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In 1986, Apple launched their final main Apple II laptop, the Apple IIgs. Constructed on virtually a decade of 8-bit legacy, the IIgs featured a 16-bit microprocessor, superior graphics, improved sound capabilities, and built-in peripherals. It additionally featured near-100% backward compatibility with the earlier 8-bit Apple IIs. Most paperwork attribute this backward compatibility to an software particular circuit (ASIC) known as the Mega-II. It condenses about 80 units price of 7400-series digital logic right into a single built-in chip. Some sources go as far as to name it an “Apple IIe on a chip.” This description acquired me pondering, is it attainable to construct an Apple IIe-compatible laptop from it? So, on this element14 Presents undertaking video, I managed to Construct (and Boot) the World’s First “Single-Chip” Apple II.
The element14 video reveals my progress whereas constructing three printed boards containing a contemporary WDC 65c02 microcontroller, a 512 kilobyte electrically erasable ROM, 128 kilobytes DRAM organized in two 64 kilobyte banks, a video circuit, and the Mega-II. Utilizing this minimal {hardware}, I found some particulars concerning the Mega-II.
Mega-II?
Mega-II combines the reminiscence controller, IO controller, clock era, glue logic, and an “80 column card” from the Apple IIe right into a single 84-pin chip. In the event you evaluate the identify of the pins to an Apple IIe schematic, you will discover many similarities. Nonetheless, there are additionally a number of important variations!
Mega-II in Rev 1 Prototype (📷: Bald Engineer)
For instance, the Mega-II has a sign known as “Mega-II Choose (M2SEL).” The IIgs reminiscence controller (CYA/FPI) makes use of it to disable the Mega-II’s decode logic. In different phrases, no matter legacy 8-bit or newer 16-bit software program is operating, the IIgs’s chipset handles all RAM, ROM, IO, and delicate switches, not the Mega-II! One other instance is that Mega-II has no IOSEL alerts. Due to this fact, it’s not capable of straight entry a person enlargement slot. As a substitute, Mega-II has to speak to a different IIgs ASIC known as Slotmaker over a devoted bus to speak to IO units.
I wouldn’t have definitive proof, however I believe that is what occurred. Apple’s engineers meant the Mega-II to enter a standalone Apple II laptop. Nonetheless, its design modified to accommodate the altering laptop world. Consequently, Mega acquired relegated to being an IO controller and Apple II video generator. The IIgs’s chipset and ROM deal with all the legacy code compatibility! Mega-II shadows reminiscence from the IIgs quick facet to breed the distinctive Apple II video modes. Mega makes use of its personal RAM financial institution to generate a video bitstream that the IIgs’s extra superior Video Graphics Controller ASIC converts to RGB values.
The video modes are the one “backward compatibility” that the IIgs design makes use of! (To be clear, the Mega-II has different features, however they aren’t “Apple IIe” associated.) In different phrases, regardless of having all the II, II+, and IIe logic constructed into it, the IIgs design solely makes use of a small fraction of the Mega-II.
So I began to marvel then might this single-chip even grow to be an Apple IIe?
Single-Chip?
As any astute particular person would level out, the Mega-II is just not really a single-chip something! At a minimal, you must add RAM, ROM, and a CPU to make use of it. As well as, as I discussed, the Mega wants the Slotmaker, or a bunch of glue logic, for card IO. Moreover, you want *one thing* for video output and one thing else for keyboard enter. So then why is it ever known as a “Single-Chip?”
Slotmaker in a prototype board (📷: Bald Engineer)
The outline is historic. Within the Eighties, many designers thought-about their customized logic design to be “the pc.” They hooked up their “laptop” to an off-the-shelf CPU. For instance, whereas Commodore, Atari, and Apple all used a 6502 processor, little or no software program is suitable. That incompatibility is the customized chips constructed across the CPU (and ROM.)
Sadly, as a result of the Mega-II wants at the least one different ASIC or a bunch of glue logic, I’d agree it’s not a “Single-Chip Apple IIe.” Once I realized this, I began to fret, might the Apple IIe even run standalone? What else is lacking?
Standalone?
After at the least 80 hours of effort and lots of failures, I began to query if the Mega-II might even boot as a pc. There have been no paperwork or examples to counsel engineers completed the IIe logic. The one trace was a associated chip known as Gemini. It powers the Apple IIe compatibility card for the Macintosh LC line.
Apple IIe Compatbility Card (📷: Bald Engineer)
Nonetheless, I wouldn’t have a doc itemizing the distinction between Mega-II and Gemini. For all I knew, the engineering crew named the brand new chip Gemini as a result of it mounted recognized points with the Mega-II. (Or perhaps the identify symbolized the complementary Yin and Yang of 8-bit Apple II and 32-bit Macintosh know-how.)
To not spoil the video’s climax, however the chip finally boots. There’s a small diagnostic routine constructed into the Apple IIe ROM. When no keyboard is hooked up, it runs routinely. Since I had no keyboard, the check ran after which concluded my pile of wires was a useful Apple IIe.
However, how a lot performance does it have? Nicely, that’s the subsequent part of the undertaking.
For extra data
The aim of Rev 2 is to fill out the remainder of the first I/O units: keyboard, joystick, disk, and digital video. Then, Rev 3’s focus is making a miniature model of the {hardware} with the common-or-garden Mega-II at its core.
In case you are to study extra or wish to observe alongside, there are a few sources for extra data. First is this GitHub repo. It accommodates all the KiCad design information for the Rev 1 prototype I confirmed within the video and the Rev 2 prototype I designed since reaching the primary milestone.
One other place to examine is on the element14 Group. These posts are associated to the undertaking, and I will be including extra particulars as I transfer towards the ultimate aim.
Final, virtually the whole undertaking has been streamed and archived on Twitch. There are effectively over 100 hours of content material out there now for nearly each facet of the undertaking!
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