Gregory's Server|
133 comments
  Meme image: good news bad news.jpg I have mainly reverse engineered things with user interfaces, so there’s always strings I can work backwards from. Doing something like a hard drive controller is playing the game on extra hard mode. so last night I identified the power on self test routines by inspection. it's not too hard to identify a checksum routine or a memory test routine. this helped me fill in the memory map. also, the POR test function stores the results at a particular memory location, and the codes match up with the POR error codes in the DBA-ESDI spec! the next step is to search the whole ROM for any instructions that read this memory location--this should identify the functions that generate the status block.  @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  @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 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 What is that yellow tape? See that sometimes on Electronics but don't know where to buy or whats special about it. |
@tubetime you don’t drink enough coffee. ;-)