Gregory's Serverdrive firmware is turning into a bit of a slog so i switched over to the IBM BIOS. having a spec is nice, but the code will cover a bunch of corner cases.
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  @tubetime I've lost count of the things I've done that "sounds easy but the process uses DMA."  @tubetime  @tubetime Almost makes you want to lovingly caress it's slightly sticky chemically degraded surface. 😉   The logic analyzer from the perspective of the PCB: https://www.youtube.com/watch?v=nYPX-QQR9Tg @tubetime it’s been so long since this experiment started I forget what you’re trying to do! so i don't know how this flag is getting set. my hack is to preemptively clear the flag right before starting DMA, and so far, it seems to be working. i think this code was "working" with the real ESDI drive because that one uses burst mode DMA and it finishes up very quickly, before the irq14wait routine can exit early.  @tubetime When you find yourself asking, "How did this ever work?" and it turns out the answer is "It didn't." @tubetime I've used that library before, yes it really is easy. You probably won't set any throughput records, but it was great for writing diagnostic logs that were later read back and uploaded. @tubetime What’s the end goal here, boot the laptop using a SD card that’s emulating a Micro Channel hard drive interface, via Teensy? @tubetime I know it’s not the point of your project, but didn’t this laptop have PCMCIA? Would one of these work as a boot drive? Digigear SD SDHC SDXC to PCMCIA PC Card, Adapter Supports, ATA Flash Memory https://a.co/d/agF7LO9 my drive doesn't implement this weird feature called pseudo RBAs--it's a way to artificially limit the maximum possible block address, presumably so they can hide the partition data. i suspect the BIOS checks this, so i'll have to implement it. ugh. that means i need to figure out this incomprehensible diagram. @tubetime how does that even work? After the first iteration isn't the refresh bit always set, so it's stuck in an infinite loop if it never IRQs? @SteveSyfuhs oh sorry REFRESH_BIT is a macro that grabs an IO port bit that toggles with the DRAM refresh signal every 15us. @tubetime @tubetime I implemented PS/2 DMA in an emulator good enough to get a Sound Blaster working a while ago and god help us all @luigithirty nice, you didn't even have to get burst mode working 😉 |
@tubetime Out of curiosity, does the newer IBM BIOS have any similarity to the original IBM PC AT BIOS that they published in the AT Tech Reference from March 1984? Wondering if they ever did a full rewrite at some point. http://bitsavers.trailing-edge.com/pdf/ibm/pc/at/1502494_PC_AT_Technical_Reference_Mar84.pdf