Monday, December 28, 2020

Repairing and bootstrapping an IBM PC/AT 5170, Part 3

So! In Parts 1 and 2 I covered getting this old thing cleaned up, getting it booting, hacking together a boot floppy disc and getting a working file transfer onto a work floppy disc.

Today I'm going to cover what it took to get it going off of a "hard disk", which in 2020 can look quite a bit different than 1990.

First up - what's the hard disk? Well, in the 90s we could still get plenty of MFM and early IDE hard disks to throw into systems like this. In 2020, well, we can get Very Large IDE disks from NOS (like multi hundred gigabyte parallel ATA interface devices), but BIOSes tend to not like them. The MFM disks are .. well, kinda dead. It's understandable - they didn't exactly build them for a 40 year shelf life.

The IBM PC, like most computers at the time, allow for peripherals to include software support in ROM for the hardware you're trying to use. For example, my PC/AT BIOS doesn't know anything about IDE hardware - it only knows about ye olde ST-412/ST-506 Winchester/MFM drive controllers. But contemporary IDE hardware would include the driver code for the BIOS in an on-board ROM, which the BIOS would enumerate and use. Other drive interconnects such as SCSI did the same thing.

By the time the 80386's were out, basic dumb IDE was pretty well supported in BIOSes as, well, IDE is really code for "let's expose some of the 16 bit ISA bus on a 40 pin ribbon cable to the drives". But, more about that later.

Luckily some electronics minded folk have gone and implemented alternatives that we can use. Notably:

  • There's now an open-source "Universe IDE BIOS" available for computers that don't have IDE support in their BIOS - notably PC/XT and PC/AT, and
  • There are plenty of projects out there which break out the IDE bus on an XT or AT ISA bus - I'm using XT-IDE.
Now, I bought a little XT-IDE + compact flash card board off of ebay. They're cheap, it comes with the universal IDE bios on a flash device, and ...

... well, I plugged it in and it didn't work. So, I wondered if I broke it. I bought a second one, as I don't have other ISA bus computers yet, and ...

IT didn't work. Ok, so I know that there's something up with my system, not these cards. I did the 90s thing of "remove all IO cards until it works" in case there was an IO port conflict and ...

.. wham! The ethernet card. Both wanted 0x300. I'd have to reflash the Universal IDE BIOS to get it to look at any other address, so off I went to get the Intel Etherexpress 8/16 card configuration utility.

Here's an inside shot of the PC/AT with the XT-IDE installed, and a big gaping hole where the Intel EtherExpress 8/16 NIC should be.

No wait. What I SHOULD do first is get the XT-IDE CF card booting and running.

Ok, so - first things first. I had to configure the BIOS drive as NONE, because the BIOS isn't servicing the drive - the IDE BIOS is. Unfortunately, the IDE BIOS is coming in AFTER the system BIOS disks, so I currently can't run MFM + IDE whilst booting from IDE. I'm sure I can figure out how at some point, but that point is not today.

Success! It boots!

To DOS 6.22!

And only the boot sector, and COMMAND.COM! Nooooo!

Ok so - I don't have a working 3.5" drive installed, I don't have DOS 6.22 media on 1.2MB, but I can copy my transfer program (DSZ) - and Alley Cat - onto the CF card. But - now I need the DOS 6.22 install media.

On the plus side - it's 2020 and this install media is everywhere. On the minus side - it's disk images that I can't easily use. On the double minus side - the common DOS raw disk read/write tool - RAWREAD/RAWRITE - don't know about 5.25" drives! Ugh!

However! Here's where a bit of hilarious old knowledge is helpful - although the normal DOS installers want to be run from floppy, there's a thing called the "DOS 6.22 Upgrade" - and this CAN be run from the hard disk. However! You need a blank floppy for it to write the "uninstallation" data to, so keep one of those handy.

I extracted the files from the disk images using MTOOLS - "MCOPY -i DISK.IMG ::*.* ." to get everything out  - used PKZIP and DSZ to get it over to the CF card, and then ran the upgrader.

Hello DOS 6.22 Upgrade Setup!

Ah yes! Here's the uninstall disc step! Which indeed I had on hand for this very moment!

I wonder if I should fill out the registration card for this install and send it to Microsoft.

Ok, so that's done and now I have a working full DOS 6.22 installation. Now I can do all the fun things like making a DOS boot disk and recovery things. (For reference - you do that using FORMAT /S A: to format a SYSTEM disk that you can boot from; then you add things to it using COPY.)

Finally, I made a boot disk with the Intel EtherExpress 8/16 config program on it, and reconfigured my NIC somewhere other than 0x300. Now, I had to open up the PC/AT, remove the XT-IDE and install the EtherExpress NIC to do this - so yes, I had to boot from floppy disc.

Once that was done, I added a bunch of basic things like Turbo C 2.0, Turbo Assembler and mTCP. Now, mTCP is a package that really needed to exist in the 90s. However, this and the RAM upgrade (which I don't think I've talked about yet!) will come in the next installment of "Adrian's remembering old knowledge from his teenage years!".

Thursday, December 17, 2020

Repairing and bootstrapping an IBM 5170 PC/AT, part 2

Ok, so now it runs. But, what did it take to get here?

First up - I'm chasing down a replacement fusable PROM and I'll likely have to build a programmer for it. The programmer will need to run a bit at a time, which is very different to what the EPROM programmers available today support. It works for now, but I don't like it.

I've uploaded a dump of the PROM here - .

Here's how the repair looks so far:

Next - getting files onto the device. Now, remember the hard disk is unstable, but even given that it's only DOS 5.0 which didn't really ship with any useful file transfer stuff. Everyone expected you'd have floppies available. But, no, I don't have DOS available on floppy media! And, amusingly, I don't have a second 1.2MB drive installed anywhere to transfer files.

I have some USB 3.5" drives that work, and I have a 3.5" drive and Gotek drive to install in the PC/AT. However, until yesterday I didn't have a suitable floppy cable - the 3.5" drive and Gotek USB floppy thingy both use IDC pin connectors, and this PC/AT uses 34 pin edge connectors. So, whatever I had to do, I had to do with what I had.

There are a few options available:

  • You can write files in DOS COMMAND.COM shell using COPY CON <file> - it either has to be all ascii, or you use ALT-<3 numbers> to write ALT CODES. For MS-DOS, this would just input that value into the keyboard buffer. For more information, Wikipedia has a nice write-up here: .
  • You can use an ASCII only assembly as above: a popular one was TCOM.COM, which I kept here:
  • If you have MODE.COM, you could try setting up the serial port (COM1, COM2, etc) to a useful baud rate, turn on flow control, etc - and then COPY COM1 <file>. I didn't try this because I couldn't figure out how to enable hardware flow control, but now that I have it all (mostly) working I may give it a go.
  • If you have QBASIC, you can write some QBASIC!
I tried TCOM.COM, both at 300 and 2400 baud. Both weren't reliable, and there's a reason it isn't - writing to the floppy is too slow! Far, far too slow! And, it wasn't enforcing hardware flow control, which was very problematic for reliable transfers.

So, I wrote some QBASIC. It's pretty easy to open a serial port and read/write to it, but it's not AS easy to have it work for binary file transfer. There are a few fun issues:

  • Remember, DOS (and Windows too, yay!) has a difference between files open for text reading/writing and files open for binary reading/writing.
  • QBASIC has sequential file access or random file access. For sequential, you use INPUT/PRINT, for random you use GET and PUT.
  • There's no byte type - you define it as a STRING type of a certain size.
  • This is an 8MHz 80286, and .. well, let's just say QBASIC isn't the fastest thing on the planet here.
I could do some basic IO fine, but I couldn't actually transfer and write out the file contents quickly and reliably. Even going from 1200 to 4800 and 9600 baud didn't increase the transfer rate! So even given an inner loop of reading/writing a single byte at a time with nothing else, it still can't keep up.

The other amusingly annoying thing is what to use on the remote side to send binary files. Now, you can use minicom and such on FreeBSD/Linux, but it doesn't have a "raw" transfer type - it has xmodem, ymodem, zmodem and ascii transfers. I wanted to transfer a ~ 50KB binary to let me do ZMODEM transfers, and .. well, this presents a bootstrapping problem.

After a LOT of trial and error, I ended up with the following:

  • I used tip on FreeBSD to talk to the serial port
  • I had to put "hf=true" into .tiprc to force hardware handshaking; it didn't seem to work when I set it after I started tip (~s to set a variable)
  • On the QBASIC side I had to open it up with hardware flow control to get reliable transfers;
  • And I had to 128 byte records - not 1 byte records - to get decent transfer performance!
  • On tip to send the file I would ask it to fork 'dd' to do the transfer (using ~C) and asking it to pad to the 128 byte boundary:
    • dd if=file bs=128 conv=sync
The binary I chose (DSZ.COM) didn't mind the extra padding, it wasn't checksumming itself.

Here's the hacky QBASIC program I hacked up to do the transfer:

OPEN "RB", #2, "MYFILE.TXT", 128

' Note: LEN = 128 is part of the OPEN line, not a separate line!
OPEN "COM1:9600,N,8,1,CD0,CS500,DS500,OP0,BIN,TB2048,RB32768" FOR RANDOM AS #1 LEN = 128

size# = 413 '413 * 128 byte transfer
DIM c AS STRING * 128 ' 128 byte record
FOR i = 1 TO size#
  GET #1, , c
  PUT #2, , c

Now, this is hackish, but specifically:
  • 9600 baud, 8N1, hardware flow control, 32K receive buffer.
  • 128 byte record size for both the file and UART transfers.
  • the DSZ.COM file size, padded to 128 bytes, was 413 blocks. So, 413 block transfers.
  • Don't forget to CLOSE the file once you've written, or DOS won't finalise the file and you'll end up with a 0 byte file.
This plus tip configured for 9600 and hardware flow control did the right thing. I then used DSZ to use ZMODEM to transfer a fresh copy of itself, and CAT.EXE (Alley Cat!)

Ok, so that bootstrapped enough of things to get a ZMODEM transfer binary onto a bootable floppy disc containing a half-baked DOS 5.0 installation. I can write software with QBASIC and I can transfer files on/off using ZMODEM.

Next up, getting XT-IDE going in this PC/AT and why it isn't ... well, complete.

Wednesday, December 16, 2020

Repairing and bootstrapping an IBM 5170 PC/AT, part 1

 I bought an IBM PC/AT 5170 a few years ago for a Hackerdojo project that didn't end up going anywhere.

So, I have a PC/AT with:

  • 8MHz 80286 (type 3 board)
  • 512K on board
  • 128K expansion board (with space for 512K extended RAM, 41256 RAM chip style)
  • ST4038 30MB MFM drive with some gunk on head or platter 3 (random head 3 read failures, sigh)
  • 1.2MB floppy drive
  • CGA card
  • Intel 8/16 ethernet card

Ok, so the bad disk was a pain in the ass. It's 2020, DOS on 1.2MB floppy disks isn't exactly the easiest thing to come across. But, it DOES occasionally boot.

But, first up - the BIOS battery replacement had leaked. Everywhere. So I replaced that, and had to type in a BASIC program into ROM BASIC to reprogram the BIOS NVRAM area with a default enough configuration to boot from floppy or hard disk.

Luckily someone had done that:

So, I got through that.

Then, I had to buy some double high density 5.25" discs. Ok, well, that took a bit, but they're still available as new old stock (noone's making floppy discs anymore, sigh.) I booted the hard disk and after enough attempts at it, it booted to the command prompt. At which point I promptly created a bootable system disc and copied as much of DOS 5.0 off of it as I could.

Then, since I am a child of the 80s and remember floppy discs, I promptly DISCCOPY'ed it to a second disc that I'm leaving as a backup.

And, for funsies, DOSSHELL.

Ok, so what's next?

I decided to buy an alternate BIOS - the Quadtel 286 image that's floating about - because quite frankly having to type in that BASIC program into ROM BASIC every time was a pain in the ass. So, in it went. Which was good, because...

Well, then it stopped working. It turns out that my clean-up of the battery leakage wasn't enough. The system booted with three short beeps and "0E" on the screen.

Now we get into deep, deep PC history.

Luckily, the Quadtel BIOS codes are available here:

.. but with the Intel BIOS, it didn't beep, it didn't do anything. Just a black screen.

What gives?

So, starting with PC/AT and clone machines, the BIOS would write status updates during boot to a fixed IO port. Then if you have a diagnostic card that monitors that IO port, you'd get updates on where the system go to during boot before it hit a problem. These are called POST (power on self test) codes.

Here's a write-up of it and some POST cards:

Luckily, the Quadtel BIOS just spat it out on the screen for me. Phew.

So! 0xE says the 8254 interval timer wasn't working. I looked on the board and ... voila! It definitely had a lot of rusty looking crap on it. U115, a 32 byte PROM used for some address line decoding also was unhappy.

Here's how it looked before I had cleaned it up - this is circa July:

I had cleaned all of this out and used some vinegar on a Q-tip to neutralise the leaked battery gunk, but I didn't get underneath the ICs.

So, out they both came. I cleaned up the board, repaired some track damage and whacked in sockets.

Then in went the chips - same issue. Then I was sad.

Then! Into the boxes of ICs I went - where I found an 8254-2 that was spare! I lifted it from a dead PC clone controller board a while ago. In IT went, and the PC/AT came alive again.

(At this point I'd like to note that I was super afraid that the motherboard was really dead, as repairing PC/AT motherboards is not something I really wanted to do. Well, it's done and it works.)

Rightio! So, the PC boots, CGA monitor and all, from floppy disc. Now comes the fun stuff - how do I bootstrap said PC/AT with software, given no software on physical media? Aha, that's in part 2.