- VCF South West - June 14 - 16, Davidson-Gundy Alumni Center at University of Texas at Dallas
- VCF West - Aug 2 - 3, Computer History Museum, Mountain View, CA
- VCF Midwest - Sept 7 - 8 2024, Schaumburg, IL
- VCF SoCal - Mid February 2025, Location TBD, Southern CA
- VCF East - April 2025, Infoage Museum, Wall NJ
-
Please review our updated Terms and Rules here
VAX binary compatibility question.
- Thread starter Teletech
- Start date
nadaveiron
Member
- Joined
- Dec 6, 2019
- Messages
- 39
It depends what you have on the disk. uVAX II support came into the main line VMS releases (as opposed to MicroVMS being a separate product) with 5.0, so 5.0 and later are no-brainers. I'm not sure about a 4.x disk. MicroVMS is VMS but it may require some cajoling. Earlier (3.7 or earlier) didn't support uVAX at all. Try on simh?
mcguire
Experienced Member
I expect that it would boot. If it's a UNIX, the device naming should be the same (in /etc/fstab), and you should be able to match the unit numbers.
If it's VMS, it should boot but may have trouble due to differences in memory size, and sysgen parameters based on that, as VMS doesn't autosize memory at boot time on a VAX. Memory sizes in MicroVAX-IIs are usually odd, due to the 1MB present on the CPU board, so 5MB, 9MB, 13MB, etc. If you have 16MB on separate memory boards, the 1MB on the processor board is disabled so you'll get 16MB. But VAX-11/750s tended to have more "even" memory sizes like 8MB.
I've moved a MicroVAX-II VMS system disk to a VAX-11/750, but not in the other direction.
-Dave
If it's VMS, it should boot but may have trouble due to differences in memory size, and sysgen parameters based on that, as VMS doesn't autosize memory at boot time on a VAX. Memory sizes in MicroVAX-IIs are usually odd, due to the 1MB present on the CPU board, so 5MB, 9MB, 13MB, etc. If you have 16MB on separate memory boards, the 1MB on the processor board is disabled so you'll get 16MB. But VAX-11/750s tended to have more "even" memory sizes like 8MB.
I've moved a MicroVAX-II VMS system disk to a VAX-11/750, but not in the other direction.
-Dave
g4ugm
Veteran Member
The machines are not instruction set compatible. The 11/750 has pdp-11 emulation, the uVAX does not. The uVax may be deficient in other ways. Pre-4.x versions on VMS made use of this mode. Not sure if it was used to boot, but certainly for some utilities. Perhaps this old thread I started may help...
comp.os.vms.narkive.com
VAX PDP11 Compatibility Mode
OK, that *is* interesting. I recall the 750 updating it's microcode as part of booting, so there's that aspect of things. I wonder how deep that emulation goes, could you run RT-11 or RSTS on a 780 for example if you had the right loader?The machines are not instruction set compatible. The 11/750 has pdp-11 emulation, the uVAX does not. The uVax may be deficient in other ways. Pre-4.x versions on VMS made use of this mode. Not sure if it was used to boot, but certainly for some utilities. Perhaps this old thread I started may help...
VAX PDP11 Compatibility Mode
Anyway, it suggests that later software might be easier to dual-port boot on a drive and I defiantly shouldn't assume I can interchange boot media for a VAX in a different family.
Good to know that at least sometimes it works based on the report of moving a uVAX VMS disk over to a 750.
mcguire
Experienced Member
I don't believe running PDP-11 OSs is possible in VAX-11 family's PDP-11 compatibility mode.
It's true that the VAX-11/750's microcode is updated at boot time. The OS bootstrap checks to see if the system being booted is an 11/750, and if it has the PCS board (L0008) rather than the CCS board (L0005). (It's said that all L0005 CCS boards in the field were replaced by L0008 PCS boards, but there are some floating around.) If these conditions are met, the microcode patches in pcs750.bin are loaded into the PCS board's RAM.
Very generally speaking, it's fine to assume you can interchange boot media between VAXen if you're using the same devices and you're not running pre-V4 VMS. You may not get all the way up, but you'll get somewhere, and can usually recover from there. The two OSs you're likely to see are VMS and various flavors of UNIX. As I mentioned above, the former may need some adjustments to sysgen parameters, and for the latter, most of the important stuff is scaled at boot time. (modulo things like MAXUSERS in the configuration files for BSD UNIX kernels, which is used to set various sizes at build time but usually won't keep a machine from coming up)
-Dave
It's true that the VAX-11/750's microcode is updated at boot time. The OS bootstrap checks to see if the system being booted is an 11/750, and if it has the PCS board (L0008) rather than the CCS board (L0005). (It's said that all L0005 CCS boards in the field were replaced by L0008 PCS boards, but there are some floating around.) If these conditions are met, the microcode patches in pcs750.bin are loaded into the PCS board's RAM.
Very generally speaking, it's fine to assume you can interchange boot media between VAXen if you're using the same devices and you're not running pre-V4 VMS. You may not get all the way up, but you'll get somewhere, and can usually recover from there. The two OSs you're likely to see are VMS and various flavors of UNIX. As I mentioned above, the former may need some adjustments to sysgen parameters, and for the latter, most of the important stuff is scaled at boot time. (modulo things like MAXUSERS in the configuration files for BSD UNIX kernels, which is used to set various sizes at build time but usually won't keep a machine from coming up)
-Dave
Bitly
Experienced Member
You have to execute a specific instruction (or sequence of instructions) to switch from VAX to PDP-11 mode, so I can't see any way for a PDP-11 OS to boot.
CW
mcguire
Experienced Member
Yes, and that would be pretty easy to add to the bootstrap code. But I think (I don't know for sure) that there would be problems elsewhere. I've not looked very deeply into how PDP-11 compatibility mode is implemented. One area that might have issues is the I/O page; the VAX side would have to map device CSRs into the "PDP-11"'s address space somehow. That would involve a fair bit of VAX-specific setup. And then such an approach would only work for peripherals on the Unibus or Qbus; things like the RH750 would have no direct analog in the PDP-11 world. (the RH11 implements the same interface but is a very different beast)
-Dave