 'We didn't have the personnel when it was needed,' says Russell. 'We could do a simple 80-column display chip, but we couldn't do an integrated one.'" Truly, Robert Bernardo Fresno Commodore User Group www.dickestel.com/fcug.htmJuly 18-19 Commodore Vegas Expo v11 2015 - www.portcommodore.com/commvex
|
|