On a side note, if the Mega II was NOT responsible for Apple IIe emulation, and mainly just used as an I/O controller that bottlenecked
the IIGS bus and video draws to 1 MHz, one questions why on earth
Apple didn't scrap the Mega II and design a replacement chip for the
IIGS in all those years? It seems like it was added to the IIGS simply because it happened to be sitting unused in Apple's development
tool box (the Mega II was originally developed for other purposes).
---
It's possible that everybody, including Apple, has been wrong for 37
years but extraordinary claims require extraordinary evidence.
Kelvin Sherlock wrote:
---
It's possible that everybody, including Apple, has been wrong for 37
years but extraordinary claims require extraordinary evidence.
I found articles going back to the 1986 release of the IIgs that described the Mega II as a "complete Apple II on a chip." Most likely someone made
an
assumption about the chip, and it spread around, was repeated, and stuck
for 37 years.
Question: How *exactly* does an Apple IIGS emulate the 8-bit Apple IIe?
From page 27 of the Custom ICs document:
"The Mega II, shown in Figure 10, has virtually all the characteristics of
an Apple II on a chip; it supports a slotted architecture and has built-in >peripherals."
-+-
But I think it got corrupted into "a complete Apple IIe on a chip," which >definitely isn't correct.
D Finnigan <dog_cow@macgui.com> wrote:
From page 27 of the Custom ICs document:
"The Mega II, shown in Figure 10, has virtually all the characteristics of >>an Apple II on a chip; it supports a slotted architecture and has built-in >>peripherals."
-+-
But I think it got corrupted into "a complete Apple IIe on a chip," which >>definitely isn't correct.
The Mega II *is* a complete Apple IIe on a chip. It's just, for the
most
part, missing the CPU and ROM firmware.
The Mega II apparently just sits in the IIGS performing unrelated I/O tasks, NOT emulating an Apple IIe (the fact that it can, is wasted). A
part of it is used for providing classic Apple II text and video modes,
but
that may be it. Which again begs the question....if it isn't the Mega II, what logic allows the Apple IIGS to emulate an Apple IIe?
Question: How *exactly* does an Apple IIGS emulate the 8-bit Apple IIe?
For decades the answer has seemingly been the Mega II chip, essentially
an entire 8-bit Apple IIe computer on a single chip (minus the CPU, RAM
and ROM). In fact Apple's marketing and technical documentation always >pointed to the Mega II as how the Apple IIGS was backwards compatible.
I've always know the Mega II is what provides classic Apple II video
modes for the IIGS (40/80 ASCII, including Mousetext and international >symbols, LR, HR, DLR, DHR). And while some functions are not used
such as its keyboard control and mouse support, I just assumed the rest
was responsible for the Apple IIGS's 8-bit emulation mode....
Then I saw James Lewis' project to build a single chip Apple II, and
read his claim the Mega II's primary (sole?) function is to provides 8-bit >video modes. So, that got me curious. What, exactly, is allowing the
IIGS to emulate the Apple IIe? Such as its sound generation, or replicating >the MMU, IOU and various other components? Is it simply the FPI/CYA
chipset and some other TTL logic recreating the Apple IIe? If none of it
is coming from the Mega II, I'm looking for the real nitty gritty in terms of >technical details on how the Apple IIGS emulates an Apple IIe.
On a side note, if the Mega II was NOT responsible for Apple IIe
emulation, and mainly just used as an I/O controller that bottlenecked
the IIGS bus and video draws to 1 MHz, one questions why on earth
Apple didn't scrap the Mega II and design a replacement chip for the
IIGS in all those years? It seems like it was added to the IIGS simply >because it happened to be sitting unused in Apple's development
tool box (the Mega II was originally developed for other purposes).
Mitchell Spector
Kelvin Sherlock wrote:
---
It's possible that everybody, including Apple, has been wrong for 37
years but extraordinary claims require extraordinary evidence.
I found articles going back to the 1986 release of the IIgs that described >the Mega II as a "complete Apple II on a chip." Most likely someone made an >assumption about the chip, and it spread around, was repeated, and stuck for >37 years. This happens in every knowledge domain, not just with vintage >computers.
Here's a post from September 1986 that asks if the Mega II is an emulator >chip:
https://macgui.com/usenet/?group=6&id=3010
Here's a reference to an October 1986 BYTE Magazine article about the Mega >II:
https://macgui.com/usenet/?group=1&id=3626
THE MEGA II
The Mega II is a custom CMOS chip containing about 3000 gates and a 2K-byte >by 8 ROM (for the character generator). It replaces the following chips from >the Apple IIe and IIe: character generator ROMs for eight languages. >several TTL chips that perform logic functions. and the MMU (memory >management unit). IOU (input/output unit). TMG (timing generator). and GLU >(general logic unit) custom chips.
In previous Apple II designs, the refreshing of memory was tied directly to >the Apple II video mode. The Mega II includes an 8-bit counter for >refreshing the 128K bytes of (slow) memory associated with the Apple >IIe/IIc model; it does five cycles of RAM refresh during the horizontal >retrace of each video scan line and refreshes the 128K bytes of memory in >3.25 milliseconds. By taking care of RAM refresh the Mega II chip opens the >Apple II design to new video modes that were impossible before.
In article <kvrgsipj5orb244msd7nk4l0agt76gr3c7@4ax.com>,[snip]
Mitchell Spector <mitch2gs@hotmail.com> wrote:
Then I saw James Lewis' project to build a single chip Apple II, and >>read his claim the Mega II's primary (sole?) function is to provides 8-bit >>video modes. So, that got me curious. What, exactly, is allowing the
IIGS to emulate the Apple IIe?
Here's how the IIgs works:
So, the FPI is the magic that talks to the slow Apple II side. The Mega II >is just one of the things it talks to, which is just most of Apple //e
logic rolled into one chip to take less board space.
The Mega II is MMU, IOU, and non-SHR video generation on a IIgs, but
only for banks $E0 and $E1. So the FPI has to ALSO have the MMU logic
in it since it affects fast side memory as well. One way to think of
the Mega II is it controls the memory in banks $E0 and $E1, and that is
the Apple //e part, so it's doing the MMU features for that memory. But
the FPI is handling all other banks, so it has replicated the MMU logic,
too (aux/main and LC switches are all in the FPI, too, as well as the
Mega II). The FPI just replicates the needed logic to get the Apple //e >compatibility right. The FPI handles shadowing, passing writes to the
video memory in banks $00 and $01 to also go to the Mega II to update
the bank $E0 and $E1 memory. So this is likely where some confusion >arises--the FPI also has to do lots of things for Apple //e
compatibility, and does it in parallel, duplicating some Mega II >functionality.
This is unlike the Gemini chip on the Apple IIe PDS card. In that situation this Apple IIe-on-a-chip takes over the host machine and
the Macintosh literally becomes an Apple IIe (for the most part, video
of course is emulated by QuickDraw, and there is still a custom GUI
control panel accessible when it's suspended).
I think the misconception has been: when the IIGS goes into 8-bit emulation mode, the Mega II kicks in and takes over full control like
the Gemini--it does NOT. Likely why, perhaps, some IIGS-specific
features can still be accessible under Applesoft BASIC (through peeks
and pokes, I've seen the Ensoniq and SHR graphics used). Or how
there's been hybrid games possible, essentially 8-bit software with
some glorified audio/visual effects mixed in (e.g. Paperboy, Gauntlet,
John Madden's Football).
In article <dog_cow-1708091984@macgui.com>,
D Finnigan <dog_cow@macgui.com> wrote:
Here's a reference to an October 1986 BYTE Magazine article about the Mega >>II:
https://macgui.com/usenet/?group=1&id=3626
THE MEGA II
The Mega II is a custom CMOS chip containing about 3000 gates and a
2K-byte
by 8 ROM (for the character generator). It replaces the following chips >>from
the Apple IIe and IIe: charÂacter generator ROMs for eight lanÂguages. >>several TTL chips that perÂform logic functions. and the MMU (memory >>management unit). IOU (inÂput/output unit). TMG (timing generaÂtor). and
GLU
(general logic unit) custom chips.
In previous Apple II designs, the reÂfreshing of memory was tied directly >>to
the Apple II video mode. The Mega II includes an 8-bit counter for >>refreshÂing the 128K bytes of (slow) memory associated with the Apple >>IIe/IIc model; it does five cycles of RAM refresh during the horizontal >>retrace of each video scan line and refreshes the 128K bytes of memory in >>3.25 milliseconds. By taking care of RAM refresh the Mega II chip opens
the
Apple II design to new video modes that were impossible before.
I don't know the source of the above paragraph and what it's trying to
say,
but I do not believe it is correct.
Kent Dickey wrote:[snip]
In article <dog_cow-1708091984@macgui.com>,
D Finnigan <dog_cow@macgui.com> wrote:
In previous Apple II designs, the reÂfreshing of memory was tied directly >>>to
the Apple II video mode. The Mega II includes an 8-bit counter for >>>refreshÂing the 128K bytes of (slow) memory associated with the Apple >>>IIe/IIc model; it does five cycles of RAM refresh during the horizontal >>>retrace of each video scan line and refreshes the 128K bytes of memory in >>>3.25 milliseconds. By taking care of RAM refresh the Mega II chip opens >>>the
Apple II design to new video modes that were impossible before.
I don't know the source of the above paragraph and what it's trying to
say,
but I do not believe it is correct.
The source is the October 1986 BYTE Magazine article by Gregg Williams, >senior technical editor at BYTE.
It's on page 86 of the issue of BYTE magazine dated October 1986.
--
]DF$
The New Apple II User's Guide:
https://macgui.com/newa2guide/
Sorry, I didn't mean to imply I doubted that BYTE magazine published those statements in 1986. I meant I'm not sure what his source was, but I
believe he is mistaken.
Kent Dickey wrote:I found this to help understand what it all means. Thanks. A2ForEver 73 ve3myz James
Sorry, I didn't mean to imply I doubted that BYTE magazine published those statements in 1986. I meant I'm not sure what his source was, but I believe he is mistaken.What was the author's source? Let's assume that Gregg Williams wrote his article in September 1986. Apart from his own direct investigation, how many sources were even available in September 1986? He could have asked some Apple employees, but he also could have referred to the Cortland documents or similar pre-release developer's documentation.
--
]DF$
The New Apple II User's Guide:
https://macgui.com/newa2guide/
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