- cctalk - classiccmp.org

by Robert Armstrong

Can someone explain how the Intel 8008 (yes, the 8008, NOT the 8080!) handled saving and restoring the flags during an interrupt? The 8008 has four flag bits - CZS&P - but there doesn't seem to be any explicit equivalent to the PSW register nor any "PUSH PSW" or "POP PSW" type instructions. Other than testing the flag bits with the JUMP/CALL/RET instructions there doesn't seem to be any way to access them. Interrupts work pretty much like the 8080 - during the interrupt response the peripheral forces an instruction onto the bus, typically an RST opcode but in theory it could be anything. RST and CALL however only appear to save the PC on the internal stack, and RET only restores the PC. So how do you save and restore the flag bits? It seems like interrupts would be pretty much useless without this, so I must be missing something. Thanks,

23 hours, 31 minutes

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Re: i8008 and interrupts

by trp

I didn’t see anyone post this clever way to save and restore flags in interrupt for the 8008. This is from https://stevemorse.org/8086history/8086history.pdf APPENDIX 1 SAVING AND RESTORING FLAGS IN THE 8008 Interrupt routines must leave all processor flags and registers unaltered so as not to contaminate the processing that was interrupted. This is most simply done by having the interrupt routine save all flags and registers on entry and restore them prior to exiting. The 8008, unlike its successors, has no instruction for directly saving or restoring flags. Thus 8008 interrupt routines that alter flags (practically every routine does) must conditionally test each flag to obtain its value and then save that value. Since there are no instructions for directly setting or clearing flags, the flag values must be restored by executing code that will put the flags in the saved state. The 8008 flags can be restored very efficiently if they are saved in the following format in a byte in memory. Most significant bit = bit 7 bit 7 = original value of CARRY bit 6 = original value of SIGN bit 5 = original value of SIGN bit 4 = 0 bit 3 = 0 bit 2 = complement of original value of ZERO bit 1 = complement of original value of ZERO bit 0 = complement of original value of PARITY With the information saved in the above format in a byte called FLAGS, the following two instructions will restore all the saved flag values: LDA FLAGS ;load saved flags into accumulator ADD A ;add the accumulator to itself This instruction sequence loads the saved flags into the accumulator and then doubles the value, thereby moving each bit one position to the left. This causes each flag to be set to its original value, for the following reasons: The original value of the CARRY flag, being in the leftmost bit, will be moved out of the accumulator and wind up in the CARRY flag. The original value of the SIGN flag, being in bit 6, will wind up in bit 7 and will become the sign of the result. The new value of the SIGN flag will reflect this sign. The complement of the original value of the PARITY flag will wind up in bit 1, and it alone will determine the parity of the result (all other bits in the result are paired up and have no net effect on parity). The new setting of the PARITY flag will be the complement of this bit (the flag denotes even parity) and therefore will take on the original value of the PARITY flag. Whenever the ZERO flag is 1, the SIGN flag must be 0 (zero is a positive two's- complement number) and the PARITY flag must be 1 (zero has even parity). Thus an original ZERO flag value of 1 will cause all bits of FLAGS, with the possible exception of bit 7, to be 0. After the ADD instruction is executed, all bits of the result will be 0 and the new value of the ZERO flag will therefore be 1. An original ZERO flag value of 0 will cause two bits in FLAGS to be 1 and will wind up in the result as well. The new value of the ZERO flag will therefore be 0. The above algorithm relies on the fact that flag values are always consistent, i.e., that the SIGN flag cannot be a 1 when the ZERO flag is a 1. This is always true in the 8008, since the flags come up in a consistent state whenever the processor is reset and flags can only be modified by instructions which always leave the flags in a consistent state. The 8080 and its derivatives allow the programmer to modify the flags in an arbitrary manner by popping a value of his choice off the stack and into the flags. Thus the above algorithm will not work on those processors. A code sequence for saving the flags in the required format is as follows: MVI A,0 ; move zero in accumulator JNC L1 ; jump if CARRY not set ORA 80H ; OR accumulator with 80 hex (set bit 7) L1: JZ L3 ; jump if ZERO set (and SIGN not set and PARITY set) ORA 06H ; OR accumulator with 06 hex (set bits 1 and 2) JM L2 ; jump if negative (SIGN set) ORA 60H ; OR accumulator with 60 hex (set bits 5 and 6) L2: JPE L3 ; jump if parity even (PARITY set) ORA 01H ; OR accumulator with 01 hex (set bit 0) L3: STA FLAGS ; store accumulator in FLAGS

23 hours, 47 minutes

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Re: Hot Video Shift Register on VT100

by Rob Jarratt

I found some time to look at this again today. A few answers below. > -----Original Message----- > From: Martin Bishop <mjd.bishop(a)emeritus-solutions.com> > Sent: 30 November 2025 23:12 > To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts > <cctalk(a)classiccmp.org> > Subject: RE: [cctalk] Re: Hot Video Shift Register on VT100 > > A short ground lead is 10 mm, from probe tip to ground spring see eg > https://www.aliexpress.com/item/1005007465068388.html > > However, ground springs require ground planes surrounding test points, or > test / ground point PEC layouts - very unlikely on a volume PCB. > > A 100 mm ground lead is neither short nor likely to be free of ringing. To > eliminate CRO probe ringing seriously short grounds are essential. > I pre-emptively bought an oscillator (24MHz and not 24.07342Mz) and tried it on a breadboard. This gave me the same signal with a negative spike on the falling edge. Although I don't have a ground spring I managed to shorten the ground connection considerably and this did indeed clear up the signal and it looked a lot squarer, without the negative spike. I lifted the original oscillator, but the leads on it are too short to work on my breadboard so I can't verify the same behaviour, but it seems highly likely that you are right that the negative spike is not real. That leaves me with the 74S299 still getting hot for reasons I don't understand. Maybe it is just the design, but the same chip in a VT102 doesn't get nearly so hot. Someone else suggested replacing it with a 74F299, so maybe I will try this now that I have socketed it. Regards Rob > Regarding some of the other points raised: > - 24.072 MHz is likely to be a magic number for video rates, with 24 MHz > YMMV > - the negative excursion may be clamped somewhere by diode(s) to gnd, > hence -1v2 .. - 1v4 excursion > - fast edges can be softened by a series resistor (22 .. 33 R say) following the > driver, triksy to retrofit > - are the power rails clean, on a scope, only pertinent if the spikes are > asynchronous to the signal(s) > > HtH; Martin >

1 day, 6 hours

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Editing files in VMS over a telnet connection

by Peter Ekstrom

I feel like I have tried everything, although I'm sure I haven't... I have my real MV 3100 up and running with a serial terminal on the console port. It works great but I don't want to sit by the machine all the time to do stuff. I have DECnet running and am able to connect to it. I have an old raspberry pi running a minimal linux setup with the decnet stuff installed. So I ssh to the pi (named gatewy) and from there I use dnlogin to talk DECnet and connect to the machines. This way I can connect to my PDP-11 machines running RSX11M and M+ and use EDT to edit files in screen mode. When I try the same under VMS, EDT starts, loads the file and enters screen mode but just shows a blank screen with the cursor in the top-left corner. If I go to line mode I can walk through the file and edit using line mode. But as soon as I go back to the screen mode (the change command) I just get the blank screen. I have made sure TERM in each shell is vt100 and I have tried forcing VMS to VT100 but it stays with a vt200_series terminal. I also have a SIMH MV3100 also running VMS 5.3-1 (they both run on that version) and it acts the same way as the hardware MV3100. If I go onto the console connection in SIMH EDT works fine, as on the physical terminal on the real machine. Anyone have any pointers or ideas I can try? - Peter

1 day, 22 hours

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Early delivery from Santa's sleigh

by Brian L. Stuart

So Santa had his sleigh out for some test drives tonight. For ballast (the real gifts won't be loaded until Wed night), he used a big stack of engineering drawings from the 1940s. Fortunately for us, his "spirited" driving included one turn that was too tight and out flew the drawings, where they landed on my lap. For anyone who's as eager as I am to geek out over 1940s computer design, I share them with you here: http://cs.drexel.edu/~bls96/eniac/drawings/ There are over 900 drawings in the set. Most of these images come from scans of microfilm taken of the patent trial exhibits. There are also about 100 copies that the Smithsonian had imaged and put up on their web site. I've processed them all to approximate something like we remember from our technical drawing classes in college (well those of us old enough to have taken them). The quality varies widely, but a large subset are readable down to the measurements for the panels and the component values in the circuits. If you've read Kathy Kleiman's excellent book (Proving Ground) about the original ENIAC programmers, you may have found yourself wondering what the block diagrams from which they figured out how to program the machine looked like. Well, those are among the drawings here. So feel free to try your hand at figuing out how to program it from them. One subset of the drawings is conspicuous by its absence, the PX-15 series. From what I can tell, those were drawings of a delay line memory unit that was never built. Those drawings were not part of the trial exhibits and were not microfilmed. However, some physical copies do exist, and I am hoping to get back into the archives to scan them at some point in the future. Beyond that, there are a number of drawing numbers that are skipped, and I suspect that these are drawings that do (or did) exist, but were not microfilmed for whatever reason. So if anyone has or comes across any drawings that are not on this list, or better copies of those that are, feel free to send scans my way. For anyone wanting to grab copies of the full set, it should be pretty easy to extract a file name list from the index.html file. I'll leave the sed command for extracting the list as an exercise for the reader. For printing, all of them are sized in the PDF files for 8.5x11 or 11x17 paper. Be warned, printing all of them without duplexing and folding the 11x17s to fit into a normal binder, ends up filling 2 4-inch binders. By the way, if anyone has access to the adapter shown in drawing PX-4-119, I think there's an error in the drawing and I'd like to check to see if either there really is or if I'm missing something. Thanks and have fun, BLS

2 days

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DEC VT05 available in Germany

by hupfadekroua

Hi all, there is a DEC VT05 available in Germany. Please look at kleinanzeigen.de and search for DEC VT05. I already talked to the seller and he‘ll ship worldwide. Best

4 days, 3 hours

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Re: IBM 2315 Cartridges

by brian＠quarterbyte.com

> No. The only DEC drive that had removable plus fixed on the same spindle > was the RC25. > If I remember correctly it was Diablo that sold an RK05-compatible drive with one fixed platter and one removable cartridge. I encountered it on a PDP 11/34 running Mini Unix and then, hmmm, Unix v<something small>. (Or maybe had to move to a larger disk system when we moved up from Mini Unix. The details have gotten hazy.)

4 days, 4 hours

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Computer Collection Dissolution

by Jochen Kunz

Hello. So it is happening. Finally. What a relieve. I am giving away my (almost) entire classic computer collection. Free of charge (almost). But local pickup only due to health reasons. Read: Due to a physical disability I am not able to ship any computers. Location: Kaiserslautern, Germany. Almost because I will keep my beloved SPARCstation 1+, my SGI O2 and a few spare parts for sentimental reasons. I am still tempted to keep a QBus with a LSI11 and MicroVAX III... Anything else has to go. I don't wane make a profit out of it. I want the stuff go into good hands with absolut minimum effort at my side. As compensation I only ask for your help to get rid of some other stuff that has accumulated over the years. (Loads of disk, CDROM and tape drives. Cables, adapters, expansion cards, RAM modules, ... and some junk in my basement. More cables, 14" VGA monitor, glass-TTY, 19" rack mounts, empty boxes, ...) Either you take some of the junk with you, or you help me bring the junk to the nearby recycling center. I don't care, as long as the junk is gone. I may also request your help to rearrange existing or install new furniture in my appartement. (My desk uses the Sun 3/260 as support...) You want it all? Perfect. Rent a moving truck. Come and get it. You just want some pieces? Fine. Come, get what you want and take some junk out of the pile. The more nice stuff you take, the more junk you have to take. (Sounds worse then it is.) Almost complete list of stuff available below. Almost all stuff can be picked up and transported by a singe, strong person. Only a few pieces will require an additional hand, that I can not provide my self due to health reasons. Contact me off list with your machines of interest to get things arranged. Rationale: I haven't done anything with this stuff in 15+ years. After 15 years, I am quite confident that this will not change. I tried to get back into it in 2010 by acquiring a PDP-11/34A. After a few weeks I lost interest and now it is sitting partly disassembled and waiting for repairs. (The PSU bricks need new electrolytic capacitors...) Sitting there for 15 years. Untouched. I haven't really read any postings to this list in 10+ years. I am basically only still subscribed to nag myself to write this email. For me, my machinery is like books that I have read. I had a lot of fun reading those books. I learned a lot out of them. Most important are the people that I got to meet via them. Like this very mailing list and the legendary VAXpower meetings... :-) But now I know all the books. There is nothing new to discover. The books just sit there on shelves, collecting dust. It reached a point where all this stuff is getting in my way to live a happy live. The collection has become an obligation, a burden. Especially as: At the end of 2009 my back broke and I needed an emergency surgery. Now I have to be very careful with lifting and carrying stuff. I don't move anything over maybe 6 kg anymore. (That is 13ish pounds for you imperialists. ;-) ) This is also the reason why I can not ship anything. I am simply not physically able to package the stuff and bring it to the post office without risking breaking my back again. And when my back brakes again, I may end up in a wheelchair. I escaped that just barely when my back broke initially in 2009... Needless to say that this physical damage also caused mental issues. I will not take any risk breaking my body again. Also: I live in an old appartement building. My landlord will renovate the appartement that I rent in late summer 2026. I may move out of this appartement into an other one in spring 2026, so that the appartement is uninhabited during renovation. I will not move all of this stuff to a new appartement. And if I don't move, I wane get rid of the collection anyway. It will ease the renovation process a lot. Anything that is not already gone when the renovation starts, respectively on moving day, will be recycled. If this means the PDP-11/34A goes into the dumpster to be shredded, than it will happen. Machinery: DEC PDP-11/34A + RK05, optionally in low boy 19" rack. Shares a RL02 and a Cypher 9-track Pertec tape drive with the 11/73. Tektronix 4000-something storage tube vector graphics terminal. I think I still have the accompanying Tek printer. Originally part of the 11/34A system. DEC PDP-11/73, the "franken-11" I pieced this together over time in a BA23 desk side box. Runs 2.11BSD. Has 4 MB RAM, 2 x MSCP ESDI disks (IIRC 150 MB each), TK50, RL02, SMD and Pertec adapters. Comes with a 9" 1 GB E-SMD disk. Shares a RL02 and a Cypher 9-track Pertec tape drive with the 11/34A. DEC VAX 4000-300, upgraded to a -400 DEC VAX 4000-200 DEC VT1300 2 x DEC VAXstation 2000 (At least one has patched ROMs to boot SCSI.) DEC VAXstation 3100m76 DEC VAXstation 4000 VLC DEC VAXstation 4000-60 DEC VAXstation 4000-90 DEC MicroVAX 3100-95 DEC DECstation 3100 DEC DECstation 5000-240 DEC AlphaStation 200 4/166 DEC 3000 300x DEC 3000 600 DEC Alpha PWS 500au DEC Alpha DS20E (I don't know for sure. It has no model number on the front. It looks like a DS20E. I have never, ever even powered up this thing.) Tektronix 8560 Multi-User Software Development Unit This is a DEC PDP-11/23 CPU in Tek box. It runs a customized UNIX V7. At my parents place there should still be MicroVAX III MicroVAX 3500. 2 x 9" NEC E-SMD disks, 1 GB Tektronix 4000-something text terminal with storage tube. Additional logistics will be required if you wane retrieve this stuff. IBM PS/2 Modell 95 XP 486 IBM PS/2 80 IBM RS/6000 43P-140 IBM RS/6000 43P-150 IBM RS/6000 44P-170 SGI Indy SGI Octane, SSE GFX, PCI card cage with SCSI and FDDI cards. IIRC 300 MHz R12k and 2 GB RAM. There must be an additional SI or SE graphics somewhere. I run this machine dual headed. This was my main desktop from 2001 to 2006. SGI Personal IRIS, probably a 4D/25, I can't remember. SGI Indigo IIRC R3000 SGI Indigo IIRC R4000 SGI Indigo 2 teal R4000, Extreme GFX SGI Indigo 2 magenta R10k, Impact GFX SGI Octane carcass (Empty main box with front plane, no CPU, GFX, ...) SGI Origin 200 Sun 3/60 Sun 3/60 box with Sun 4/600 board inside, -12V hack for console RS232. Sun 4/110 Sun 3/260 Sun SPARCstation IPX Sun SPARCstation 2 Sun SPARCstation 5 Sun SPARCstation 10 Sun SPARCstation 20 Sun Ultra 1 creator3D Sun Ultra 2 Sun Ultra 5 ("franken 5" build from parts into a PC box.) Sun Ultra 10 Sun Netra X1 Sun Netra T1 Sun Fire V240 Apple Macintosh IIci Apple MacMini G4 Apple PowerMac G5, single 1,8 Ghz. Motorola MVME Type 3200, desktop box, 4 x 6U VME, IIRC MVME162 CPU. Sony NEWS NWS-3410 HP Apollo 400 425t HP 9000 750 (It may be 400s, but IIRC it is a 750.) HP 9000 B2000 Loads of (mostly SCSI) disk, CDROM and tape drives. Cables, adapters, expansion cards, RAM modules, hubs, switches, FDDI concentrator, ... IBM P200 / Sony Trinitron 19" monitor. Does sync on green. I used it with my SGIs. HP LaserJet 4 with JetDirect LAN (The VF-Display is dead, but otherwise works.) 2 x HP Scanjet IIc, one has an Automated Document Feeder. Works great with Impresario from SGI IRIX. Test equipment: Dolch Logic Instruments logic analyzer. IIRC 48 channels @ 300 MHz. HP Digital Storage Oscilloscope 54710 with 3 x 1 GHz / 2 Msamels/s Y-modules. Analog photo gear: Durst Laborator 1000 4"x5" enlarger, table top version. -- tschüss, Jochen

6 days, 23 hours

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Beginning January 2026, all ACM publications and related artifacts in the ACM Digital Library will be made open access.

by Christian Liendo

Dear Members of the Computing Community, We are pleased to share an important milestone for our field. Beginning January 2026, all ACM publications and related artifacts in the ACM Digital Library will be made open access. This change reflects the long-standing and growing call across the global computing community for research to be more accessible, more discoverable, and more reusable. https://dl.acm.org/openaccess

1 week, 1 day

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[OT] Digikey's project of the week is impressive

by me＠scifidan.com

A dude decided to do a hackodore 64 laptop in the most expensive way possible, but it's a beautiful result. Nice design ethic. I don't typically share youtube links but this one is a nice exception. https://www.youtube.com/embed/H5QQ0ECfwyE I hope this inspires perifractic into doing a c128 laptop after their new fpga64 ships. Daniel sysop | Air & Wave BBS finger | calcmandan(a)bbs.erb.pw

1 week, 1 day

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