Gregory's Serverdecided to look at the real drive. and guess what--it's not using burst mode. the POS registers have it turned off by default. it's also slow to read the data from the spinning disk, so IBM must have figured that it wasn't really necessary.
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84 comments
 @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 so here's what i think happened:  figured it out and fixed it. i forget to set the "transfer request" flag to kick off DMA. in another routine, it sees that this flag is clear and assumes that a word has already been read using DMA, so it reads a crap value and then sets the transfer request flag again to start the next DMA transfer. that "crap value" pushes the valid data forward by one word. on to the next issues: randomly the ATN register mailbox flag gets set but the data in it is stale. also, the status interface register will randomly get read from by the host. I think these are two facets of the same problem: the mailbox flags sometimes respond when you access a register that they are not supposed to be monitoring! the problem? the line (la_addr == REG_ATN) creates a bunch of gates that are slightly slower than the simple AND gates in the previous part of the line. so la_mca_op=1, ~la_s0_w_l=1, and (la_addr == REG_ATN) *is also a 1 for a very short time!!!* this is because the previous value of la_addr WAS a REG_ATN. what i need to do is take that entire wire and turn it into a latch (a reg) and clock it on cmd. so here's the solution: all the signals in the MCA bus domain go to a latch clocked in that domain (the first "always" block). then *without any combinational logic* the output of that latch goes *directly* to another latch (the second "always" block) located in the main clock domain. (i have another flip flop in main clock domain just for detecting the edge) @tubetime This was really enjoyable to follow along with! Congrats on the progress  @tubetime "No command can not access" so.. uh.. commands can access? 🤯 @tubetime Your project is an ESDI drive emulator, right? (specific drive type, but ESDI interface) Would it work in another computer that had an ESDI controller and understood the IBM drive? (I assume so, but then you mentioned microchannel which confused me -- I'm assuming the "creaky old IBM laptop" interface is ESDI?)  @tubetime Your recent posts make me believe you’re a time traveller - perhaps the great-great-grandson of an IBM engineer who as a child found an old notebook complaining about this problem your grandpa just couldn’t solve and that he was fired for and altered the arc of his life. You studied your whole life as an engineer for this moment to come back to today when the hardware was still available, fix the problem, then go back and help your grandpa. Let us know how it turns out. @amart not too far off! my grandfather worked there. i once reverse engineered a prototype floppy drive he worked on, and got it working again. https://twitter.com/TubeTimeUS/status/1617703291483467776 |
@tubetime When you find yourself asking, "How did this ever work?" and it turns out the answer is "It didn't."