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Tube🍂Time

managed to reverse engineer enough that I was able to read the defect map out of one of the original hard drives. sounds easy but the process uses DMA.

120 comments
🇺🇦 haxadecimal replied to Tube🍂Time

@tubetime I've lost count of the things I've done that "sounds easy but the process uses DMA."

Tube🍂Time replied to Tube🍂Time

working through a nasty timing hazard with the mailbox flags on the command port. sometimes you write data and the "data available" flag never gets set.

so now i am digging through the logic that yosys generated to see if it even makes sense.

Tube🍂Time replied to Tube🍂Time

having good test programs is important. here's the status interface register dropping values. the Teensy program is just writing an incrementing number, and the diagnostics program is checking for gaps.

Douglas King replied to Tube🍂Time

@tubetime
If a computer can't count straight, it's not a good sign.

Tube🍂Time replied to Tube🍂Time

got that all sorted out. it was a synchronization issue with the flags between the two interfaces.

this is the "seek" command successfully completing! this is a *major* step since it requires 4 working mailboxes and interrupts.

Tube🍂Time replied to Tube🍂Time

another important step today--i got the data port and data port mailbox flags working. it can also detect 8-bit vs 16-bit transfers. getting very close to working PIO transfers.

Tube🍂Time replied to Tube🍂Time

nice! I managed to get PIO data transfers working well enough for the buffer test routine to pass.

Tube🍂Time replied to Tube🍂Time

ok this is fantastic--I've managed to transfer my first actual sector! it's just using PIO and the data is not from a real filesystem, but this is another big step forward!

Darryl Ramm replied to Tube🍂Time

@tubetime Almost makes you want to lovingly caress it's slightly sticky chemically degraded surface. 😉

Tube🍂Time replied to Tube🍂Time

DMA on Micro Channel is really hard. i'm running a bunch of simulations first, making adjustments to the logic as needed.

so many moving parts.

Briala replied to Tube🍂Time

@tubetime I wonder if IBM expected you to use a special chip that did a lot of that for you.

Tube🍂Time replied to Tube🍂Time

wow, got four bytes to transfer successfully over DMA! not sure why it got stuck after that.

Tube🍂Time replied to Tube🍂Time

just ran the same test again and it transferred the whole sector over DMA!!

so at least read transfers are working partially. writes just hang the machine after transferring half a sector. it's probably time for the logic analyzer.

Tube🍂Time replied to Tube🍂Time

not sure why I always end up in front of a logic analyzer, but here we are.

Tube🍂Time replied to Tube🍂Time

several issues. this first issue, during a host to device write, holds the arbitration bus too long. it should release immediately after the second arb/gnt pulse

Tube🍂Time replied to Tube🍂Time

had a theory and it reproduces in simulation. the transfer request flag isn't getting cleared soon enough. la_dma_selected is what can clear this flag and it is changed on the falling edge of cmd, which is too late to catch the ARB/GNT pulse.

Tube🍂Time replied to Tube🍂Time

yes, that solves the crashing problem. but data isn't getting transferred correctly, so I've got more work to do.

Tube🍂Time replied to Tube🍂Time

weirdly enough, it works the second try!!! something on the host was prematurely turning off DMA. maybe a bug in difdiag.

Jonathan Flusser replied to Tube🍂Time

@tubetime it’s been so long since this experiment started I forget what you’re trying to do!

Tube🍂Time replied to Tube🍂Time

so the interrupt_detected flag is supposed to be set in the irq14 handler, and it is *supposed* to be set only when DMA is done. but somehow interrupt_detected is set without the IRQ handler ever being called! then the DMA operation is broken down prematurely.

Tube🍂Time replied to Tube🍂Time

using the logic analyzer, i proved that the irq14 handler never gets called. the only code that *ever* sets the interrupt_detected flag exists in this handler. it's declared as a volatile so it can't be cached in a register.

Tube🍂Time replied to Tube🍂Time

I wrote the flag value out to an unused IO port, 0x4F, so I can see it on the logic analyzer. a neat trick!

Tube🍂Time replied to Tube🍂Time

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.

Tube🍂Time replied to Tube🍂Time

decided 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.

Brian Danger Hicks replied to Tube🍂Time

@tubetime When you find yourself asking, "How did this ever work?" and it turns out the answer is "It didn't."

Tube🍂Time replied to Tube🍂Time

now I'm reading up on accessing SD cards from the Teensy 4.1. looks like SdFat is the library? could it be so easy?

Chris Hammond replied to Tube🍂Time

@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.

Tube🍂Time replied to Tube🍂Time

turns out it's easy but I had to reformat the SD card using the official sdcard.org utility. anyway, I've read the first sector from a real disk image!

Tube🍂Time replied to Tube🍂Time

the drive now gives the POS ID. let's try to boot!

Tube🍂Time replied to Tube🍂Time

hmm 01048200 is a drive select acknowledgement error.

Tube🍂Time replied to Tube🍂Time

the BIOS runs faster than the DIFDIAG utility, and so it seems like it is hitting a timing problem that i didn't hit before.

my drive code seems to randomly hang up and not respond correctly.

Tube🍂Time replied to Tube🍂Time

it's occasionally getting a spurious end-of-interrupt command which is really odd and points to an issue with the mailboxes (again, sigh).

but it's SO DARN CLOSE. it's transferring sectors from the IML region in the disk image.

Daniel Schwarz :toad: replied to Tube🍂Time

@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?

Daniel Schwarz :toad: replied to Tube🍂Time

@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 a.co/d/agF7LO9

Tube🍂Time replied to Tube🍂Time

figured out one problem. the disk boot routines slam the drive with an ATN and the first command word in 5.5us. the Teensy code takes too long to see the ATN and clears the command register full flag, which drops the first word. oops.

Tube🍂Time replied to Tube🍂Time

so it *almost* boots now. in fact it successfully loads the IML sectors from the hidden partition on the drive, and no longer throws an I999... error code!

Tube🍂Time replied to Tube🍂Time

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.

Tube🍂Time replied to Tube🍂Time

holy crap it's booting I can't believe it sdfadfsdfsdfsffasdf

Tube🍂Time replied to Tube🍂Time

well, it's working well enough to run qbasic. right now the drive is read-only.

Steve Syfuhs replied to Tube🍂Time

@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?

Tube🍂Time replied to Steve

@SteveSyfuhs oh sorry REFRESH_BIT is a macro that grabs an IO port bit that toggles with the DRAM refresh signal every 15us.

Steve Syfuhs replied to Tube🍂Time

@tubetime ahhhh that makes way more sense now

Netux replied to Tube🍂Time

@tubetime
Ok, I'm a bit confused. You have an fpga you are worthing on, but also a hard drive you are trying to read and another cup you have a white paper for? The drive controller chip?
I looks like you are writing notes for yourself.
I'm guessing you are an EE? Or are you a CE? Us normal CS guys don't ever get to the logic analyzer.

Tube🍂Time replied to Netux

@Netux it's a solid state hard drive replacement that I'm designing. EE but I do some digital stuff too

Katherine the Sixth replied to Tube🍂Time

@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

Tube🍂Time replied to Katherine the Sixth

@luigithirty nice, you didn't even have to get burst mode working 😉

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