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    Early Macintosh home brew 4MB memory upgrade board development

    Haha, I hear you on the 'Mac Plus mode' satisfaction. It's like those old-school RPGs where you grind for hours to learn a super-powerful spell, even though a basic attack works just fine. I guess there's just something about the challenge and the sense of accomplishment, right? Glad we were...
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    Early Macintosh home brew 4MB memory upgrade board development

    Don't toss it yet! The RAM refresh configurator board I published is able to generate 1,024 refresh cycles according to the encoding you produce. I think we should test the 1024 refresh mode with a PCB prototype for the 2 MB RAM ICs we both have.
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    Macintosh 128/512 board resurrection - checkerboard pattern

    sorry, didn't read that you already did all that.
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    Early Macintosh home brew 4MB memory upgrade board development

    I have uploaded an updated PCB design (Version 2.1) for the expansion RAM board to provide adequate clearance for the integration of a MacSnap SCSI board. No annotations or functional changes have been made to the core circuitry. BOM files have also been added.
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    Early Macintosh home brew 4MB memory upgrade board development

    Finally, I can share the fabrication files for my RAM expansion board. You can now build your own 4MB RAM upgrade and breathe new life into your Early Mac!. Check out the full guide and download the files here I made my best effort to assure it will work on a Macintosh 128K with all LB...
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    Early Macintosh home brew 4MB memory upgrade board development

    Yes, I posted the patched ROM here
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    Macintosh 128K board with custom 512KB upgrade

    Definitively the IC on the floating board is a custom chip (PAL) for demultiplexing and for /CAS signals generation. For 1MB of RAM with 32 256Kb x 1 bit ICs it needs 2 pairs of CAS signals. Each pair controls a 512KB memory bank of 16 bit address bus. Identify the inputs from the LB. For sure...
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    Macintosh 128K board with custom 512KB upgrade

    Consider identifying the input and output signals directly from the logic board (LB) and the floating board.
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    Macintosh 128K board with custom 512KB upgrade

    The differences you've observed compared to the Dr. Dobbs article are expected, as your LB is an earlier revision and lacks some components layout like there is no footprint for U13G and R42. Additionally, RP3 has been removed to provide clean access to /CAS0 and /CAS1. Please note that RP1 is...
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    Early Macintosh home brew 4MB memory upgrade board development

    I finally had the time to fully assemble the second version of the prototype boards. Stock and MacPlus-style refresh cycle modes work flawlessly as before, now with full 4MB! I promise to make the fabrication files for version 2 of these boards available on GitHub soon. Just for fun, I'm...
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    Early Macintosh home brew 4MB memory upgrade board development

    Your interpretation is correct! So in summary, the non-contiguous memory problem in early Macs was primarily due to a limitation in the original ROM design. The introduction of the 128K ROM, with its ability to dynamically adjust the screen buffer pointer, addressed this issue. Therefore, it...
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    Early Macintosh home brew 4MB memory upgrade board development

    Thank you, it works as expected with 2MB of RAM. What the app does is setting a RAM disk of the preferred size and copying a selected folder into it during boot-up. It also lets you reserve a desired RAM size for cache.
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    Early Macintosh home brew 4MB memory upgrade board development

    Quite a mystery, those memory ranges. I will try to dig into the Mac Plus disassembly looking for them. ** I haven't yet populated the RAM ICs to reach 3.5MB of RAM expansion; that's probably why it's working with 1.5 MB and the 74259 enabled. I read something here about memory map differences...
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    Early Macintosh home brew 4MB memory upgrade board development

    Dove only implemented the 74259 in 'E' board models (524E, 548E), which were advertised for the Macintosh 512KE. No software for configuration came with those boards. As JoopMac pointed out, the only software that came with all MacSnap models was an app to create RAM disks.
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    Early Macintosh home brew 4MB memory upgrade board development

    The macSnap board doesn't use pull-ups for A21, A22, A23. So, I guess I use it at some point testing the circuit. I eliminate them and everything works fine.
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    Early Macintosh home brew 4MB memory upgrade board development

    A little update: The 74259 sub-circuit can now be activated without causing issues. The Mac boots normally, recognizes the RAM expansion, and seems to function correctly. The only changes I made since the prototype build were to add pull-ups to A22 and A23 and patch the code of the ROM-INATOR...
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    Early Macintosh home brew 4MB memory upgrade board development

    Golden Potato, I've tried your proposed ROM patch and can confirm that it works as intended! Now my Mac 512K boots with a 1.5MB RAM expansion and the ROM-INATOR. Once again, you nailed it! Joopmac, here is the patched ROM image and its full disassembly. I burn it with Flash Tool 1.5 directly on...
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    Early Macintosh home brew 4MB memory upgrade board development

    I've actually got some experience programming Intel microcontrollers in assembler from the 80s, but I'm a bit lost when it comes to Mac ROMs. I'd love to learn more about the specific software and tools needed to decode a Mac ROM and make the necessary changes. Could you point me in the right...
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