Classic computing fanatic Ted Fried has launched one other Teensy 4.1-powered drop-in substitute for a basic computing chip, this time turning his focus to the Motorola 68000 and its native bus interface.
“The MCL68+ is a Motorola 68000 drop-in substitute emulator. It makes use of the Teensy 4.1 to emulate each the microprocessor and the native bus interface at full velocity,” Fried explains of his newest creation. “It’s the most formidable drop-in emulator I’ve designed to this point, and I’m happy with the progress I used to be in a position to obtain.”
Considered one of Fried’s hobbies is to make use of comparatively high-performance however low-cost microcontroller boards to create drop-in replacements for processors from basic computer systems, generally to work round shortages of unique elements however usually to crank up the velocity and enhance efficiency or add lacking options. Earlier tasks have included the MCL64 improve chip for the Commodore 64, the MCL86jr for the IBM PCjr, and the MCL65+ — uncommon for having truly “downgraded” an Apple II into an Apple I appropriate.
This time, Fried targeted on the Motorola 68000 — a component greatest identified for driving Apple’s Macintosh household of computer systems in addition to Commodore’s Amiga vary. Given the complexity of the half, it proved a problem — leading to an emulation which Fried says is “the quickest bit-banged native bus velocity that I’ve ever achieved” on the full 7.8MHz of the unique {hardware}.
“It required numerous methods resembling pre-calculating addresses and utilizing arrays to map GPIO [General-Purpose Input/Output] values to the Teensy’s output registers,” Fried explains. “The Motorola 68000 additionally required extra pins than had been out there on the Teensy 4.1, so I used a sequence of byte-wide latches to shift out the tackle and knowledge busses. I additionally wanted to multiplex just a few different indicators to squeeze all of it onto the Teensy’s pins.”
The board is designed to sit down instead of an unique Motorola 68000 processor in a basic Apple Macintosh. (📷: Ted Fried)
The {hardware} facet of the venture is already full, however there are just a few kinks to iron out within the emulator working atop it. “The progress all the time stops right here,” Fried says of a video displaying an MCL68+ booting an Apple Macintosh 512k part-way into the working system, “however I’m able to transfer the cursor round with the mouse and reboot the machine, so the system just isn’t locked up. I traced the error to an tried subroutine leap to a misaligned tackle, however I’ve not had the time to hint the code backwards to search out the place this tackle comes from.”
Extra particulars on the MCL68+ can be found on the Microcore Labs weblog; the venture supply code and {hardware} design recordsdata are printed to GitHub below an open supply license.